Dissolved oxygen is determined by the Winkler method: $O X_{2}$ oxidises $Mn X^{2 +}$ , which then liberates iodine from iodide in acid; the iodine is titrated with thiosulphate, with overall stoichiometry $O X_{2} \equiv 4 S X_{2} O X_{3} X^{2 -}$ . A 200 mL water sample consumes 8.0 mL of 0.0125 M $Na X_{2} S X_{2} O X_{3}$ . The dissolved oxygen in mg per litre is (O = 16):

Answer: B

mmol thiosulphate =

8.0 \times 0.0125 = 0.100

, so mmol

O X_{2}

= 0.100/4 = 0.025. Mass = 0.025 x 32 = 0.80 mg in 0.200 L, i.e.

4.0

mg/L.