Relative atomic mass and relative molecular mass — AQA A-Level Chemistry
Test yourself on Relative atomic mass and relative molecular mass with AQA A-Level practice questions.
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Relative atomic mass and relative molecular mass explained
Relative atomic mass is a ratio, so it has no units.
Read the full explanation
It is the weighted mean mass of an atom of the element divided by one-twelfth the mass of an atom of carbon-12. Both halves of that definition carry a mark, and the two halves must be consistent: atoms compared with atoms, or moles compared with moles, never one of each. Carbon-12 is the reference because its mass is defined as exactly 12 on this scale. Relative molecular mass is the same idea applied to a molecule: the mean mass of one molecule of the substance divided by one-twelfth the mass of one atom of carbon-12, obtained in practice by adding the relative atomic masses of every atom in the formula.
The term relative formula mass will be used for ionic compounds.
Ionic compounds are giant lattices, not collections of molecules, so there is no molecule whose mass could be quoted. The term relative formula mass is used instead: it is the sum of the relative atomic masses of the ions in the simplest formula unit. Sodium chloride is 23.0⁺ 35.5 = 58.5, and calcium chloride is 40.1⁺ 2 × 35.5 = 111.1. Where a formula contains a bracketed compound ion, everything inside the bracket is multiplied by the subscript outside, so calcium hydroxide is 40.1⁺ 2(16.0⁺ 1.0) = 74.1. Hydrated salts include the water: the dot in the formula means add the mass of that many water molecules. Like relative molecular mass, the value is a ratio with no units.
Your focus
- Define relative atomic mass as a two-part fraction, naming the reference isotope and the one-twelfth factor.
- Calculate the relative molecular mass of a compound by summing the relative atomic masses in its formula.
- Explain why relative atomic masses have no units and why most are not whole numbers.
Show all 6 objectives
- Calculate the relative formula mass of an ionic compound whose formula contains a bracketed compound ion.
- Explain why the term relative molecular mass is inappropriate for sodium chloride.
- Include water of crystallisation correctly when calculating the relative formula mass of a hydrated salt.
Relative atomic mass and relative molecular mass exam tips
Marking Points
- one mark for the numerator given as the average or mean mass of an atom of the element
- one mark for the denominator given as one-twelfth the mass of one atom of carbon-12
- one mark for relative molecular mass defined as the mean mass of a molecule on the same scale
- one mark for keeping atoms with atoms or moles with moles throughout the definition
- one mark for adding the relative atomic masses of all the ions in the formula unit
- one mark for multiplying everything inside a bracket by the subscript outside it
- one mark for including the water of crystallisation shown after the dot in a hydrated formula
- one mark for the value quoted without units
Examiner Tips
- 💡Write the definition as a fraction with a horizontal line so the examiner can mark each line separately.
- 💡Say carbon-12 explicitly rather than just carbon; ordinary carbon is a mixture of isotopes.
- 💡Add average or weighted mean to the top line even when the question seems to be about a single atom.
- 💡Expand any bracketed formula on the page before you add anything up.
- 💡Use the relative atomic masses printed in the data booklet, not rounded whole numbers, when the answer needs decimals.
- 💡Say relative formula mass rather than relative molecular mass whenever the substance is ionic.
Common Mistakes
- mixing moles in one half of the definition with atoms in the other, which is capped at one mark
- writing mass of one atom without the word average or mean
- comparing with the mass of a carbon-12 atom while forgetting the one-twelfth
- giving units such as grams to a quantity that is a ratio
- multiplying only the first atom inside a bracket, so calcium hydroxide comes out too light
- treating sodium chloride as a molecule and calling the answer a relative molecular mass
- leaving out the water of crystallisation in a hydrated salt
- adding the subscripts instead of using them as multipliers