What you'll learn
Chemical reactions never create or destroy atoms — they only rearrange them. That single idea, the conservation of mass, underpins all the calculations in this topic. For AQA GCSE Chemistry you need to calculate relative formula mass (Mr) from a chemical formula, use conservation of mass to balance equations and work out unknown masses, and explain the situations where the mass in a container appears to change. This guide covers relative atomic mass and relative formula mass, how to calculate Mr for compounds with brackets, the law of conservation of mass, apparent mass changes involving gases, and percentage composition. By the end you should be able to calculate Mr confidently and explain any mass change in a reaction.
Key terms and definitions
Relative atomic mass (Ar) — The average mass of an atom of an element compared with one twelfth the mass of a carbon-12 atom; found in the periodic table.
Relative formula mass (Mr) — The sum of the relative atomic masses of all the atoms shown in the formula of a compound.
Law of conservation of mass — No atoms are lost or made during a chemical reaction, so the total mass of the products equals the total mass of the reactants.
Reactant — A substance present at the start of a reaction.
Product — A substance formed by a reaction.
Balanced equation — An equation with the same number of each type of atom on both sides.
Closed system — A container that does not allow substances, including gases, to enter or escape.
Percentage composition — The percentage of the total mass of a compound made up by a particular element.
Core concepts
Relative atomic mass
Every element has a relative atomic mass (Ar), which you read from the periodic table — it is the larger of the two numbers shown for each element. For example, hydrogen has an Ar of 1, carbon 12, oxygen 16, and magnesium 24. These values compare the mass of atoms to one another, which is why they have no units.
Calculating relative formula mass
The relative formula mass (Mr) of a compound is found by adding up the relative atomic masses of every atom in its formula. Work through the formula element by element, multiplying each Ar by the number of atoms:
- Water, H₂O: (2 × 1) + 16 = 18
- Carbon dioxide, CO₂: 12 + (2 × 16) = 44
- Magnesium oxide, MgO: 24 + 16 = 40
Formulae with brackets
When a formula contains brackets, everything inside the brackets is multiplied by the number after them. For calcium hydroxide, Ca(OH)₂, the bracket contains one oxygen and one hydrogen, and there are two of these groups:
Mr = 40 + 2 × (16 + 1) = 40 + 34 = 74
Take care to multiply every atom inside the bracket, not just the last one — this is a very common source of lost marks.
The law of conservation of mass
In a chemical reaction, atoms are rearranged but never created or destroyed. This means the total mass of the reactants always equals the total mass of the products. If 24 g of magnesium reacts with 16 g of oxygen, the mass of magnesium oxide produced must be 24 + 16 = 40 g. This law is also why chemical equations must be balanced: the same number of each type of atom must appear on both sides.
Apparent changes in mass
Sometimes the mass measured in a container seems to change during a reaction. Mass is never really lost or gained — a gas has simply entered or escaped:
- Mass appears to decrease when a gas is produced and escapes into the air. For example, when a metal carbonate is heated, carbon dioxide escapes, so the solid left behind weighs less.
- Mass appears to increase when a gas from the air is a reactant. For example, when a metal is heated in air, oxygen from the air combines with it, so the product weighs more than the original metal.
If the reaction is carried out in a closed system, where no gas can enter or escape, the total mass stays exactly the same. Explaining apparent mass changes in terms of gases entering or escaping is a very common exam question.
Percentage composition
You can find the percentage of an element in a compound using:
percentage = (Ar × number of atoms of that element ÷ Mr of compound) × 100
For example, the percentage of oxygen in water is (16 ÷ 18) × 100 = 88.9%.
