GCSE Combined Science revision · Chemistry: atoms, bonding and amounts

Conservation of mass and formula mass

The idea

No atoms are lost or made in a chemical reaction, so the mass of the products equals the mass of the reactants. Balanced equations have the same number of each atom on both sides. Relative formula mass (Mr) is the sum of the relative atomic masses in the formula. If a gas escapes from an open container, the mass appears to fall; if a gas is taken in (such as oxygen when a metal burns), the mass appears to rise.

Balance atoms by changing the large numbers in front only.
Add up Ar values for Mr.
Explain apparent mass changes with gases.

Watch out Never change the small numbers in a formula to balance an equation.

See it worked

Worked example

Calculate the relative formula mass of calcium carbonate, CaCO₃ (Ca = 40, C = 12, O = 16).

40 + 12 + (3 × 16).
= 40 + 12 + 48 = 100.

Answer 100

Your turn

Question 1Calculator allowed2 marks

Calculate the Mr of sulfuric acid, H₂SO₄ (H = 1, S = 32, O = 16).

Question 2Calculator allowed1 mark

Why does the mass fall when calcium carbonate is heated in an open tube?

Question 3Calculator allowed2 marks

12 g of magnesium burns completely in oxygen to make 20 g of magnesium oxide. What mass of oxygen reacted, in grams?

In past papers

56 official questions in 38 papers since 2018. The latest:

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