Unit 1-3 Flashcards
How much do protons and neutrons weight?
1 atomic mass unit (AMU)
Notes: The nucleus is the heavy part of the atom
Electrons have negligible mass
What does the atomic number represent?
The number of protons
Also the number of electrons if there’s no charge
Note: Identifies an element because its unique and never changes
What does the mass number represent?
Total number of protons and neutrons
An average of all isotopes therefore not always whole
What is an isotope?
Same element (atomic number) but different number of neutrons (mass number)
What is the relative atomic mass (Ar)?
Average mass of one atom relative to 1/12 the mass of one atom of C-12 because 1/12 the mass of a C-12 atom is 1 AMU
Example: Oxygen is 16 times heavier than 1/12 a C-12 atom
What is the relative formula mass (Mr)?
The total Ar of a molecule or compound
Average mass of one molecule relative to 1/12 the mass of one atom of C-12
What is the relative isotopic mass?
Mass of one isotope relative to 1/12 the mass of one atom of C-12
Example: Cl-35 is 35 times the mass of C-12 but Cl-37 is 37 times its mass
What is the mole?
The amount of any substance that contains the same number of particles as there are atoms in exactly 12g of C-12 (6.02*10^23 atoms)
n=m/mr
What is molar mass?
Mass per unit amount of a substance, gmol^-1 (The mr expressed as gmol^-1)
How is the number of particles calculated?
Number of particles= nL
Moles= n
Avogadro’s number (6.02*10^23 ) = L
What is the empirical formula?
Simplest whole number ratio of elements in a compound
Example: Ethane = CH3
What is the molecular formula?
Actual ratio of elements in a compound
Example: Ethane = C2H6
What is mass spectrometer used for?
Used to determine relative atomic mass by measuring the mass of each isotope and the relative abundance of each isotope
(Step 1) What must happen to an element before entering a mass spectrometer?
The element is vaporised
Not heated if they are gases at room temp
(Step 2) Why are samples bombarded with high energy electrons from the electron gun of a mass spectrometer?
Positive ions are obtained as 1 electron is knocked off
X(g) → X(g)+ + e-
Note: If the energy is too high then too many electrons will be knocked off
(Step 3) How are ions accelerated in a mass spectrometer?
Ions are accelerated using the electrical field created by the charged plates
(Step 4) In mass spectroscopy the instrument is evacuated so a vacuum pump can remove air molecules. Why?
So the movement of positively charged ions is unaffected
(Step 5) What is the purpose of the magnetic field in mass spectroscopy?
Ions are deflected using the magnetic field
Ions with a small mass/charge ratio are defected more
(Step 6) What is the purpose of changing magnetic field strength in mass spectroscopy?
Ions are detected at different strengths
(Step 7) What is produced when ions are detected in mass spectroscopy?
An electrical impulse
(Step 8) What is fed into the recorder in mass spectroscopy
An amplified electrical impulse
What does each peak in a mass spectra represent?
An isotope
What is the atomic ion region in a mass spectra?
The region where the lone ions are detected
The value for mass/charge is the same as the Ar for that isotope
What is the molecular ion region in a mass spectra?
The region where molecules are detected
Different or similar isotopes bonded together
Note: m/z is greater as it is a combination of the bonded isotopes
What does atom economy determine?
The theoretical yield of a product
What is the purpose of reacting masses of solids?
To determine the mass of a product using info of the reactants
What is the purpose of titrations?
To determine the mass of a product using info of the reactants
Can also determine molar mass, % purity etc once concentration is known
What is the standard solution in titration?
A solution with known concentration
What is a primary standard?
- Readily available in a high state of purity
- Stable in air at room temperature
- Readily soluble in water
- High molar mass
- Must undergo a complete and rapid reaction when used in volumetric analysis
What is the stoichiometric/equivalence point?
The point where reaction is completed and chemically equivalent quantities of reactants have been mixed
What is the end point?
When the indicator used in titration changes colour
What is titre?
The volume used from the burette in titration
What are the steps for preparing a standard solution? (9)
- Weigh the bottle containing a substance that will react with the solution of unknown concentration
- Transfer the substance to 50cm^3 of water in a 250cm^3 flask
- Reweigh the empty bottle and calculate the mass of substance added
- Stir to dissolve the solid and add more water if necessary
- Transfer the solution to a volumetric flask
Rinsing the beaker ensures that all the solution enters the flask - Add distilled water so the level is within 1cm of the mark; shake thoroughly
- Add water to raise the bottom of the meniscus to the mark with a pipette
The solution is now 250cm3 in volume - Insert the stopper and invert 10 times to mix the contents
- Concentration can be calculated
What are the steps for acid base titration (8)
- Rinse the burette with the solution of unknown concentration
- Fill the burette to 0.0 with this solution, rinsing and filling the tap
- Rinse the pipette with standard solution using the pipette filler
- Transfer 25cm3 of standard solution to a 250cm3 conical flask
- Add 2-3 drops of indicator i.e phenolphthalein
- Run the burette over the conical flask while swirling until slight colour change
Trial run since the end point will likely be overshot - Record the total volume of the solution in the burette used
This is titre - Repeat while ignoring the trial run to find mean titre
Mean titre is more reliable
Repeat until readings are concordant
What is accuracy?
Closeness of agreement between a result and accepted value
What is error?
The difference between measurement and true value
What is precision?
Closeness of agreement between measurements taken in identical conditions
What is uncertainty?
A range of values where the true value is said to lie
Often taken as half a unit on either side of the smallest unit on the scale
What is the maximum error of class A and class B glassware respectively?
Class A glassware has a maximum error of half a division
Class B glassware has a maximum error of a full division
What is reliability?
The opposite of uncertainty
Inversely proportional to the size of the range of values where the true value lies
What is the effect of temperature on glassware?
Effects the volume of solution the glassware can hold
What is the difference between digital and non-digtial measuring devices
Digital devices record all the digits shown
Non-digital devices record all the figures that are known for certain
What is the maximum error of a volumetric flask (250cm^3)?
±0.2cm^3
What is the maximum error of a burette?
±0.1cm^3
Half a division (±0.05cm^3) read twice
What is the maximum error of a pipette?
±0.06cm3
What is the maximum error of a balance?
Balances have a max error depending on resolution
Multiplied by 2 in titration since we measure the bottle twice
Example: ±0.005g for a scale that measures to two-decimal places
In titration ±0.01g
How are burette measurements recorded in titrations?
Recorded to two-decimal places ending in 0 or 5
