What is pH? Calculate the pH of (i) 0.01 mol L⁻¹ nitric acid and (ii) 0.001 mol L⁻¹ sodium hydroxide at 25 °C. Explain two ways in which pH matters in daily life.

1. 1 · Define. pH = −log[H⁺], the negative logarithm of the hydrogen ion concentration. Below 7 is acidic, 7 neutral, above 7 basic.
2. 2 · Nitric acid.
   • Strong acid, so [H⁺] = 0.01 = 10⁻² mol L⁻¹
   • pH = −log(10⁻²) = 2
3. 3 · Sodium hydroxide.
   • Strong base, so [OH⁻] = 0.001 = 10⁻³ mol L⁻¹
   • pOH = −log(10⁻³) = 3
   • pH = 14 − 3 = 11
4. 4 · Daily life, point 1. Digestion: extra stomach acid causes acidity, and an antacid such as milk of magnesia neutralises it.
5. 5 · Daily life, point 2. Tooth decay begins below pH 5.5, and acid rain (below 5.6) harms life in rivers.
6. 6 · Close in one line. So a small change in pH can upset living things, and pH must stay in the right range.

Scoring tip: Write the formula, the value of [H⁺] or [OH⁻] with its unit, and then the answer. The steps carry marks even if the last line slips.