A sample containing HAsO 2 (mol. wt. = 108) and weighing 3.78 g is dissolved and diluted to 250 mL in a volumetric flask. A 50 mL sample (aliquot) is withdrawn with a pipet and titrated with 35 mL of 0.05 M solution of I 2 . Calculate the percentage HAsO 2 in the sample :

Step-by-step solution

I 2 + AsO 2 - + 2 H 2 O → HAsO 4 2 - + 2 I - + 3 H + m-eq. of HAsO 2 (in 50 mL) = m-eq. of I 2 = 3 5 × 0.05 × 2 = 3.5 m-eq. of HAsO 2 in 250 mL = 3.5 × 2 5 0 5 0 = 17.5 wt. of HAsO 2 in sample = 17.5 2 × 1 0 8 × 1 0 - 3 = 0.945 g % of HAsO 2 in the sample = 0.945 3.78 × 1 0 0 = 25%