How Much?

Question 1

What are isotopes?

Answer 1

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

Question 2

What is relative atomic mass?

Answer 2

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

Question 3

What is relative formula mass?

Answer 3

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

Question 5

What is a mole?

Answer 5

The amount of substance in the relative atomic/formula mass of a substance in grams

Question 6

What do you need to do if asked to work out the percentage composition of a substance?

Answer 6

Work out the RAM (A_r) of each element in the substance

Then use those to work out the RFM (M_r)

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)

Question 7

What is empirical formula?

Answer 7

The simplest ratio of elements in a compound

Question 8

How do you work out the empirical formula of a compound?

Answer 8

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

Question 9

What is percentage yield?

Answer 9

Amount of product collected ÷ maximum possible x 100

Question 10

Why is it not possible to get 100% yield from a reaction?

Answer 10

• Reactions may not go to completion
• Other reactions may occur
• Some product may be lost when it is seperated or collected from the apparatus

Question 11

Why should chemical manufacturers use reactions with high yields?

Answer 11

• To conserve energy resources
• To reduce waste
• This reduces pollution
• And makes production more sustainable

Question 12

What is a reversible reaction?

Answer 12

A reaction in which the products can re-form/react to make the original reactants

Question 13

Give an example of a reversible reaction

Answer 13

Heating ammonium chloride

Ammonium chloride ⇌ Ammonia + Hydrogen chloride

Question 14

Why might food be checked by chemical analysis?

Answer 14

To check that only safe, permitted additives have been used

Question 15

What methods can be used to analyse what food additives have been added to food?

Answer 15

Paper chromatography and mass spectrometry

Question 16

Describe the process of gas chromatography

Answer 16

• 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

Question 17

What can a mass spectrometer be used for?

Answer 17

To give the relative molecular mass of a compound

Question 18

What is the molecular ion peak?

Answer 18

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