Topic 2: Quantitative chemistry (physical chem) Flashcards
Define solubility
the property of a solid, liquid or gaseous chemical substance called solute to dissolve in a solid, liquid or gas solvent
what is the state symbol of an element if it is soluble in water
aqueous (aq)
what are the aqueous substances
(SNACKS)
-All common Na+, K+, NH4+ salts
-All nitrates
-Most chlorides except for silver and lead
-Most sulfates except for lead, barium and calcium
what substances are insoluble (solids)
-Most carbonates
-Most hydroxides
-most oxides
what happens to hydroxides and sulphates down group 2
-Hydroxides are more soluble and sulphates are less soluble down group 1 and 2
what equation links base + acid
base + acid –> water + salt
what does MASH stand for
metal + acid –> salt + hydrogen
What does CAWCS stand for
metal carbonate + acid –> water + carbon dioxide + salt
when does a metal carbonate form
when a carbonate ion reacts with a metal cation
what is a metal carbonate described as
a base
what is the formula for hydrochloric acid
HCL
what is the formula for sulfuric acid
H2SO4
what is the formula for nitric acid
HNO3
what is a decomposition reaction
AB –> A + B
heat is required
if C and O is present CO2 is formed in products
what are the 4 observations in reactions
-if solid is in reactants but not in products then the solid dissapears/dissolves
-If a gas is produced bubbling/effervescence occurs
-If an alkali is formed litmus paper will turn blue
-If an acid is formed the litmus paper will turn red
what is the amount of a substance measured in
moles
define molar mass
the mass of 1 mol of a chemical
what does 1 mol equal
12g of carbon
how do you calculate mol
mass / mr
how do you calculate the number of atoms/molecules
mol x avogadros constant
define mole
the total number of atoms present in the molar mass of the substance
calculate the amount in mole of water with 9.36 x 10^24 of atoms
9.36 x 10^24 / 6.022 x 10^23
Answer / 3 as there is 3 moles
How many atoms are there in a mole of neon
1 x 6.022 x 10^23
Calculate the number of chloride ions in 1 mol of MgCl2
Total = 3 moles (1 mole of Mg / 2 mole of Cl
Chloride ion = 2 x 6.022 x 10^23
define empirical formula
the simplest whole number ratio of atoms of each element in a compound
define molecular formula
the actual number of atoms of each element in a compound
how do you calculate empirical formula
1) Find the mr of each element (mass number)
2) Do mass of the element told in question divided by mr
3) Divide the amounts by the smallest amount to find ratio
how do you calculate molecular formula
1) calculate mr of empirical formula e.g C3H7 = 43
2) Divide the mass given in the question by mr of empirical formula e.g 86 / 43 =2
3) Multiply the atoms by the number e.g C6H14 is the molecular formula
what does 24dm3 equal
1 mol
how do you convert mg to g
divide by 1000
calculate the mass of a single atom of this isotope of sodium
23 / 6.022 x 10^23
what is the mass of 1mol of oxygen
16.0g
what equation links moles, concentration and volume
mol = concentration x volume
what is the mass of 1 atom of carbon
12 / 6.022 x 10^23 = 1.99 x 10^23
Define combustion
a chemical reaction where a fuel is burnt in oxygen
what is the equation for complete combustion
hydrocarbon + oxygen –> Water + carbon dioxide
what is combustion analysis
calculating the empirical formula of a hydrocarbon by using the masses of the combustion products
how do you calculate combustion analysis
1) Calculate moles of carbon dioxide then moles of carbon from that
2) Calculate moles of water and then moles of hydrogen from that
3)Work out masses of carbon and hydrogen to calculate mass of oxygen
4) Calculate moles of oxygen
5) Calculate empirical formula of the compound
Give the formula of the hydroxide of the element in group 2 from Mg to Ba that is least soluble in water
Mg(OH)2
which compound has the same empirical formula and molecular formula
propane
how do you calculate the amount of protons in 6.0g of nitrogen gas
6.0 / 14
(Ans x 6.022 x 10^23) x 7
