Unit 1.3 - Chemical Calculations Flashcards
Relative atomic mass symbol
Ar
Why can atoms not be weighed?
They’re too small
What do we do as atoms are too small to be weighed?
Compare them to a standard - the C-12 isotope
What is the standard we compare atoms to?
The Carbon 12 isotope
What’s the mass of C-12 isotopes?
12 units
What can atoms of the same element have due to the existence of isotopes?
Different masses
What do we have to do to define relative atomic masses due to the existence of isotopes?
Take an “average” mass of the atoms
Does relative atomic mass (Ar) have units? Why?
No, as they’re ratios of units
What do all of the “relative….mass” definitions have reference to?
The C-12 isotope
Relative atomic mass (Ar)
The ratio of the average mass of an atom of the natural nuclidic composition of the element to 1/12 of the mass of an atom of the C-12 isotope
Relative isotopic mass
The ratio of the average mass of an atom of the element to 1/12 of the mass of an atom of the C-12 isotope
Relative molecular mass symbol
Mr
Relative molecular mass (Mr)
The ratio of the mass of a molecule of a chemical compound to 1/12 of the mass of an atom of the C-12 isotope
How do you calculate the relative molecular mass (Mr)?
Add up all of the relative atomic masses (top numbers), multiplying only by the small numbers and ignoring big ones, using the molecular formula
How are relative formula mass and relative molecular mass the same?
-Same definition
-Same symbol (Mr)
-Same calculation
How is relative formula mass different to relative molecular mass?
Relative molecular mass = Molecular formula
Relative formula mass = Empirical formula
What do we measure the amount of a substance in?
Moles
Mole
One mole is the amount of a substance which contains the same number of particles as there are atoms in 12g of the C-12 isotope
What is Avogadro’s Constant?
The number of atoms in one mole of an element
Avogadro’s constant symbol
NA
What is the number of atoms in one mole of an element?
Avogadro’s constant
How do we calculate the mass of 1 mole of an element in grams
Ar in grams (super easy!)
What’s the ratio of the average mass of an atom of the natural nuclidic composition of the element to 1/12 of the mass of an atom of the C-12 isotope?
Relative atomic mass (Ar)
What’s the ratio of the average mass of an atom of an element to 1/12 of the mass of an atom of the C-12 isotope?
Relative isotopic mass
What’s the ratio of the mass of a molecule of a chemical compound to 1/12 of the mass of an atom of the C-12 isotope?
Relative formula mass (Mr)
What’s the amount of a substance which contains the same number of particles as there are atoms in 12g of the C-12 isotope?
A mole
What does the mass spectrometer do?
Accurately measures relative atomic masses
What do we obtain from a mass spectrometer?
A mass spectra, which has a wide variety of uses in labs
What’s the order of the processes that take place within the mass spectrometer?
-Vaporisation
-Ionisation
-Acceleration
-Deflection
-Detection
Where does the vaporised sample enter in order to be ionised in the mass spectrometer?
The ionisation chamber
Why must our sample into the mass spectrometer be vaporised?
You can only ionise vapour
How does the ionisation stage within the mass spectrometer work?
The electron gun gives out high energy electrons, which knock the electron out of an atom in the gaseous sample due to its speed, thus ionising them
Why is ionisation necessary in the mass spectrometer in order to accurately measure relative atomic masses?
-Only ions will create a current when hitting the detector
-We want ions, not molecules to interact with the magnetic field
What happens during the acceleration stage to ions within the mass spectrometer?
They’re sped up by a magnetic field until they’re all at the same speed into a finely focused beam
How are ions sped up in the mass spectrometer?
By a magnetic field
Until when are the ions sped up in a mass spectrometer?
Until they’re all travelling at the same speed into a finely focused beam
Draw and label the mass spectrometer
(Check notes)
What’s the purpose of the vacuum pump on the mass spectrometer?
To prevent the ions colliding with air molecules
What are ions deflected by in the mass spectrometer?
A magnetic field (an electromagnet)
What is at the detection stage of the mass spectrometer?
-Amplifier
-Chart recorder
Why is it the MASS that impacts how much the ions are deflected by the electric field of the electromagnet in a mass spectrometer?
As the ions are all equal charge so this doesn’t effect it
What are ions deflected according to in a mass spectrometer?
