Question

A 45.00 mL solution of 0.3500 M AgNO3 was added to a solution of AsO43–. Ag3AsO4 precipitated and was filtered off. Fe3 was added and the solution was titrated with 0.2150 M KSCN. After 34.50 mL of KSCN solution had been added, the solution turned red. What mass of AsO43– was in the original solution?

Answer 1

moles of AgNO3 = 45 x 0.35 / 1000 = 0.01575 mol

moles of KSCN = 34.50 x 0.2150 / 1000 = 7.418 x 10^-3 mol

the added KSCN reacts with excess Ag+.

moles of AgNO3 react with AsO43- = 0.01575 - 7.418 x 10^-3

                                                        = 8.3325 x 10^-3 mol

moles of AsO43- = 8.3325 x 10^-3 mol

mass of AsO43- = 8.3325 x 10^-3 x 138.917

                          = 1.1575 g

mass of AsO43– was in the original solution = 1157.5 mg