Question

You have 1 L of a solution containing 0.01 M of both Sr+ and Ca 2+ ions. You slowly add K2C2O4.

a) which precipitates first CaC2O4 of SrC2O4? CaC2O4

at what C2O4-2 concentrarion does this molecule precipitate? answer given by professor is 4X10^-7 M

what mass of K2C2O4? 6.6 mg

i need help getting those answers i was doing ksp divided by 0.01 solution to get the concentration but my numbesr are way off i used ksp + 2.3X10^-9

Answer 1

Your professor used a rounded value for Ksp, CaC2O4 = 4x10^-9

a. Ksp, SrC2O4 = 5x10^-8   >    Ksp, CaC2O4 = 4x10^-9

CaC2O4 pp. first

b.

The precipitation starts at

[C2O4-2] = Ksp/[Ca2+] = 4x10^-9 / 0.01 M = 4x10^-7M

To obtain this concentration (to start pp) , add

4x10^-7mol/L x 1 L x 166.22 g K2C2O4 /mol = 6.6x10^-5 g = 0.066 mg K2C2O4

To obtain a relatively complete precipitation you need to have a criterion for what means complete precipitation. Here it was [Ca2+] < 1x10^-4 M (less than 1% remaining in solution).

The concentration [C2O4-2] has to be

[C2O4-2] = Ksp/[Ca2+] = 4x10^-9 / 1x10^-4 M = 4x10^-5M

To obtain this concentration (for complete pp) , add

4x10^-5mol/L x 1 L x 166.22 g K2C2O4 /mol = 6.6x10^-3 g = 6.6 mg K2C2O4