Their mass/charge ratio
If an ion is lighter, how does this effect how it’s deflected?
Deflected more
What type of ions reach the detector first in a mass spectrometer?
The lighter ones
Which ions have the lowest m/z value in a mass spectrometer?
The lightest ones
What happens to ions with different mass to charge ratios in the mass spectrometer?
They’re deflected and measured separately
What do we look at for an ions m/z number?
Its mass number
Mass to charge ratio symbol
m/z or m/e
List what happens at the detection stage on the mass spectrometer
-Beam of ions detected electrically
-Electric current = amplified + recorded
-More ions = more current
-Results analysed by a computer and displayed on the visual display unit of the computer
How does the amount of ions effect the current in a mass spectrometer?
More ions = higher current
How are the results from a mass spectrometer analysed on a computer?
Displayed on the visual display unit of the computer
What two things do we need to know in order to calculate the relative atomic masses of elements?
- The relative isotopic mass (top number)
-The relative abundance of the isotopes
Relative abundance
The fraction of that isotope found in the natural nuclidic composition of the element
What’s the fraction of an isotope found in the natural nuclidic composition of an element?
Its relative abundance
What’s the formula for calculating the relative atomic mass of an element?
(%abundance X isotopic mass of element - top number) + (next one) + (next one) …
———————————————————————————————————————
100
How can we confirm if the relative atomic mass that we’ve calculated is correct?
Is says it on the periodic table (the top number)
What do we need to do if the abundance given to us for a relative atomic mass formula is in decimals?
We do NOT have to divide it by 100
What do we do if relative abundance isn’t given in decimals or percentages and simply as a number?
Add them together and divide by this total
What’s the highest possible relative abundance?
100%
What’s relative abundance (simply)?
How much of that isotope is in that element
What’s goes along the x and y axis on a graph for calculating relative atomic mass?
X - mass/charge (m/z)
Y - relative abundance
What’s really important to show on the isotopes we’re using for relative atomic mass calculations?
Their charges
What do we do if peak height is given instead of relative abundance to calculate the relative atomic mass?
Relative abundance = peak height
——————
Total height
Equation for the ionisation of Cl-35 in the mass spectrometer
Cl2 (g) + e- ——> Cl2+ (g) +2e-
Equation for the ionisation of Chlorine
Cl2 (g) + e- ——> Cl2+ (g) + 2e-
What can molecular ions do in the mass spectrometer?
Undergo fragmentation to give us monatomic chlorine ions
What does the fragmentation of chlorine molecular ions give us?
Monatomic chlorine ions
Why does fragmentation happen to molecular chlorine ions in the mass spectrometer?
Due to the instability of Cl2+ ions
What ions does Chlorine come out as out of the ionisation chamber and what’s wrong with this?
Cl2+ are unstable = undergo fragmentation
Fragmentation
the dissociation of energetically unstable molecular ions formed from passing the molecules in the ionization chamber of a mass spectrometer
Chlorine Cl2+ ions fragmentation equation
Cl2+ (g) ——> Cl+ (g) + Cl (g)
How many isotopes does chlorine have and what are they?
Two
35Cl
37Cl
What is the abundance of the Cl35 Chlorine isotope in its natural nuclidic composition?
75%
What is the abundance of the Cl37 Chlorine isotope in its natural nuclidic composition?
25%
What is the ratio in the atomic ion region of chlorine?
3:1
What gives us the molecular ion region on chlorines graph?
The fact that chlorine exists as a diatomic molecule - it consists of molecules, not individual atoms (think of the possible combinations of Cl35 and Cl37)
What type of molecule does chlorine exist as and what does this give us?
Diatomic, giving us the molecular ion region on the graph
What are the two ion regions we have o chlorine’s mass spectrum?
-atomic ion region
-molecular ion region
What do we call the graph with relative abundance and m/z?
The mass spectrum
What does chlorine’s atomic ion region peak in?
3:1 ratio
What does chlorine’s molecular ion region peak in?
9:6:1 ratio
Which molecular ions do we have for chlorine?
M/z = m70, m72 and m74
What’s the possibility of an atom being 35Cl?
3/4
What’s the possibility of an atom being 37Cl?
1/4
How do we get the fractions for figuring out the ratio of the molecular ion region on chlorine’s mass spectrum?
