Amount of substance

Question 1

Relative formula mass

Answer 1

The weighted mean mass of the formula unit of a compound compared with one-twelfth of the mass of an atom of carbon-12.

Question 2

Relative molecular mass

Answer 2

The weighted mean mass of a molecule compared with one-twelfth of the mass of an atom of carbon-12.

Question 3

Water of crystallisation

Answer 3

Water molecules are part of the crystalline structure

Question 4

Ideal gas

Answer 4

Assumptions:
- random motion
- elastic collisions (don’t stick together upon impact)
- negligible size
- no intermolecular forces

Question 5

Percentage yield

Answer 5

Reasons:
- reaction may not have gone to completion
- other reactions (side) may have taken place alongside the main reaction
- purification of the product may result in loss of some of the product