C1.254SD0 · Section C1: Principles of chemistry

Chemical equations and relative formula mass

Key learnings

Balance with coefficients, never subscripts
You may only place or change the large coefficient in front of a formula; never change a small subscript inside one, because that changes the substance — $\text{H}_2\text{O}$ is water but $\text{H}_2\text{O}_2$ is hydrogen peroxide. A coefficient multiplies every atom that follows it: $2\text{Ca(OH)}_2$ is 2 Ca, 4 O and 4 H. Count each element on both sides, adjust coefficients until they match, then re-check. A balanced equation obeys conservation of mass.
Word equation, symbol equation, state symbols
A word equation lists reactants on the left and products on the right, joined by an arrow meaning "changes into". A balanced symbol equation replaces the names with formulae and adds coefficients so there is the same number of each atom on both sides — required by conservation of mass. State symbols follow each formula: (s) solid, (l) pure liquid, (g) gas, (aq) aqueous. Example: $\text{2Mg(s)} + \text{O}_2\text{(g)} \rightarrow \text{2MgO(s)}$ .
Relative atomic mass and relative formula mass
Relative atomic mass ( $A_r$ ) is the average mass of an atom relative to 1/12 of a carbon-12 atom (it accounts for isotopes and their abundances). Relative formula mass ( $M_r$ ) is the sum of the $A_r$ of every atom in the formula — multiply each element's $A_r$ by its count, then add. On the 4SD0 Edexcel data sheet every $A_r$ is a whole number except copper ( $63.5$ ) and chlorine ( $35.5$ ), so $\text{CuSO}_4 = 159.5$ (not 160) and $\text{NaCl} = 58.5$ .

Common mistakes

Drawn from real examiner reports.

Oxygen is O₂; sulfur dioxide is SO₂

Use the correct formula for each substance before balancing. Oxygen is diatomic — $\text{O}_2$ , not O — and sulfur dioxide is $\text{SO}_2$ , not "SO". A wrong formula makes the equation impossible to balance and loses the mark. Learn the diatomic elements ( $\text{H}_2$ , $\text{O}_2$ , $\text{N}_2$ , $\text{Cl}_2$ ) and build compound formulae from their correct ratios.

Nov24 Q11ai — oxygen written as O and sulfur dioxide as SO, not O2 and SO2

Count the oxygen already in the molecule

When balancing a combustion equation, count the oxygen atoms already inside the fuel molecule (e.g. the O in an alcohol) before deciding how many $\text{O}_2$ are needed — missing this is a persistent error. Fractions or half-coefficients are acceptable since coefficients only show a ratio; double the whole equation afterwards if you want whole numbers.

Nov24 Q9di — did not count the O already in the alcohol; Jun23 Q8d — fractions/half-coefficients are acceptable

Never change a subscript to balance

The commonest balancing error is changing a small subscript inside a formula to make atoms match — e.g. writing $\text{H}_2\text{O}_2$ to gain an oxygen. That changes the substance: $\text{H}_2\text{O}_2$ is hydrogen peroxide, not water. Balance using only the large coefficients in front, keeping every formula chemically correct.

Bracket subscript multiplies every atom

In an $M_r$ calculation, do not ignore the number after a bracket. For calcium hydroxide $\text{Ca(OH)}_2$ the subscript 2 multiplies both the O and the H inside it: $40 + 2\times(16+1) = 74$ , not $40 + 16 + 1 = 57$ . The same slip happens with the number after a dot in a hydrated salt and with a coefficient in front. Expand the bracket fully before multiplying by $A_r$ .

State symbols: (aq) is not (l)

State symbols are often left out or wrong. A dissolved substance is (aq), not (l) — (l) is only for a pure liquid such as molten metal or water. Sodium chloride solution is $\text{NaCl(aq)}$ , never $\text{NaCl(l)}$ . When a question asks for state symbols, every formula must carry one to earn the mark.

Ar (one atom) ≠ Mr (whole formula)

Do not confuse $A_r$ , the relative atomic mass of a single atom, with $M_r$ , the relative formula mass of a whole formula unit. The $A_r$ of an oxygen atom is 16, but oxygen gas $\text{O}_2$ has $M_r = 32$ . Read whether the question wants the atom or the molecule/formula unit, and quote the value that matches.

Exam tips

Balancing routine
Write the correct formula for each substance, then tally each element on both sides. Balance the most complex formula first with a coefficient, leaving elements that appear alone (like $\text{O}_2$ ) until last. Re-tally, add state symbols if asked, and never touch a subscript.
Show Mr working element by element
List each element with its count and $A_r$ (e.g. Ca $1\times40$ , O $2\times16$ , H $2\times1$ ), multiply, then add. Writing it out shows your working so a slip costs only one mark, and it forces you to apply every bracket subscript rather than skipping it.
Read exactly what the command asks
Check the command before answering: is it asking for a name or a formula, for $A_r$ or $M_r$ , and are state symbols required? Giving a formula when the name was asked, or the $A_r$ of an atom when the $M_r$ was wanted, throws away easy marks.

Revision notes