1.254CH1 · Section 1: Principles of chemistry

Chemical formulae and equations

Key learnings

Ionic formula = charges balanced to zero
Look up each ion's charge, then balance the charges so the compound is neutral (cross-over / swap-the-charges). Monatomic charges follow the group: Group 1 = $1+$ , Group 2 = $2+$ , Group 6 = $2-$ , Group 7 = $1-$ ; a Roman numeral fixes a transition-metal charge (iron(III) is $\text{Fe}^{3+}$ ). Polyatomic ions keep their identity and take a bracket when more than one is needed, e.g. $\text{Ca}^{2+}$ with $\text{NO}_3^{-}$ gives $\text{Ca(NO}_3)_2$ .
Coefficients balance; state symbols do not
A word equation names reactants and products either side of an arrow. A balanced symbol equation has the same number of each atom on both sides — atoms are neither created nor destroyed. Balance only by changing the large numbers (coefficients) in front of formulae, never a subscript inside a correct formula. State symbols (s), (l), (g), (aq = aqueous) describe physical state only; adding them does not balance an equation.
Mr = sum of the Ar values in the formula
Find the relative formula mass ( $M_r$ ) by adding the relative atomic masses ( $A_r$ ) of every atom in the formula. A subscript multiplies the atom before it; a subscript outside a bracket multiplies everything inside it. Example: $M_r(\text{Ca(OH)}_2) = 40 + 2\times(16 + 1) = 74$ . On the 4CH1 data sheet every $A_r$ is a whole number except copper ( $\text{Cu} = 63.5$ ) and chlorine ( $\text{Cl} = 35.5$ ) — never quote $\text{Cu} = 64$ .

Common mistakes

Drawn from real examiner reports.

Ionic formulae without using the charges

Two recurring errors: writing $\text{Zn}_2\text{O}_2$ instead of the simplest ratio $\text{ZnO}$ , or $\text{NH}_3\text{SO}_4$ instead of $(\text{NH}_4)_2\text{SO}_4$ ; and forgetting to bracket a polyatomic ion that appears more than once. Fix: write each ion's correct charge, balance the charges, then simplify to the lowest whole-number ratio.

June 2024 Paper 1CR Q11(b) — ammonium sulfate was the least well-answered; candidates wrote NH3SO4 and forgot to bracket NH4 and multiply by 2. Examiner advice: "Use the charges to balance the ions."

Balancing by adding state symbols

Balance an equation only by adjusting the coefficients (large numbers) in front of formulae. Two traps: changing a subscript in a correct formula makes a substance that does not exist; and adding state symbols hoping they balance the atoms — they describe physical state, not atom count. If atoms differ side to side, change a coefficient, never a formula.

June 2023 Paper 1CR Q9(a)(iii) — many candidates struggled to balance the equation and a few added state symbols instead of balancing the equation.

Wrong charge on a polyatomic ion

Putting the wrong charge on a polyatomic ion wrecks the formula. The nitrate ion is $\text{NO}_3^{-}$ (charge $1-$ ), not $\text{NO}_3^{2-}$ . Learn the standard charges — nitrate $1-$ , hydroxide $1-$ , sulfate $2-$ , carbonate $2-$ , ammonium $1+$ — so you build the neutral formula correctly first time. A wrong charge feeds a wrong subscript and $M_r$ .

June 2023 Paper 1CR Q9(a)(i) — a common mistake was giving the nitrate ion a charge of 2- instead of 1-.

Naming when a formula is asked

When a question asks for a formula, a name scores zero — give the correct symbols and subscripts. "Copper oxide" is not $\text{CuO}$ , and "sodium carbonate" is not $\text{Na}_2\text{CO}_3$ , until you write it. Read the command word: formula means chemical symbols, name means words. Build the formula from the ion charges rather than recalling it.

June 2023 Paper 1CR Q4(b)(i) — candidates named copper oxide but did not give its formula when asked.

Subscript outside a bracket multiplies all

A subscript outside a bracket multiplies everything inside, not just the nearest atom. In $\text{Al}_2(\text{SO}_4)_3$ the $3$ multiplies both S and the four O, giving 3 S and 12 O — so $M_r = (2\times27) + 3\times(32 + 4\times16) = 342$ . Count atoms group-by-group before summing the $A_r$ values.

Confusing Ar (atom) with Mr (formula)

$A_r$ and $M_r$ are not interchangeable. Relative atomic mass ( $A_r$ ) is per single atom of an element; relative formula mass ( $M_r$ ) is the sum of the $A_r$ values for a whole formula. Use $M_r$ for any compound: for $\text{CO}_2$ , $M_r = 12 + 2\times16 = 44$ , not $A_r = 12$ . Dividing a mass by $A_r$ instead of $M_r$ is a classic mole-calculation slip.

Exam tips

Balance by inspection, oxygen last
Routine: write the correct formula of each reactant and product (never change one later); tally each element both sides; adjust the coefficients to balance, leaving oxygen and hydrogen last; re-count, then add state symbols (s), (l), (g), (aq) as a final step.
Use the Mr to sanity-check a formula
After writing a formula, a quick $M_r$ check catches slips: $M_r(\text{MgCl}_2) = 24 + 2\times35.5 = 95$ confirms two chlorines, not one. Show the sum — examiners credit the method even if the number is wrong. The 4CH1 data sheet gives $\text{Cl} = 35.5$ , $\text{Cu} = 63.5$ .
Fractions are allowed in equations
Half-coefficients are fine — coefficients show only a ratio, so $\text{H}_2 + \tfrac{1}{2}\text{O}_2 \rightarrow \text{H}_2\text{O}$ is valid (or double to clear it). In combustion, oxygen is diatomic ( $\text{O}_2$ ) — count the oxygen already in the reactant.

Revision notes