AQA Combined Science (8464) 5.3.1.2 · Quantitative
Relative formula mass
A formula tells you which atoms are in a substance and how many. Can you turn it into a mass, and use that mass to say how much of a compound is each element?
Combined Science · Higher tier
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Three atoms against two.
A water molecule, H₂O, has three atoms. An oxygen molecule, O₂, has two. Atoms of different elements have different masses: on the periodic table, hydrogen is 1 and oxygen is 16.
If you had to guess, which molecule has the greater mass: the water, or the oxygen?
Each element has a relative atomic mass, Ar, printed on the periodic table. It compares the masses of atoms, so it has no units. 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. So the whole skill is counting atoms correctly. A small number after a symbol counts that atom only. A small number after a bracket multiplies everything inside the bracket.
Ar used here: H 1 · C 12 · N 14 · O 16 · Mg 24 · Al 27 · S 32 · Ca 40
The formula unpacker
Count the atoms. Then watch them add up to Mr.
0 of 4 unpacked
Commit first. How many oxygen atoms are in one CO₂, carbon dioxide?
In a balanced equation atoms are only rearranged, so the sum of the relative formula masses of the reactants, in the quantities shown, equals the sum for the products. A number in front of a formula counts whole formula units: 2MgO is two units of Mr 40, a total of 80. The Mr of one MgO is still 40.
Load the pans
2Mg + O₂ → 2MgO
0 of 2 loaded
Commit first. Which pan will be heavier once each side holds its formula units?
Because the totals balance, the equation also gives the ratio of masses. For 2Mg + O₂ → 2MgO: 2 × 24 : 32 : 2 × 40 = 48 : 32 : 80. So 48 g of Mg reacts with 32 g of O₂ to produce 80 g of MgO, and any other mass of magnesium scales every number by the same factor.
Your call
Calculate6 g of magnesium burns completely: 2Mg + O₂ → 2MgO. What mass of magnesium oxide forms?
Mr also tells you how much of a compound's mass comes from each element. Divide the mass of that element's atoms by Mr and multiply by 100. Multiply the element's Ar by the number of its atoms first: hydrogen is 2 ÷ 18 of the mass of water, not 1 ÷ 18.
Equations · chemistry has no equation sheet
each Ar times the number of those atoms. No units.
% by mass of an element = (Ar × number of atoms of that element) ÷ Mr × 100
mass ratio = ratio of (number in front × Mr)
read from the balanced equation, then scaled to the mass you are given.
Units first
Ar, Mr and percentages have nothing to convert. A mass does: when the answer is wanted in grams, change kg to g (× 1000) before you insert it.
Command words in this lesson
- Calculate
- Show the count, each Ar × number, then the total. Mr has no unit; a percentage or a mass does.
- Show that
- The answer is given to you. The marks are for writing every step that reaches it.
- Explain
- Give the reason, here the counting rule behind a number, not just the number.
Key fact
A number after a bracket multiplies everything inside it: Mg(OH)₂ is one Mg, two O and two H, Mr = 58. Mr has no units.
Key note · AQA 5.3.1.2
Relative formula mass
Photograph this card. Tomorrow, cover it and say each line first.
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Ask MrBadmus
Stuck counting the atoms in a formula with brackets?
The unpacker draws atoms larger for larger Aᵣ only as a cue; real atomic sizes do not follow mass. The mass bar is drawn to one scale for every formula. The pans show the totals of relative formula masses, which have no units; they are not weighing real samples.