Multiply the possibility of getting each isomer
E.g - 35Cl-35Cl
3/4 x 3/4
= 9/16
(The 9 in the ratio)
What’s it important to do with all of the fractions when working out the ratio of a molecular ion region?
Keep them relative (same bottom number) so as to get the correct ratio - the calculator might simplify!
What do we do if we’re asked to work out the percentage of an isotope in a molecule, or the mass number?
Use algebra - the same method as before with the relative atomic mass but with an ‘x’ where relevant - remember you can find the relative atomic mass for the start of the equation on the periodic table
How do you work out the ratio of the molecular ion region of an element?
Multiply the possibilities of getting each isotope (abundance) with it’s possible pair - make sure all fractions have the same bottom number
(Check notes for chlorine example, it’ll look confusing here)
What isotopes does Bromine have?
79Br and 81Br
What is the ratio of 79Br and 81Br isotopes in a Bromine molecule?
50:50
How many peaks does a diatomic molecule have on its mass spectrum? How do we know this?
5
Think - we have 2 isotopes, which is 2 peaks in the atomic ion region
Then, we have 3 different possibilities of combinations of these isotopes for the molecular ion region = 3 more peaks
How many peaks does Bromine’s mass spectrum have?
5
Which diatomic molecules do we need to remember their mass spectrums and why they look like this (know the ratios, abundances, isotopes e.t.c.)?
Mostly Chlorine, an idea of Bromine too
Empirical formula
The simplest ratio of atoms in the molecule
Molecular formula
The actual number of each atom present in the molecule
How do we calculate the relative molecular mass?
Sum of Ar’s in the molecule (multiply only by the little numbers, ignoring the big ones)
Relative molecular mass symbol
Mr
Molecular formula of glucose molecules
C6H1206
Molar mass unit
gmol-1
Molar mass of glucose molecules
180gmol-1
Empirical formula of glucose molecules (how is this worked out?)
CH20
(Molecular formula divided by 6)
What do we do if working out the empirical formula from a diagram?
Ignore ions
Are ions part of empirical formulas?
No
What do we have to know in order to calculate empirical formulas?
The % composition by mass of each element in the molecule in order to divide it by their relative atomic masses
Steps in working out Empirical formulas
- Write symbols of elements
- Write masses/percentages from the question
- Divide each mass/percentage by the mass number of that element (Ar)
- Divide each answer with the smallest answer to find the ratio
Do we ever round up or down when working out empirical formulas?
No, instead we multiply them up if the ratios are in decimals (individual elements)
What would you multiply a 0.2 decimal in a ratio for an empirical formula by?
5
What would you multiply a 0.3 decimal in a ratio for an empirical formula by?
3
What would you multiply a 0.5 decimal in a ratio for an empirical formula by?
2
What do we have to know in order to work out the molecular formula of a molecule?
The relative molecular mass
How do we calculate the molecular formula of a molecule?
- Determine the relative mass of empirical formula
(Add up the Ar numbers, multiplying only by the small numbers and ignoring the big ones) - Divide the Mr (given in question) by mass of empirical formula (our last answer) to get a multiple
- Multiply empirical formula by the multiple
How do we determine the relative mass of an empirical formula?
Add up all of the Ar numbers (relative atomic masses of the atoms), multiplying only by the small numbers and ignoring the big ones
What’s the most likely mass of a chlorine molecule?
70
Define the mole
The standard unit (an SI unit) for the amount of a substance, which is Avogadro’s constant (NA)
Avogadro’s constant symbol
NA
Simple way of defining the mole in terms of particles
One mole has (Avogadro constant) amount of atoms
What type of quantity is molar mass and what does this mean for it?
A physical quantity, therefore it has units
Molar mass units (Mr)
gmol-1
Define the molar mass of an element
The mass of one mole of the element in grams
What is molar mass equal to?
Mr
Triangle showing the relationship between molar mass, mass and amount of moles
m
—
n I M
What does M stand for?
Molar mass
What does m stand for?
Mass
What does n stand for?
Amount in moles
Amount in moles unit
Mol
What do we use as our molar mass in equations and when?
If we get no information on the moles or mass, make the molar mass equal to the Mr
Number of particles/atoms formula
Number of moles x NA
(Remember - this can be rearranged!)
