AQA GCSE Chemistry Foundation (8462), Foundation tier · Quantitative chemistry › Conservation of mass and chemical measurements
Practise Relative formula mass. 9 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Relative formula mass (Mr) is the sum of the relative atomic masses of all the atoms in a formula. You need it for almost every calculation in quantitative chemistry. Questions also ask you to work out the percentage by mass of an element in a compound, or to show that the Mr values balance in an equation.
Key facts
Ar = relative atomic mass (from the periodic table)
Mr = sum of the Ar values of all the atoms in the formula
Ar and Mr have no units
% by mass of element = (Ar × number of atoms) ÷ Mr × 100
Balanced equation: total Mr of reactants = total Mr of products
The relative atomic mass (Ar) of an element is an average mass of its atoms that takes account of the abundance of its isotopes. Read it from the periodic table you are given in the exam: it is the larger number in each element's box.
The relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.
Ar and Mr have no units, because they are relative values.
Elements made of molecules have an Mr too: O2 = 2 × 16 = 32.
Working out Mr
Count the atoms of each element, multiply by its Ar, then add everything up.
Example: H2SO4 = (2 × 1) + 32 + (4 × 16) = 98.
With brackets, work out the group first: Al2(SO4)3 = (2 × 27) + 3 × (32 + 4 × 16) = 54 + 288 = 342.
Percentage by mass of an element
diagram
% by mass of an element = (Ar × number of atoms of that element) ÷ Mr × 100
Example: nitrogen in ammonium nitrate, NH4NO3. Mr = 14 + (4 × 1) + 14 + (3 × 16) = 80. There are 2 nitrogen atoms, so % N = 28 ÷ 80 × 100 = 35%.
Each element's share of Mr = 80 gives its percentage by mass; the shares add up to 100%.
The percentages of all the elements in a compound add up to 100%.
Mr in balanced equations
In a balanced equation, the total Mr of the reactants (in the quantities shown) equals the total Mr of the products. This is conservation of mass again.
Work out the Mr of one formula first, then multiply by the balancing number.
How to answer each type of question
Calculate the relative formula mass
1 to 2 marksGrade 4
Count the atoms of each element, taking care with brackets.
Multiply each count by the Ar given.
Add them up and show the working. Do not give a unit.
Example. Calculate the relative formula mass (Mr) of magnesium nitrate, Mg(NO3)2. Relative atomic masses (Ar): N = 14; O = 16; Mg = 24 [2 marks]
Show the model answerHide the model answer
24 + 2 × (14 + 3 × 16) (1) = 24 + 124 = 148 (1)
Don’t lose marks
Multiplying only the first atom after a bracket: in Ca(OH)2 both the O and the H are doubled.
Using the atomic number instead of the relative atomic mass from the periodic table.
Counting only one atom of the element for % by mass, e.g. using 14 instead of 28 for nitrogen in NH4NO3.
Including the balancing number when asked for the Mr of a substance: the Mr of water is 18, even if the equation shows 2H2O.
Rounding Ar values: use 35.5 for chlorine and 63.5 for copper, as printed.
Giving a unit such as g with an Mr.
More tips
Memory tricks
Learn the Mr values you use most (water 18, carbon dioxide 44, calcium carbonate 100) to save time and check answers.
Brackets first: work out the group (e.g. NO3 = 62), then multiply by the number outside.
Sense check for % by mass: it must be less than 100%, and all the elements together add up to 100%.
Type the whole calculation into your calculator in one go, using brackets, to avoid rounding errors.
Exam technique
Use the Ar values given in the question, or on the periodic table, exactly as printed.
Show each step: a mark is often given for the working even if the final answer is wrong.
Look for 'Give your answer to 3 significant figures' in percentage by mass questions.
In 'show that' questions, write out both totals in full and say that they are equal.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Read relative atomic masses from the periodic tableAr is the larger of the two numbers in each box, e.g. O = 16, Na = 23.
Grade 4
Calculate Mr of a simple formulaAdd the Ar of every atom, e.g. CO2 = 12 + (2 × 16) = 44.
Grade 5
Calculate Mr for formulae with bracketsMultiply everything inside the bracket, e.g. Ca(OH)2 = 40 + 2 × (16 + 1) = 74.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Calculate the relative formula mass (Mr) of calcium carbonate, CaCO3.
100
Calculate the relative formula mass (Mr) of sodium chloride, NaCl.
58.5
Calculate the relative formula mass (Mr) of urea, CO(NH2)2.
60
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Relative atomic masses (Ar): H = 1, C = 12, O = 16, Mg = 24, Ca = 40
(a) Calculate the relative formula mass (Mr) of calcium carbonate, CaCO3.[2]
(b) Calculate the relative formula mass (Mr) of magnesium hydroxide, Mg(OH)2.[2]
(c) What is meant by the relative formula mass of a compound?[1]
Show the answer and mark scheme
(a)Answer: 100
40 + 12 + (3 × 16)
100
(b)Answer: 58
24 + 2 × (16 + 1)
58
(c)
the sum of the relative atomic masses of the atoms in the numbers shown in the formula
Question 2Medium7 marks
Ammonium sulfate, (NH4)2SO4, and ammonium nitrate, NH4NO3, are both used as fertilisers. Relative atomic masses (Ar): H = 1, N = 14, O = 16, S = 32
(a) Calculate the relative formula mass (Mr) of ammonium sulfate.[2]
(b) Calculate the percentage by mass of nitrogen in ammonium sulfate. Give your answer to 3 significant figures.[2]
(c) The percentage by mass of nitrogen in ammonium nitrate is 35.0%. A farmer needs to add 70 kg of nitrogen to a field. Calculate the mass of ammonium nitrate the farmer needs.[2]
(d) Suggest one reason why the farmer might choose ammonium nitrate rather than ammonium sulfate.[1]
Show the answer and mark scheme
(a)Answer: 132
(2 × 14) + (8 × 1) + 32 + (4 × 16)
132
(b)Answer: 21.2%
(28 ÷ 132) × 100
21.2 (%)
(c)Answer: 200 kg
70 × (100 ÷ 35.0) / 70 ÷ 0.350
200 (kg)
(d)
ammonium nitrate has a higher percentage of nitrogen (so a smaller mass is needed / less to transport and spread)