how do you calculate ionic equations
1) rewrite solid, liquid, gases how they are
2) split ionic aq substances into their components
3) eliminate spectator ions
How do you calculate the volume of molecules
1) find moles through mass/mr
2) mol x 6.022 x 10^23
3) ans x volume
How do you find the maximum mass of Na2CO3 from 800g from 2NaCl
1) find moles of other compounds
2) find moles of Na2CO3
3) moles x mr of Na2CO3
true or false - when calculating empirical formula of compounds they are molecules not atoms
false –> only atoms e.g Na2 will be calculated as Na
How do you calculate the number of water molecules
1) calculate change in mass
2) calculate final mass
3) calculate moles
4) calculate ratio
what type of reaction is thermal decomposition
endothermic
how would you calculate the mole of y when moles of X is present
Moles X = Moles Y x (coefficient of x / coefficient of y)
how do you calculate moles
number of particles / avogadro’s constant
what is an ideal gas
a gas that has zero interactions with other particles and occupies zero space
what is the equation for ideal gas
pV = nRT
P = pressure (pascals)
V = volume (m3)
N = moles
R = gas constant (8.31 J/mol/K)
T = temperature (Kelvin)
how do you convert KPa to Pa
multiply by 10^3
how do you convert from Cm^3 to M^3
divide by 10^6
how do you convert from Dm^3 to m^3
divide by 10^3
how do you convert from degrees Celsius to kelvin
Add 273
if two moles of CaCO3 decompresses how many moles of CO2 are produced
2 moles
how do you calculate the molar mass of gases from ideal gas law
1) n = pV / RT
2) mr = mass / moles
how do you calculate percentage uncertainty
(uncertainty / mass added) x 100
how can gas be collected
-Gas can be collected and measured using either an inverted measuring cylinder or a gas syringe is used to measure the volume of gases produced. (gas syringe is more accurate)
what is 1 mole equivalent to
avogadro’s constant
what is avogadro’s law
equal volumes of gases under the same conditions of pressure and temperature contain the same number of molecules
what is molar volume
the volume occupied by one mole of gas at a specific temperature and pressure
what equation involves volume 24dm^3 and moles
volume = moles x 24dm^3
how do you do calculations involving mass
1) Moles = Volume / 24dm^3
2) mr = mass / moles
3) identify the gas from the mr (can be diatomic)
define molar mass
the mass in grams of 1 mol of substance
what is the volume to volume ratio the same as
mole to mole ratio
how do you find volume from an equation
1) calculate moles of Mg
2) look at mole to mole ratio
3) find moles of H2
4) moles x 24 = volume
volume of gas equation –>
V1 / N1 = V2 / N2
how do you calculate volumes from equations
1) state mole of mole ratio
2)find out how much we have of each molecule
3) calculate total volume of gas produced
how do you work out gas volumes
1) work out volume of reactant 2 that would react with the given volume of reactant 1
2) determine which reactant is in excess and which fully reacts
3) use the equation to work out the reacting volumes of the other reactants
4) work out the volumes of excess reactant and the volumes of product produced (add together to get total volume)
what mass of iron can be produced from 20Kg of iron oxide
1) find mole to mole ratio
2) Find mr of each compound
3) calculate moles from mass of iron oxide
4) multiply moles by mole to mole ratio
2) mr x moles = mass of Fe
do you use the reactant that is in excess or limiting to calculate moles on products
limiting
state a reason for the difference in experimental value and calculated value
solution was not heated to a constant mass
define mr (molecular mass)
the average mass of one molecule
State why the student should use sodium hydroxide solution rather than
water for the final rinse of the burette
water would dilute the NaOH
what is the mr of nitrogen gas (halogen)
28 (diatomic element so 14 x 2)
What is the final volume of gas remaining at RTP when 10cm3 of methane is completely burned in 30cm3 of oxygen?
