Question

A solution contains 1.53 mg of CoSO4 (155.0 grams/mole) per mL. Calculate the volume (in mL) of 0.009623 M Zn2+ needed to titrate the excess complexing reagent after the addition of 80.00 mL of 0.008642 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 = 1.53 / 155 X 1 = 0.0099 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 = 0.0099 X 20 / 1000 = 0.000198

So moles of EDTA used = 0.000198

Volume of EDTA used = Moles / Molarity of EDTA = 0.000198 / 0.008642 = 0.02291 L = 22.91 mL

So volume of EDTA left = 80mL - 22.91 L = 57.09 mL

So moles of EDTA left = Molarity X volume in L = 0.008642 X 57.09 / 1000 = 0.000493

Moles of EDTA left = Moles of zn+2 reacted

so volume of zn+2 reacted = Moles / Molarity = 0.000493 / 0.009623 = 0.05132 L = 51.23 mL