15. Solving a chemistry problem

51. Give the reaction equations for the quantitative determination of phenoxymethylpenicillin by iodimetry. Calculate the content (%) if a sample weighing 0.0636 g was dissolved and brought to the mark with water in a 50 ml volumetric flask. To a 2.5 ml aliquot was added 20.0 ml of 0.01 mol/l (UCH 1/2 I2) iodine solution (K = 0.98), the excess of which was titrated in the main experiment with 12.8 ml 0.01 mol / l solution of sodium thiosulfate (K = 1.02), in the control experiment - 19.6 ml of the same titrant. Sample moisture - 1.5%.
The titer of phenoxymethylpenicillin at a temperature under experimental conditions of 18°C is 0.0004367 g/ml (T B/A).