138. Задачка по химии №013 834

99. A sample of phenol equal to 2.4560 g is dissolved in a volumetric flask with a capacity of 500 cm3, 10.00 cm3 of a 10% NaOH solution is added, brought to the mark with water, mixed thoroughly. Take 100.00 cm3 of the analyzed solution into a cone with a ground-in stopper, add another 2 cm3 of 10% NaOH solution, heat to 50 C, introduce 45.00 cm3 of I2 solution with C (1 / 2I2) = 0.1000 mol / dm3 and keep at 60 C until the reaction is complete;
C6H5OH + 3I2 → C6H2I3OH + 3HI
Then the reaction mixture is acidified with 5% HCl and unreacted I2 is titrated with 17.65 cm3 of 0.09906 M Na2S2O3 solution. Determine the mass fraction of phenol in the sample.