Question

A solution contains 1.569 mg of CoSO₄ per milliliter.

a) Calculate the volume of 0.007840 M EDTA needed to titrate a 20.00-mL aliquot of this solution

b) If 50.00 mL of 0.007840 M EDTA is added to a 25.00 mL aliquot of this solution, calculate the volume of 0.009275 M Zn²⁺ needed to titrate the xcess EDTA.

c) An unmeasurered excess of ZnY^2- is added to a 25.00-mL aliquot of the CoSO₄ solution. Zinc is displayed by the Co²⁺ in the following reaction:
Co²⁺ + ZnY^2- -----> CoY^2- + Zn²⁺
Calculate the volume of 0.00784 M EDTA needed to titrate the displaced Zn^2+.

d) Why might you need to perform a displacement titration for Co²⁺?

e) What is an appropriate indicator and pH range for the titration in part (c)?

Answer 1

I'll answer (for now) a) and b) because I have a doubt with some calculations in part c), there's a number that it's not correct, and I don't want you to get an incorrect answer. So, if you like, post part c), d) and e) in another question thread.

a) first, let's calculate the moles per liter:
moles CoSO4 = 1.569x10^-3 g/mL * 1000 mL/L * 1 mol/155 g = 0.0101 mol/L

Now, let's calculate the volume of EDTa according to the following reaction::
Co2+ + H4Y >> CoH2Y + 2H+

VEDTA = 0.0101 * 20 / 0.007840 = 25.77 mL

b) moles EDTa = M * v
moles EDTA = 0.00784 * 0.050 = 0.000392 moles
moles EDTA excess = moles EDTA (50) - moles EDTA (25)
moles EDTA excess = 0.000392 - (0.0101*0.025) = 0.0001395 moles = moles Zn²⁺ needed in the solution.
VZn = 0.0001395 / 0.009275 = 0.01504 L or 15.04 mL

Hope this helps