AQA GCSE Combined Science Chemistry - Quantitative chemistry (paper 1)

What is relative formula mass (Mr)?

The sum of the relative atomic masses (Ar) of all atoms in a compound.

How do you calculate Mr of a compound?

Add together the Ar values of each atom in the formula.

What is the Ar of oxygen?

16

What is the Mr of CO₂?

12 + (2 × 16) = 44

What is the law of conservation of mass?

Mass is conserved – the total mass of products equals the total mass of reactants.

Why might mass increase in a reaction?

A gas (e.g. oxygen) is added from the air.

Why might mass decrease in a reaction?

A gas is produced and escapes into the air.

What is a mole?

The amount of a substance containing 6.02 × 10²³ particles (Avogadro’s number).

What is the formula to calculate moles?

Moles = Mass (g) ÷ Mr

What is the formula to calculate mass?

Mass = Moles × Mr

What is the formula to calculate Mr?

Mr = Mass ÷ Moles

What is the limiting reactant?

The reactant that is completely used up first in a reaction.

What is meant by excess reactant?

A reactant that is not completely used up – some is left over.

Why is it important to use a limiting reactant?

To make sure the reaction stops at a predictable point and waste is minimised.

What is concentration?

The amount of solute dissolved in a given volume of solution.

What is the formula for concentration (in g/dm³)?

Concentration = Mass of solute (g) ÷ Volume of solution (dm³)

How do you convert cm³ to dm³?

Divide by 1000 (e.g. 250 cm³ = 0.25 dm³)

How do you increase the accuracy of a concentration calculation?

Use precise equipment (e.g. pipettes, burettes), and repeat to find a mean.

What is the unit for concentration ?

Usually g/dm³

What is the purpose of a titration?

To find out how much acid or alkali is needed to neutralise the other.

What indicator is used in titrations?

methyl orange

What equipment is used to measure volumes accurately in titration?

Burette and pipette.

Why are titrations repeated?

To improve accuracy and find consistent, reliable results.

Why are no universal indicators used in titrations?

They don’t show a clear end point – too many colour changes.

What do you always record from a titration?

The burette reading at the start and end, and the volume used.