Number of particles unit
It doesn’t have one
What is necessary in calculations such as ones discovering the number of particles?
Combining both the molar mass, mass, moles triangle equation and the number of particles, moles and NA one
What do the same conditions of temperature and pressure cause any gas to do?
Occupy the same volume
What conditions are require to be kept the same for one mole of any gas to occupy the same volume?
Temperature
Pressure
When does one mole of any gas occupy the same volume?
Under the same conditions of temperature and pressure
How much of any gas will occupy the same volume under the same conditions of temperature and pressure?
1 mole
What is the molar volume of a gas?
Under the same conditions of temperature and pressure, one mole of any gas will occupy the same volume
What’s the name for the fact that under the same conditions of temperature and pressure, one mole of any gas will occupy the same volume?
Molar volume
Stp
Standard temperature and pressure
What is the standard temperature and pressure?
273K
1 atm (1.01x10^5 Pa)
What is 1 atm?
1.01 x10^5 Pa
22.4dm^3 in litres
22.4 litres (the same)
What’s the difference between the two molar volumes given in the data sheet?
Different temperatures
What happens to a gas with increased temperature, providing that the pressure remains constant?
Gas occupies a bigger volume
When does a gas occupy a bigger volume?
With increased temperature, providing that the pressure remains constant
What do we need to check before answering any of the questions surrounding molar volume?
The temperature, to get our molar volume based off of information on the data sheet
Volume unit
dm^3
Moles unit
Mol
Triangle for the equations on molar volume
V
—
n I Vm
Amount in moles symbol
n
Volume symbol
V
Molar volume symbol
Vm
How do we transfer between cm^3 and dm^3?
cm^3 ———> dm^3
÷1000
What does the volume always have to be in molar volume questions?
dm^3
How do we figure out how much one mole of an element is?
Ar in grams (super easy)
What do we do if a molar volume question gives us the temperature in degrees Celsius?
Add 273 to the number and it’ll be in Kelvin
(It says this on the front of the data sheet though)
How do we know whether to combine the molar volume equations with the molar mass equations?
Just think of the information that we have - for example, if it’s asking us to calculate the mass with data on the moles, use the molar mass equations, but if we’re given the volume, we need to use the molar volume equation for the amount of moles and then use the molar mass equations to go back to the mass - I’ve made it sound really confusing. Just think what equations you know and what information you have and what makes sense together - you’ll get it.
What do you call the solution that moles are in?
Concentration
Concentration
How much of a dissolved substance is present per unit volume of a solution (number of moles in 1dm^3)
How many dm^3 and cm^3 is one litre?
1 dm^3
1000cm^3
How many litres and dm^3 is 1000cm^3?
1 litre
1 dm^3
How many cm^3 and litres is 1 dm^3?
1000cm^3
1 litre
What is the main solvent used in chemistry?
Water
What is concentration measured in?
gdm^-3 OR moldm^-3
How do we convert between gdm^3 and moldm^3?
gdm^-3
——-————
moldm^-3 I Mr
Moles in solution equations
n
—
c I V
Shorthand of moldm^-3
M
What is “M” the shorthand of?
Concentration (moldm^-3)
What is the mass to charge ratio (m/z) equal to and when?
When the ions generated by a mass spectrometer have a 1+ charge (like chlorine!), it’s equal to the atomic mass of the element
Concentration in parts per million (ppm) equation
Mass solute
—————— x1,000,000
Mass Solution
When do we take into account the big numbers of a molecule and when don’t we?
Molecular = ignore big numbers
Formula = use big numbers
(Mr)
What are the masses of the molecular ion region in the mass spectrum of chlorine?
70, 72 and 74
What are the masses in the molecular ion region on the mass spectrum of Bromine?
158, 160, 162
Show algebraically the relationship between the amount of moles o A, B, C and D involved in a reaction
nA nB. nC nD
— = — = — = —
a. b. c. d.
What does the algebratic equation between nA, nB, nC and nD show?
The amount in moles of A, B, C and D respectively involved in the reaction
What’s the reaction between sodium hydroxide and hydrogen chloride?
NaOH + NCl ———> NaCl + H20
How can we see if different elements have the same number of moles as each other or not?
Look at the big numbers - the same = same number of moles
What do we need to be given to calculate the mass of the products formed?
-Mass of reactants
-A balanced chemical equation
What happens in a chemical reaction?