CH4 (g) + 2O2 (g) —> CO2 + 2H2O (g)
1:2 –> 10:30
1 = 10
2 = 20 (oxygen is in excess) –> 10cm3 excess
CO2 = 10cm3
2H2O = 20cm3
Total final volume –> 10 + 20 + 10 (excess) = 40cm3
what is conservation of mass
For any system that is closed to all transfers of matter and energy, the mass of the system must remain constant of time as the systems of mass cannot change
why would there be excess reactants in a reaction
-If there are no losses during a chemical reaction, the starting reactants are converted to the required products.
-Not all reactants will be converted to the required products because most of the time in industry, some reactants are added in excess to ensure the most valuable reactant is converted to as must product as possible.
how can you calculate the moles from limiting reactant and mole:mole ratio
mr of limiting x (coefficent / coefficent of limiting)
justify why a smaller target titre would not be appropriate
smaller titre will increase the percentage uncertainty
state the reason for inverting the flask several times
so the solution is homogeneous
state one advantage of using a conical flask rather than a beaker for a titration
less chance of splashing
define yield
the amount of product formed
what are the steps to calculating theoretical yield
0) Balance the equation
1) Find mol to mol ratio
2) Find moles of compound where mass is given
3) Find moles of desired product using mol to mol ratio
4) Find the mass of desired product
if all of one compound decomposes what will be the yield of the desired product
100%
what are some reasons for less product
-reactants may not be totally pure
-some of the product may be lost during transfer of chemicals from one container to another (during separation or purification)
-there may be side reactions to form other products
-some of the reactants may not react because the reaction is too slow
what assumption is made when calculating theoretical yield
no losses are made and all of the reactants have reacted
if copper oxide was impure what would happen to the yield of copper sulfate
decrease –> side reactions
how do you calculate percentage yield
actual yield / theoretical x 100
define desired product
the amount of product that is useful and required
what is atom economy
a measure of the amount of starting materials that end up as useful products
how do you calculate atom economy
(Mr of desired products / total mr of all the products ) x 100
what polymer has the highest atom economy
alkenes
what equipment is used in titrations
burette, pipette filler, pipette, conical flask
what are the steps to carrying out a titration
Carrying out a titration:
1) rinse the pipette with the solution going into the conical flask
2) Pipette a known volume of this solution into a conical flask
3) Rinse the burette into the solution it will contain and run the tap (fill the jet of the burette
4) Read the initial volume at the base of the meniscus
5) Add a few drops of phenopthalein to the conical flask
6) place conical flask on a white tile under the burette
7) Titrate rapidly then dropwise near the end point (colour change from colourless to pink) and record the final volume. Repeat until concordant titres
what is a neutralisation reaction
base + acid –> salt + water
-pH of reaction is neutralised when water is formed
what do coloured indicators do
detect the end point of acid base reactions
what are the best indicators to use for titrations
-Methyl orange or phenolphthalein are the best indicators to use for titration as they give sharp colour changes at the end point
what is the purpose of a titration
the analyst finds the volume of the sample solution that reacts with the certain volume of a reference solution that has an accurately known concentration. If we know the volume and concentration of one solution and the volume of another we can determine the concentration of an unknown acid or alkali. It works by gradually adding a base of known concentration to an unknown acid solution or vice versa until the solution is neutralised.
