Relative formula mass and conservation of mass – GCSE Combined Science Chemistry

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When you burn a candle, it seems to disappear. When you bake a cake, the mixture seems to change completely. Chemistry has a simple rule that explains what really happens to the mass.

This matters because it lets you predict masses before you do an experiment. Chemists use it to work out how much of a substance they will make. It is also the start of all the calculations in this topic.

By the end you will be able to find a missing mass. You will calculate the relative formula mass of a compound. You will also explain why a mass can seem to go up or down.

Conservation of mass

No atoms are made or destroyed in a chemical reaction. They are rearranged into new substances. So the total mass of the reactants equals the total mass of the products. This is the law of conservation of mass. Think of a Lego house rebuilt into a car. The bricks are the same, so the mass is the same.

If you know every mass but one, subtract. Say 12 g of magnesium reacts with oxygen to make 20 g of magnesium oxide. The oxygen is 20 − 12 = 8 g.

Relative atomic mass and relative formula mass

The relative atomic mass, Ar, of an element is shown on the periodic table. It compares the mass of an atom with other atoms. Use these values. H = 1, C = 12, N = 14, O = 16, Na = 23. Mg = 24, S = 32, Cl = 35.5, Ca = 40.

The relative formula mass, Mr, of a compound is the sum of the Ar values in its formula. Add one for every atom. Mr has no units. For water, H₂O, Mr = 2 × 1 + 16 = 18.

Calculating Mr

Write out each element, multiply its Ar by the number of atoms, then add. For carbon dioxide, CO₂: 12 + 2 × 16 = 44. For sodium hydroxide, NaOH: 23 + 16 + 1 = 40.

With brackets, multiply everything inside the bracket by the number outside. For calcium hydroxide, Ca(OH)₂: 40 + 2 × (16 + 1) = 40 + 34 = 74.

Think of a shopping basket. Each item has a price, and you multiply by how many you bought, then add up.

Mr and balanced equations

The masses in a balanced equation also add up. Take 2Mg + O₂ → 2MgO. The left side is 2 × 24 + 32 = 80. The right side is 2 × (24 + 16) = 80.

Multiply the Mr by the big number in front of the formula, then add. If both sides match, the equation is balanced and your Mr values are probably right. If they do not match, check your sums again.

Why mass can seem to change

Sometimes the mass appears to change. This happens when a gas is a reactant or a product, and the container is open. Gases are very light and you cannot see them. The gas is not weighed, so it seems to appear or disappear.

If a gas is a reactant, the solid seems to gain mass. If a gas is a product, the mass seems to fall because the gas escapes. It is like a fizzy drink going flat in an open glass. If the container is sealed, the total mass does not change.

Mass going up and mass going down

When magnesium burns, 2Mg + O₂ → 2MgO. The white powder is heavier than the magnesium. The extra mass is the oxygen that joined it. If 2.4 g of magnesium makes 4.0 g of magnesium oxide, the oxygen is 4.0 − 2.4 = 1.6 g.

When a metal carbonate is heated in an open tube, carbon dioxide escapes. The solid left is lighter. Say 5.0 g of a metal carbonate leaves 3.1 g of solid. The gas lost is 5.0 − 3.1 = 1.9 g.

Recap

Mass is conserved because atoms are rearranged, not made or destroyed.

Mr is the sum of the Ar values for every atom in the formula. Multiply by any number outside brackets.

In an open container, a gas joining makes the mass seem to rise. A gas leaving makes it seem to fall.

To find a missing mass, subtract the masses you know.