What are isotopes?
Atom that has the same number of protons but a different number of neutrons
This means it is the same element and has the same atomic number but a different mass number
What is relative atomic mass?
Also written as RAM and Ar
The average mass of the atoms of an element compared with carbon-12
The average mass must take into account the proportions of the naturally occuring isotopes of the element
What is relative formula mass?
Also written as RFM or Mr
The total of the relative atomic masses of a substance
These are added up in the ratio shown in the chemical formula
What is a mole?
The amount of substance in the relative atomic/formula mass of a substance in grams
What do you need to do if asked to work out the percentage composition of a substance?
Work out the RAM (Ar) of each element in the substance
Then use those to work out the RFM (Mr)
Then divide the amount of the element in the substance by its total mass
(e.g. percentage of carbon in CO2, you would divide the mass of carbon by the total mass of the CO2)
What is empirical formula?
The simplest ratio of elements in a compound
How do you work out the empirical formula of a compound?
Find the mass (either in 100g or given)
Find the moles/RAM
Divide masses by moles
Divide by the smallest number to find the simplest ratio
What is percentage yield?
Amount of product collected ÷ maximum possible x 100
Why is it not possible to get 100% yield from a reaction?
- Reactions may not go to completion
- Other reactions may occur
- Some product may be lost when it is seperated or collected from the apparatus
Why should chemical manufacturers use reactions with high yields?
- To conserve energy resources
- To reduce waste
- This reduces pollution
- And makes production more sustainable
What is a reversible reaction?
A reaction in which the products can re-form/react to make the original reactants
Give an example of a reversible reaction
Heating ammonium chloride
Ammonium chloride ⇌ Ammonia + Hydrogen chloride
Why might food be checked by chemical analysis?
To check that only safe, permitted additives have been used
What methods can be used to analyse what food additives have been added to food?
Paper chromatography and mass spectrometry
Describe the process of gas chromatography
- The mixture that needs to be seperated is carried by a gas through a long column packed with particles of a solid
- The individual compounds travel at different speeds though the column and come out at different times
- The amount of substance leaving the column at different times is recorded against time
- This shows the number of compounds in the mixture and their retention times
- The retention times can be compared with the results for known compounds to identify the unknown compounds in the mixture
What can a mass spectrometer be used for?
To give the relative molecular mass of a compound
What is the molecular ion peak?
The peak on the mass spectrum of a substance - it is the furthest to the right
It tells us the relative molecular mass of the substance
It is produced by the heaviest ion shown on the mass spectrum
