Question

A 50.0-mL solution containing Ni2+ and Zn2+ was treated with 25.0 mL of 0.0452 M EDTA to bind all the metal. The excess unreacted EDTA required 12.4 mL of 0.0123 M Mg2+ for complete reaction. An excess of the reagent 2,3-dimercapto-1-propanol was then added to displace the EDTA from zinc. Another 29.2 mL of Mg2+ were required for reaction with the liberated EDTA. Calculate the molarity of Ni2+ and Zn2+ in the original solution.

Answer 1

Total moles of EDTA used initially to bind Ni2+ and Zn2+ = 0.0452 M x 25 ml = 1.13 mmol

Excess EDTA present = 0.0123 M x 12.4 ml = 0.15 mmol

Total moles of EDTA reacted with Ni2+ and Zn2+ = 1.13 mmol - 0.0123 M x 12.4 ml = 0.98 mmol

moles of EDTA complexed with Zn2+ = 0.0123 M x 29.2 ml = 0.36 mmol

moles of EDTA complexed with Ni2+ = 0.98 - 0.36 = 0.62 mmol

Molarity of Zn2+ in the original solution = 0.36 mmol/50 ml = 0.0072 M

Molarity of Ni2+ in the original solution = 0.62 mmol/50 ml = 0.0124 M