Science (086) — AI-generated practice question

(i) $\text{HNO}_3 + \text{Ca(OH)}_2 \rightarrow \text{Ca(NO}_3)_2 + \text{H}_2\text{O}$

Count: N=1 LHS, N=2 RHS; balance by placing 2 before HNO₃ and 2 before H₂O:

$$2\text{HNO}_3 + \text{Ca(OH)}_2 \rightarrow \text{Ca(NO}_3)_2 + 2\text{H}_2\text{O}$$

(ii) $\text{NaOH} + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + \text{H}_2\text{O}$

Na=1 LHS, Na=2 RHS; balance by placing 2 before NaOH and 2 before H₂O:

$$2\text{NaOH} + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}$$

(iii) $\text{BaCl}_2 + \text{H}_2\text{SO}_4 \rightarrow \text{BaSO}_4 + \text{HCl}$

Cl=2 LHS, Cl=1 RHS; balance by placing 2 before HCl:

$$\text{BaCl}_2 + \text{H}_2\text{SO}_4 \rightarrow \text{BaSO}_4 + 2\text{HCl}$$

Source: Chapter 1, Section 1.1.2 Balanced Chemical Equations

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• Key rule: Use coefficients only — never change the subscripts inside a formula.
• For each equation, count atoms of every element on both sides and adjust coefficients to make them equal (hit-and-trial method).
• A common mistake is changing H₂O to H₂O₂ to balance oxygen — this is wrong.
• Each sub-question is 1 mark; examiners check that all atoms are balanced on both sides. Writing unbalanced equations scores zero even if the formulae are correct.