what is the pH range for an alkali
8-14
what is the pH range for an acid
0-6
standard solutions
-to accurately complete a titration a solution of known volume and concentration is needed as well as a solution of known volume but unknown concentration
-A standard solution is a solution with a known concentration
-concentration is the mass of solute dissolved in a given volume of solvent
what are the steps to making a standard solution
1) weigh the substance of solid on the weighing boat then weigh the weighing boat
2) Subtract mass of solid + boat – boat to get mass by difference
3) Dissolve solid in known volume of deionised/distilled water and stir using stirring rod
4) Transfer this to a volumetric flask and rinse washings from the breaker
5) Fill volumetric flask to the graduation line
6) invert volumetric flask
why is distilled water used instead of tap water
distilled water is purified so wont affect concentration
what is the colour change of methyl orange from acid to alkali
red to yellow
what is the colour change of methyl red from acid to alkali
yellow to red
what is the colour change of Bromothymol blue from acid to alkali
yellow to blue
what is the colour change of Phenolphthalein from acid to alkali
colourless to pink
how do you calculate titre volume
start volume - end volume
how do you get from mol/dm3 to g/dm3
x mr
what is the measure of uncertainty in a titration
+-0.05
is the known or unknown concentration in the conical flask
unknown
what is the curved upper surface of a liquid in a burette called
mensicus
what is it called the moment the indicator changes colour
end point of an acid base reaction
what is the minimum volume of a solution needed to reach the end point called
titre
what is the name of titres that are close in agreement (+-0.2)
concordant
true or false - when calculating volumes used we only use concordant titres to increase the accuracy of calculations
true
how do you calculate concentration
moles / volume
what is the procedure of dilution
-The procedure for dilution is to take a measured volume of a more concentrated solution with a pipette and run it into a graduated flask. The flask is then filled to the mark with pure water.
why is it important for there to be no impurities in pharmaceutical industry
as impurities could be dangerous and lead to dangerous side effects. In addition impurities could cause side reactions of unwanted products
what is the equation for percentage purity
(Mass of pure product / mass of impure product) x 100
or
(mass of useful product / total mass of products) x 100
how to calculate percentage purity
1) Calculate moles of substance you know volume and concentration of
2) Use mole to mole ratio
3) calculate mass (moles x mr)
4) calculate percentage purity
crystalisation of water
-When a substance crystalises it means it forms a solid crystal structure
-Water molecules are trapped within the crystal structure when a substance crystalises. This is known as the water of crystalisation.
-Crystals have very regular lattice structures
define hydrated
Hydrated –> a salt that has water incorporated into its crystal structure
define anhydrous
Anhydrous –> a salt that does not have water present in its crystalline structure
what is the notation for a hydrated salt
CuSO4 . 5H2O (copper pentahydrate)
calculations involving hydrated salts
-Calcium chloride is commonly encountered as a hydrated solid with formula CaCl2 . XH2O
-If we heat the salt, the water will evaporate off leaving the anhydrous salt behind
-To determine X the hydrated sample is weighed, heated strongly to drive off the water then reweighed. This reaction can provide us with information about the molar ratios of each compound.
steps to calculating hydrated salts
1) calculate mass of the water (initial-final)
2) calculate moles of water using mass
3) calculate moles of anhydrous salt
4) Work out mole to mole ratio to find X
how can you determine X by a titration
-We can also determine X by using titrations because many bases exist as hydrated crystalline solids
-Titrating with a standard acid solution allows us to work out the moles of a base and by difference of water
1) Use titre to calculate moles of acid
2) use equation to work out moles of the base
3) Use original mass to work out mr of hydrated base
4) use mr of anhydrous base to work out moles of water
how do you calculate percentage by mass
(mass / total mass) x 100
what is the issue with too much indicator
may affect the end point reaction so smaller volume should be used (2 drops)
why is a pipette better than a measuring cylinder for titrations
measuring cylinder has greater percentage uncertainty
what effect will rinsing have on the titre
titre value would increase as solution would be more dilute
Calculating X from titre
1) calculate moles of acid
2) calculate moles of base using mole to mole ratio
3) Moles of base x Original/new volume
4) Mass / moles = mr of base
5) Mr of acid – mr of base
6) mr / mr of water
define accuracy
-A measure of the closeness of agreement between an individual result and an accepted value or literature value
define precision
-The closeness of agreement between independent measurements obtained under the same conditions
define reliability
repeating the same method over and over again will produce the same results
what is equipment accuracy
Apparatus either holds an accurate measure e.g volumetric flask or delivers an accurate volume e.g pipette
