Conservation of mass and balanced symbol equations; relative formula mass; the mole and the Avogadro constant; apparent changes in mass in reactions involving gases.

What this dot point is asking

AQA wants you to state and explain conservation of mass, balance symbol equations, calculate relative formula mass, define the mole and the Avogadro constant, and explain why mass can appear to increase or decrease in reactions that involve a gas. These ideas are the foundation of the whole quantitative chemistry topic: every reacting-mass and yield calculation relies on the fact that mass is conserved and that a balanced equation conserves atoms.

Conservation of mass

A balanced equation has the same number of each type of atom on both sides. You balance only by changing the big numbers in front of formulae, never the small numbers inside them, because changing a subscript would change the substance itself. For example, in $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$ there is one carbon, four hydrogens and four oxygens on each side. A useful check is to count each element on both sides before trusting the balancing.

Relative formula mass

The relative formula mass ($M_r$) of a compound is the sum of the relative atomic masses ($A_r$) of all the atoms in its formula. For example, for $CO_2$, $M_r = 12 + (2 \times 16) = 44$, and for calcium hydroxide $Ca(OH)_2$, $M_r = 40 + 2 \times (16 + 1) = 74$ (note the bracket multiplies both the oxygen and the hydrogen).

The mole and the Avogadro constant

So one mole of carbon (12) has a mass of $12$ g, and one mole of water (18) has a mass of $18$ g. The mole is what lets chemists count particles by weighing: because reacting particles combine in fixed whole-number ratios, measuring amount in moles (rather than grams) is what makes the balancing numbers directly useful.

Apparent changes in mass

Mass can appear to change in a reaction in an unsealed container:

• If a gas is given off (for example $CO_2$ in a thermal decomposition) and escapes, the mass of the remaining solid appears to decrease.
• If a gas from the air is taken in (for example oxygen when a metal burns), the mass of the solid appears to increase.

The total mass is still conserved if you include the gas. In a sealed container the balance reading would not change at all, which is the clearest demonstration that no mass is truly lost or gained.

Try this

Q1. Calculate the relative formula mass of $H_2O$ ($A_r$: H = 1, O = 16). [1 mark]

• Cue. $(2 \times 1) + 16 = 18$.

Q2. Explain why the mass of a metal increases when it burns in air in an open dish. [2 marks]

• Cue. It combines with oxygen from the air, so the oxygen's mass is added to the product.

Q3. Calculate the relative formula mass of magnesium nitrate, $Mg(NO_3)_2$ ($A_r$: Mg = 24, N = 14, O = 16). [2 marks]

• Cue. $24 + 2 \times (14 + 3 \times 16) = 24 + 2 \times 62 = 148$.