139. Solving a chemistry problem

14. A sample of manganese concentrate weighing 0.2792 g was decomposed by the action of concentrated hydrochloric acid, the solution was neutralized with ammonia and diluted to 25.00 ml. For the purpose of microphotometric determination of iron, an aliquot of
An excess of potassium iodide was added to 5.00 ml, titrated in ultraviolet light with 0.01 M Na2S2O3 and the following data were obtained:
V (Na2S2O3). ml. 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Scale reading 1 3 11 18 22 23.5 24
Construct a titration curve and determine the mass fraction (%) of iron in the concentrate if iodine is the only substance that absorbs at λ = 365 μm.