what is equipment precision
-Pieces of equipment differ in terms of their precision. The precision of equipment is best represented by the number of decimal places that we can read it to
-more decimal place = more precise
-The value between divisions indicated the precision, the smaller the divisions, the more precise a piece of equipment is
how do you calculate the total number of moles of gas present in the mixture
1) Mole to mole ratio
2) Divide ratio by mass given in question
3) Add up all numbers
what is uncertainty
An estimate attached to a measurement which characterizes the range of values within which a true value is said to lie
how to calculate uncertainity
-To calculate uncertainty you take the increment of the measurement and divide by two
e.g if a piece of equipment has an increment of 0.01 you divide it by 2 = 0.005
how do you calculate percentage uncertainity
-uncertainty / value of measurement x 100
what are the exceptions for uncertainty
-Burette and balance
-You need to multiply the uncertainty by 2
-Uncertainty x 2 / value of measurement x 100
how do you reduce percentage uncertainity
-Decrease the uncertainty of the equipment by using equipment with more decimal places
-Increase the size of the measurement that is being made
what effect would rinsing have on the titre value
titre value would increase because the solution would be more dilute
what is a back titration
adding unknown concentration into known concentration that is in excess
importance of percentage yield
efficient conversion of reactants to products
importance of percentage atom economy
minimising the amount of by-products
titration
1) mean titre (volume)
2) amount of base
3) amount of acid
4) mass of acid
5) mass of product
6) percentage change
1) volume of acid
2) moles of acid
3) moles of base in new sample
4) moles of base in original using new
5) mass of base
dilution calculations
1) moles of known conc
2) mol to mol to find unknown
3) moles of original using volume (original/new)
4) moles / volume taken to be diluted
what is the mean titre
the volume of acid/base
suggest why washing does not give an incorrect result
water does not react with the alkali
the conc of ethanoic acid calulcated was lower than the actual value. Why?
rinsing the burette with distilled water before filling w acid
lower volume =
greater mr recorded
suggest one reason why calcium hydroxide is not used in the titration of a solution of an acid
dilution of acid needed or calcium hydroxide may react with CO2 in the air
in this experiment the mass of solid remaining in the crubicle was greater than expected. Why
incomplete reaction/decomposition
explain why this rinsing can improve the accuracy of the end point
returns reagent on side of the flask back into the mixture
explain why the addition of water during rinsing does not give an incorrect result
water is not a reagent
suggest why a leak of NO2 gas from the ostwald process will cause atmospheric pollution
NO2 contributes to acid rain
give one reason why excess air is used in the ostwald process
ensures complete combustion
suggest one other source of error in using the burette to carry out a titration
air bubble in the jet
relative atomic vs relative molecular mass
atomic = average mass of an atom
molecular = average mass of a molecule
density
mass / volume
number of molecules/atoms
mol x avogadro’s constant
finding X
1) find mass of water
2) calculate moles of salt and water
3) Mol to mol ratio for x
mol dm3 –> g dm3
x mr
calculating dilutions
new dilution = old dilution concentration x old/new volume
reduce uncertainity in titrations
-use pipette instead of measuring cylinder
-increase concentration of substances
reduce uncertainity in measuring mass
-balance with more decimal places
-calculate mass by difference
which suggestion decreases the percentage uncertainity in the mean titre
use a more dilute solution of sodium hydroxide in the burette
which molecule can accept an electron pair during the foormation of a coordinate bond
AlCl3
what is the purpose of increasing the concentration of solutions
increase titre value so percentage uncertainity would reduce
using mean titre to find mr
1) calculate mean titre
2) concentration x mean titre = moles
3) mass / moles = mr
calculate mass
mr / (avogadros x 1000)
identify the white precipitate and calculate the percentage mass of MgCl2 in the power
1) identify what precipitate is formed
2) find moles of BaSO4
3) find mass of BaCl2
4) (mass of powder - mass of BaCl2) / mass of powder
suggest a reason for the difference between calculated value and experimental value
solution was not heated to a constant mass
unknown concentration =
mol:mol value / initial volume
true or false -> always balance the equation before doing any calculation
true
finding X of a hydrated salt
X(18) / X(18) + CuCl2
total area in a titre
0.05 x 3 = 0.15
percentage uncertainity from gradient
(best gradient - worst gradient) / best gradient
percentage increase and decrease
increase (new - old) / old
decrease (old - new) / old
adding titre uncertainties
change in a + change in b for both volumes and the uncertainity
line of best fit
- are the data likely to be following an underlying equation (for example, a relationship governed
by a physical law)? This will help decide if the line should be straight or curved - are there any anomalous results?
