Edexcel IGCSE Chemistry · Spec 1.25-1.26
Equations & Relative Formula Mass
Writing balanced chemical equations and calculating relative formula mass.
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Equations & Relative Formula Mass
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Explained
Balancing equations, and counting up a formula mass
The rule behind every balanced equation is that atoms are never created or destroyed. Whatever goes in must come out, so each element must have the same number of atoms on both sides.
How to balance
Write the correct formulae first and then leave them alone. This is the step that decides everything: you balance by putting numbers in front of formulae, never by changing the formulae themselves.
Changing H2O to H2O2 to get another oxygen looks like it balances the equation, but it has turned water into hydrogen peroxide, which is a different substance. Putting a 2 in front, as 2H2O, means two water molecules, which is what you want.
Work through the elements one at a time, leaving oxygen and hydrogen until last because they appear in the most compounds. Then count both sides again from scratch, because balancing one element often unbalances another.
State symbols
Four are used: (s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous, meaning dissolved in water.
Deciding between (s) and (aq) is a solubility question, so the solubility rules do the work here. Sodium, potassium and ammonium salts and all nitrates are soluble, so they are (aq).
Relative formula mass
Add up the relative atomic masses of every atom in the formula. That is all it is, and the only difficulty is counting the atoms correctly.
Brackets are where people go wrong. In Ca(OH)2 the subscript applies to everything inside the brackets, so there are two oxygens and two hydrogens, giving 40 plus 32 plus 2, which is 74. In Mg(NO3)2 there are two nitrogens and six oxygens.
Relative formula mass has no unit, because it is a comparison rather than a measurement.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme splits an equation question into two marks: one for all the formulae being correct, and one for the balancing of correct formulae. The wording matters. The balancing mark is only available once the formulae are right, so an equation balanced perfectly around a wrong formula scores nothing at all.
It allows multiples and fractions, so doubling the whole equation is not penalised.
State symbols are marked separately again. On another question the marks are one for the equation being correct and balanced and one for the state symbols being correct, so the symbols are worth as much as the chemistry that produced them.
Two vocabulary rulings are worth knowing. Asked what butane contains, the mark scheme rejects hydrogen and carbon molecules, because those elements are present as atoms within the compound. And asked to name a gas in air, it allows nitrogen or N2 but ignores N, since a single N is an atom rather than the gas.
Reading an equation properly
The big number in front of a formula multiplies everything in it. The small number after an atom applies only to that atom.
So 2H2SO4 means two molecules, each containing two hydrogens, one sulfur and four oxygens, giving four hydrogens, two sulfurs and eight oxygens in total.
Getting this distinction firm is worth more than it looks, because it underlies balancing, formula mass, and every mole calculation that comes later.
Spec 1.25-1.26
What you need to know
- Write and balance symbol equations
- Use the state symbols
- Calculate a relative formula mass
Active recall
Quick check
Answer each question before opening the answer.
How do you calculate the relative formula mass (Mr) of a compound?
Add up the relative atomic masses (Ar) of all the atoms in its formula.
Why must a chemical equation be balanced?
Because atoms are not created or destroyed — there must be the same number of each atom on both sides (conservation of mass).
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