Question

A solution contains 1.49 mg of CoSO₄ (155.0 grams/mole) per mL. Calculate the volume of 0.009108 M Zn²⁺ needed to titrate the excess complexing reagent after the addition of 80.00-mL of 0.008792 M EDTA to a 20.00 mL aliquot of the Co²⁺ solution.

Answer 1

The concentration of solution of CoSO4 = Mass / molecular weight X volume

Concentration = 1.49 / 155 X 1 = 0.00961 moles / L

[I have kept mass in mg and Volume in mL , as both mill will cancel each other]

The EDTA will react with Co+2 and Zn+2 in 1:1 molar ratio

The moles of EDTA used = Moles of Co+2 present

Moles of Co+2 in 20mL = Molarity X volume in L = 0.00961 X 20 / 1000 = 0.000192

So moles of EDTA used = 0.000192

Volume of EDTA used = Moles / Molarity of EDTA = 0.000192 / 0.008792 = 0.021838 L = 22.83 mL

So volume of EDTA left = 80mL - 22.83 mL = 57.17 mL

So moles of EDTA left = Molarity X volume in L = 0.008792 X 57.17 / 1000 = 0.000502

Moles of EDTA left = Moles of zn+2 reacted

so volume of zn+2 reacted = Moles / Molarity = 0.000502 / 0.009108 = 0.0551 L = 55.1 mL