2.00 g of a spirit sample is treated with 50.0 mL of 0.100 M $K X_{2} Cr X_{2} O X_{7}$ in excess acid; ethanol is oxidised to acetic acid ( $C X_{2} H X_{5} OH CH X_{3} COOH$ ). Excess KI is then added, and the iodine liberated by the unused dichromate needs 39.0 mL of 0.200 M $Na X_{2} S X_{2} O X_{3}$ . Find the mass percentage of ethanol (molar mass 46) in the spirit, to the nearest integer.

Numerical answer type, answer in %, tolerance 0.5.

Answer: 13 %

Thiosulphate =

39.0 \times 0.200 = 7.80

mmol; unused dichromate = 7.80/6 = 1.30 mmol (1 dichromate = 6 thiosulphate). Dichromate used =

5.00 - 1.30 = 3.70

mmol =

22.2

meq. Ethanol has n = 4 (C: -2 to 0 for the carbinol carbon, 4 electrons), so mmol = 5.55, mass = 255.3 mg. Percentage = 12.77%, about 13.