- are there uncertainties in the measurements? The line of best fit should fall within error bars, if
drawn.
There is no definitive way of determining where a line of best fit should be drawn. A good rule of
thumb is to make sure that there are as many points on one side of the line as the other. Often the
line should pass through, or very close to, the majority of plotted points. Graphing programs can
sometimes help, but tend to use algorithms that make assumptions about the data that may not be
appropriate.
Lines of best fit should be continuous and drawn as a thin pencil that does not obscure the points
below and does not add uncertainty to the measurement of gradient of the line.
Not all lines of best fit go through the origin. Students should ask themselves whether a 0 in the
independent variable is likely to produce a 0 in the dependent variable. This can provide an extra
and more certain point through which a line must pass. A line of best fit that is expected to pass
through (0,0), but does not, would imply some systematic error in the experiment. This would be a
good source of discussion in an evaluation.
gradient
equation of straight line = y=mx+c
m = gradient
c = y-intercept
y = DV
x = IV
to find x in hydrated salts
divide moles by the smallest number of moles
errors using gas syringe
*gas escapes before bung inserted
*syringe sticks
* some gases like carbon dioxide or sulphur dioxide are
soluble in water so the true amount of gas is not
measured.
air bubble in jet
incorrect readings
no rinsing
. If it is not rinsed out the acid or alkali added may be
diluted by residual water in the burette or may react with
substances left from a previous titration. This would lead to
the concentration of the substance being lowered and a larger
titre being delivered.
why is a conical flask better than a beaker
A conical flask is used in preference to a beaker because
it is easier to swirl the mixture in a conical flask without
spilling the contents.
distilled water in titrations
Distilled water can be added to the conical flask during a titration to wash the
sides of the flask so that all the acid on the side is washed into the reaction
mixture to react with the alkali.
It does not affect the titration reading as water does not react with the reagents
or change the number of moles of acid added.
manganate redox reaction (purple to colourless)
MnO4-(aq) + 8H+ (aq) + 5Fe2(aq) –> Mn2+ (aq) + 4H2O (l) + 5Fe3+ (aq)
dilute sulphuric acid is used
if 10cm portions are titrated from 100cm3 total multiply answer of moles by 10
same for 25cm3 in 250cm3 total
multiply by 10
percentage by mass
calculated mass / given total mass
mol to mol ratio for metal ions
1:1
enthalphy change from experimental data
- Using q= m x cp x T calculate energy change for quantities used
- Work out the moles of the reactants used
- Divide q by the number of moles of the reactant not in excess to give H
- Add a sign and unit (divide by a thousand to convert Jmol-1
to kJmol-1
q=mct for neutralisation
mass = vol of acid + vol of base
Strontium and barium salts will not form a hydroxide precipitate on addition of
sodium hydroxide due to their high solubility. The solutions will be highly alkaline
also draw the filter paper when drawing a filtration diagram
time of flight calculations
distance = t x square root 2Ke/ m
time = d x square root m / 2Ke
state why it is good practice to rinse the pipette in this way
so titration can be done with a known concentration of NaOH
2 lone pairs on oxygen molecule
Shape of ClF4-
square planar
shape of POCl3
tetrahedral
impurity =
broad range of melting point
why would using a conical flask intead of beaker be better
reduces loss of liquid droplets
finding units
look at y and x axis
bung in condensor =
cause pressure to increase
suggest why ethyl ethanoate forms the upper layer
reason = insoluble in water = lower density than water
ester =
insoluble in water and less dense
hydrogenation reaction using
Ni and H2