Reactants are changing into products
Ratio between amount of moles of reactants and products
Stoichiometric ratios (mole ratios)
Stoichiometric ratios
Ratio between the amount of moles of reactants and products (mole ratios)
Do we multiply with the big numbers for molar mass (Mr)?
NO
What do we need to avoid doing in reacting masses calculations?
Over-round a figure too early on in the calculation (accuracy marks!!)
What do we do if asked to work out the identity of an element after we’ve figured out its mass?
Just match it with that relative atomic mass off of the periodic table
1 tonne in kilograms
1000kg
1000kg in tonnes
1 tonne
What do we do if we have an unknown mass and don’t have the moles or molar mass to work it out?
Total mass of reagents = total mass of products
e.g -
0.800g of M(OH)2 ——> 0.553g of MO + ?g of H20
0.800-0.553 = 0.247g
What is one of the main aims in green chemistry?
To reduce pollution created in a chemical reaction
What happens when a large proportion of a reagent ends up as waste? (3 things)
-Pollution contribution
-Ineffective use of resources
-Raise in production costs
What contributes to pollution, ineffective use of resources and a raise in production costs?
Large proportions of reagents ending up a waste
Atom economy
A measure of the amount of starting materials that become useful products
What is a measure of the amount of starting materials that become useful products?
Atom economy
Atom economy equation
Atom economy (%) = mass of useful product from equation
——————————————————— x100
Mass of reagents from equation
What do insufficient, wasteful processed have?
A low atom economy
What have low atom economies?
Insufficient, wasteful processes
What do efficient processes have?
High atom economies
What have high atom economies?
Efficient processes
What are efficient, high atom economy processes important for?
Sustainable development - fewer resources, less waste
What is the ideal atom economy and what would this actually mean?
100% - no waste products, everything is useful products
Do we take into account the big numbers for Mr when working out atom economies?
(Sigh) - yes.
In what equations do we take into account the big numbers for Mr?
Atom economy equations
Why is hydrogen a useful product?
Hydrogen fuel cells make electricity, with the only product being water
Name 2 non-renewable sources
Coal
Methane
What type of sources are coal and methane?
Non-renewable
% yield equation
% yield = actual mass of product obtained
————————————————
Theoretical yield
How does percentage yield working out start?
Very similar to to reacting masses sums - underline what’s important, work out the masses through comparing molar ratios etc, but use the formula at the end
Do we take into account the big numbers for Mr with percentage yields?
No
Give 2 reasons why a % yield wouldn’t be at 100%
-Some reactants not fully reacting
-Formation of side products (especially oxygen, which often reacts with reagents)
Which element often reacts with reagents?
Oxygen
Do you use the big numbers (molar ratios) in atom economy and percentage yields?
Atom economy - yes
Percentage yields - no
Is it possible for atom economy to be above 100%?
No
What do we do if a question states that something is ‘burning in air’?
Put it down as reacting with oxygen to create an oxide
What is the ratio of in empirical and molecular formulas?
Moles - in a more complex question, remember that’s what we’re trying to get the ratio of! Use the formula!
What do we do with the multiples we receive when figuring out the relative formula of a compound?
Don’t round them up - only round up the final molecular formula
How do you calculate the number of molecules in a sample?
Moles (n = m x Na (Avogadro’s number)
—
M)
How do you calculate the number of atoms in a sample?
Number of molecules x number of individual elements (e.g - SOCL2 has 4)
What do we need to remember with empirical formula?
Divide down to the SIMPLEST ratio of atoms in the molecule
Boyle’s law
PV=constant
Product of pressure and volume is a constant
-At a constant temperature, the volume of a fixed gas is inversely proportional to its pressure
Which law is PV = constant?
Boyle’s law
Charles’ law
V
— = constant
T
The volume of gas is constant as long as the pressure remains constant
Which law is V
— = constant?
T
Charles’ law
Gay-Lussac’s law
P
— = constant
T
The pressure is proportional to the temperature as long as the volume remains constant
Which law is P
— = constant
T
Gay-Lussac’s law
What has to remain constant for Boyle’s law?
Temperature
What has to remain constant for Charles’ law?
Pressure
What has to remain constant for Gay-Lussac’s law?
Volume
How did we get the gas constant and what is its symbol?