Why balanced equations respect conservation of mass
A balanced symbol equation is really just conservation of mass written in symbols. Because no atoms are created or destroyed, there must be the same number of each type of atom on the left (reactants) and the right (products). Balancing numbers are placed in front of formulae to achieve this. For example, in the reaction of hydrogen and oxygen to make water, 2H₂ + O₂ → 2H₂O, there are four hydrogen atoms and two oxygen atoms on each side. If you add up the relative formula masses, the reactant total (4 + 32 = 36) equals the product total (2 × 18 = 36), confirming that mass is conserved. Checking that the masses balance is a useful way to spot an unbalanced equation.
Worked examples
Example 1: Calculating Mr with brackets
Calculate the relative formula mass of magnesium nitrate, Mg(NO₃)₂. Given Ar: Mg = 24, N = 14, O = 16. Inside the bracket: N + 3 O = 14 + (3 × 16) = 14 + 48 = 62. There are two brackets: 2 × 62 = 124. Add the magnesium: 24 + 124 = 148.
Example 2: Using conservation of mass
In a reaction, 10 g of calcium carbonate is heated and produces 5.6 g of calcium oxide plus carbon dioxide gas. What mass of carbon dioxide is produced? By conservation of mass, mass of reactants = mass of products, so 10 = 5.6 + mass of CO₂. Mass of CO₂ = 10 − 5.6 = 4.4 g.
Example 3: Explaining an apparent mass decrease
A student heats a metal carbonate in an open crucible and finds the mass decreases. Explain why, and state whether mass has really been lost. Heating the carbonate produces carbon dioxide gas, which escapes into the air. The mass of solid left behind is therefore lower. No mass has really been lost — the total mass including the escaped gas is unchanged, and if the reaction were done in a closed system the mass would stay the same.
Example 4: Percentage composition
Calculate the percentage by mass of nitrogen in ammonia, NH₃. Ar: N = 14, H = 1. Mr of NH₃ = 14 + (3 × 1) = 17. Percentage of nitrogen = (14 ÷ 17) × 100 = 82.4%.
Common mistakes and how to avoid them
The most frequent error is mishandling brackets. In Ca(OH)₂ you must double both the oxygen and the hydrogen. Students who only double the hydrogen get 56 instead of 74. Always expand the bracket carefully.
Another common mistake is using the wrong number from the periodic table. The relative atomic mass is the larger number; the smaller one is the atomic number. Check this every time.
Students often say mass "is lost" when a gas escapes. Mass is never lost — it is conserved. The correct phrasing is that the gas has escaped from the container, so the mass of the substance remaining is lower.
When counting atoms, remember that a small number after a symbol applies only to the atom immediately before it. In H₂SO₄ there are two hydrogens, one sulfur and four oxygens.
Finally, do not forget to include every element in the formula. Missing one out entirely is an easy slip in longer formulae, so tick off each element as you go.
Exam technique for "Relative formula mass and conservation of mass"
Set out Mr calculations line by line rather than trying to do them in your head. Write each element, its Ar, the number of atoms, and the subtotal, then add. This makes it easy to check and earns method marks even if you make an arithmetic slip.
For conservation of mass questions, write the word equation or balanced symbol equation first, then set total reactant mass equal to total product mass and solve for the unknown. Show the subtraction clearly.
Questions asking you to explain a mass change are worth several marks and need a full explanation: name the gas involved, say whether it escaped or was taken from the air, and state that mass is conserved overall. Mentioning that the mass would be unchanged in a closed system usually secures the final mark. Always give your final answers to a sensible number of significant figures and include units where they are needed.
Quick revision summary
- Relative atomic mass (Ar) is the larger number in the periodic table; relative formula mass (Mr) is the sum of the Ar values of all atoms in a formula.
- Multiply everything inside brackets by the number after them: Ca(OH)₂ = 40 + 2 × 17 = 74.
- Conservation of mass: total mass of reactants = total mass of products; no atoms are created or destroyed.
- Mass appears to decrease when a gas escapes, and to increase when a gas from the air reacts.
- In a closed system the total mass never changes.
- Percentage by mass = (Ar × number of atoms ÷ Mr) × 100.