Combining Boyle’s, Charles’ and Gay-Lussac’s laws gives…
PV
— = constant for a fixed mass of gas (R)
T
So, PV=RT (for 1 mole of gas)
What is the ideal gas equation?
PV = nRT
Does every gas obey the ideal gas equation?
No gas obeys it entirely
Under what conditions does the ideal gas equation hold quite well?
Room temperature and pressure
Under room temperature and pressure, what holds quite well?
The ideal gas equation
2 features of an ideal gas
-All collisions between atoms or molecules are perfectly elastic
-No intermolecular attraction forces
What would having all collisions between atoms or molecules perfectly elastic and no intermolecular attraction forces give us?
An ideal gas
Perfectly elastic collisions between atoms or molecules
All energy is transferred - no loss of energy
If there’s no loss of energy and all is transferred during a collision between atoms or molecules, describe the collision
Perfectly elastic
SI units used with the ideal gas equation
Pressure - Pa
Volume - m^3
Temperature - K
What’s the first thing we do before anything else when dealing with the ideal gas equation questions?
Convert into the SI units
Standard pressure
1.10x10^5
(Data booklet)
atm ——> Pa
x1.10x10^5
(data booklet)
kPa ——> Pa
x1000
dm^3 to m^3
divided by 1000
cm^3 to m^3
Divided by 1,000,000
Degrees Celsius to kelvin
Celsius + 273
(on data booklet)
Standard temperature
273K
(data booklet)
What can 273K be described as?
Standard temperature
What can 1.01x10^5 be described as?
Standard pressure
What’s an alternative use for the ideal gas equation?
Calculating the volume a gas would occupy at temperatures and volumes other than those it was originally measured at
Equation for calculating the volume a gas would occupy at temperatures and pressures other than those it was originally measured at
P1V1 P2V2
—— = ——
T1 T2
What is the following equation used for?
P1V1 P2V2
—— = ——
T1 T2
Calculating the volume a gas would occupy at temperatures and pressures other than those it was actually measured at
Evolved
Created
What is titration an example of?
Quantitative analysis
Quantitative analysis
Uses statistics
What type of analysis uses statistics?
Quantitative analysis
When are titrations used?
When two solutions can be reacted and we want to determine the concentration of an identified solution
What type of apparatus do we have to use for titrations?
Accurately calibrated
Summarise the process of a titration
Take a fixed volume of a solution with a known concentration
React it with a solution with an unknown concentration
With an accurately measured volume needed to react, we can determine the solution’s concentration
What are the apparatus required for titrations?
Electronic balance
Gravitated/volumetric flask
Pipette
Burette
Two names for the type of flask used during titrations
Graduated/volumetric
What can an electronic balance weigh to within?
+-0.001g
What type of facility do most electronic balances have?
“Tare” facilities
What do we do with an electronic balance with a “tare” facility?
-Place beaker on the balance (tared)
-Required amount is placed into the beaker
-Amount weighed should be inside a specified range
What should the amount of a substance weighed on an electronic balance be?
Within a specified range
What’s the name of the balance on an electronic balance?
Tared
Tared
The balance on an electronic balance
What happens within the graduated/volumetric flask?
Make a solution of known concentration (a standard solution)
Standard solution
Solution of known concentration
Solution of known concentration
Standard solution
How is a solution of known concentration generated within the volumetric flask?
-Exact quantity of solid is weighed and dissolved in water
-Solution is placed in the graduated flask using a funnel
-Beaker is rinsed at least twice (add the water from the rinsing into the flask too)
-Add water to the flask until it reaches the mark
-Invert the flask a number of times to ensure complete mixing
How do we ensure complete mixing within the graduated/volumetric flask?
Invert a few times
How is solution placed into a graduated flask?
Using a funnel
What’s a pipette used for during titrations?
To get samples from the graduated flask - measures an exact volume of liquid
What must we use when filling up a pipette?
A pipette filler (take care when fitting it - see the instructions sheet)
What should always be forbidden in terms of pipettes?
Mouth pipetting
How much should the pipette be filled with liquid?
A little past the 25cm3 mark (the level drops slightly when removed which could give an inaccurate volume)
Important things to do when using a pipette
-Slowly and carefully to avoid bubbles
-Use a pipette filler
Why must the pipette be filled slightly past the 25cm3 mark?
The level drops slightly when removed from the liquid - inaccurate volume
What’s a burette used for during titrations?
Measuring liquid volumes accurately
What does a burette usually measure and why?
The volume of acid that reacts with alkali, as a base (alkali is a soluble base) can clog it up if left in for too long
What are the steps for using a burette during a titration?
Initially rinsed with the solution to be placed in it
Solution is placed in burette + initial reading is noted
The titration is carried out
When the reaction is complete, the burette reading is noted again
Tolerance of a burette
+-0.05cm3
How do you read the measurements on a burette?
-From the top down
-From the bottom of the miniscus
What do we do with titration calculations?
Use the same method as reacting masses (molar ratios etc)
What’s it really important not to do too early on with moles?
Over-round
What’s the name for the volume used during the titration?
Titre
Volume used during a titration
Titre
What is the first titre?
A rough titration
How do we calculate the volume used during a titration (titre)?
2nd - 1st burette reading
What can we do for the next titres after carrying out our rough titration and why?
Add the acid drop by drop, as we now have an idea of the titre
Concordent
Closeness of values
Closeness of values
Concordant
What do the volumes used/titres need to be within of each other in order to be concordant?
0.2cm3
To how many decimal places do we measure…
i) Volume used (titre)
ii) Mean reading
i) 2 d.p
ii) 3 d.p
How do we keep the burette level?
Clamp it
Why do we clamp the burette?
To keep it level
What’s in the conical flask underneath the burette?
Stuff (usually bases) measured by the pipette (exactly 25cm3) + indicator
What’s underneath the conical flask during a titration and why?
A white tile to easily see the colour change of the indicator
What does the indicator in the conical flask during a titration indicate?
The end of a titration
(The same number of acids and bases reacting)
What’s happened at the end of a titration?
The same number of acids and bases are reacting
How much indicator should we add to the stuff in the conical flask (usually bases) and why?
Only a few drops as it’s a weak acid and so it could give us inaccurate results
What could the fact that an indicator is a weak acid lead to?
Inaccurate results
What is usually the stuff in the conical flask and why?
Base, as this would clog up the burette if it were switched with the acid and left there too long
What do we have to ensure before taking a reading from a burette?
Ensure that the jet below the tap has solution in it
Where on the meniscus do we take a reading on the burette?
The bottom
What do we never start at using a burette?
Zero
What 2 things do we do with the burette after finishing?
Rinse with water
Leave the tap open when replacing the burette in the stand so that any liquid can drain
What’s the main error caused by using a burette?
Overshooting the end point
What is overshooting the end point an error with using?
A burette during titrations
Reaction between sodium hydroxide and hydrochloric acid
NaOH + HCl ——> NaCl + H20
Reaction between sodium carbonate and hydrochloric acid
Na2CO3 + 2HCL ——> 2NaCl + H20 + C02
Alkali
Soluble bases (usually hydroxide)
Soluble bases (usually hydroxide)
Alkali
Bases
All alkali is base (usually oxide)
All are base (usually hydroxide)
Alkali
Products of reacting an acid with an alkali
Salt and water
Products of reacting acid with a base
Salt and water
Products of reacting acid and a metal
Salt and hydrogen
Products of reacting acids and a carbonate
Salt and water and carbon dioxide
Describe the reaction between acid and alkali/a base
-Exothermic
-Neutralisation
-No effervescence (fizzing)
Word for fizzing
Effervescence
Effervescence
Fizzing
Describe the reaction between acid and a metal
Effervescence (fizzing)
Exothermic
Describe the reaction between acid and a carbonate
Effervescence
neutralisation
Salt
Hydrogen with the atom replaced with metal or ammonium ion e.g -
Sulphuric acid - Sulphate salt
Hydrochloric acid - Chloride salt
Nitric acid - Nitrate salt
How are the ions accelerated within the mass spectrometer?
Negatively charged plates accelerate positive ions
What happens to the volume of a gas at lower temperatures and why?
Decreases as the sample changes from gas to a liquid, reducing the volume of gas
What is the volume of a gas proportional to?
Temperature and pressure
Define an ideal gas
All collisions between atoms or molecules are perfectly elastic and there’s no intermolecular attractions
How do you calculate the percentage by mass of an element in a compound?
Mr of element
———————. x100
Mr of compound
What mass is a chlorine molecule most likely to have?
70
Define the mole and Avogadro’s constant
One mole is the amount of any substance that contains the same number of particles as here are atoms in exactly 12g of carbon-12
This fixed number is called Avogadro’s constant
What’s the volume measured in with the ideal gas equation?
metres cubed
What’s the equation for general first ionisation?
x (g) —> x+ (g) + e-
What’s an important concept to remember about emission and absorption spectra?
You cannot get an emission spectrum without getting an absorption spectrum first
Transition metal
An element whose atom has a partially filled d-sub shell
An element whose atom has a partially filled d-sub shell
Transition metal
How do we get an empirical formula from a molecular formula?
Divide by the highest common factor
Percentage error equation
Error
———. x100
Quantity being measured
What’s the ‘error’ in the percentage error equation?
The precision of which the equipment measures to/tolerance
Tolerance
The precision of which equipment measures to
Word for the precision of which equipment measures to
Tolerance
What do you do with percentage error if you have more than one piece of equipment?
The total error is the sum of the % error for each piece of equipment
To how many significant figures do we give our percentage error answer for errors between 0.1% and 1%?
3 s.f.
Give two ways that percentage error can be reduced
Make use of more accurate pieces of equipment
Arrange things so that the measurement itself is bigger
Tolerance of a pipette
0.05cm^3
Tolerance of a burette
0.05cm^3
What’s the exception for multiplying with big numbers for Mr?
a hydrate, for example CuSO4.5H20, where you’d work out the Mr of CuSO4, then the Mr of water and multiply water’s one by 5 and add these together
Why would a lid be left on whilst heating during an experiment?
Avoid spitting and thus loss of liquid
Why would a lid be left on whilst cooling during an experiment?
To avoid water being absorbed from the atmosphere
What can we do to avoid the problem of not all water being lost in a reaction?
Heat to constant mass
How many decimetres in a litre?
1
How many litres in a decimetre?
1
How do you calculate the number of particles in a sample?
Moles x Avogadro’s constant
How do we work out the value of X in front of a hydrated compound such as BaCl2.XH2O?
Mr of each
Mass of each
Compare moles as ratio
How do we know if enough of a reactant has been added to react with ALL of the other reactant by the end of the reaction process?
Allow it to settle
Add another few drops to see if precipitate still forms
If we get 0.6 in empirical and molecular formulae questions, what do we multiply it with to get a whole number?
5
What’s an alternative method for calculating concentrations?
- Work out the amount of moles in 1dm^3 of the solution
- Multiply this by the mor ratio value
For example…
As203 + 3H20 —> 2H3As03
n = m. 0.0104 moles in 100cm^3 of As203
—
M
So, in 1dm^3 —> 0.0104 x 10 = 0.104mol
Concentration of H3As03 —> 0.104 x 2 = 0.208moldm^3
Why doesn’t it matter if some solid remains in a weighing bottle?
We’re weighing by difference
Do you use mass or moles to calculate percentage yield?
Either is fine
How do we calculate the mass of a certain element present in a sample?
Total mass x Mr element
——————
Mr total
What is the error in a mass when weighing by difference?
+-0.0005g
Which impurities are removed from a sample when washed?
Soluble impurities
How do we know what our error is to use when calculating percentage error?
Half the unit of the last decimal place
What’s the uncertainty of our measurement when using a 2 d.p balance?
+-0.005
If you get 2 different mole values you could use in a % yield question, which one do we use? Why?
Figure out which one is lowest - this is the limiting factor
Use this for the theoretical yield
What type of ions does the mass spectrometer produce?
Positive ones
Under what condition is the whole system of a mass spectrometer?
Under vacuum
What’s a different way in which a percentage yield question could be asked?
Percentage purity
Why is the entire inside of a mass spectrometer under vacuum?
So that air molecules don’t interfere with the movement of the ions
How many dm^3 in a litre?
1
How do we go from gdm^-3 to moldm^-3?
Divide by Mr
Error in a 2 decimal place balance
+-0.005g
Relationship between cm^3 and grams?
(Under the correct conditions…)
1cm^3 = 1g
Do we use molar ratio (big numbers) in percentage by mass questions?
No
What do we input as our “error” with percentage errors?
Half the tolerance of the equipment
