Question

A solution contains 1.27×10^-2 M sodium hydroxide and 1.20×10^-2 M potassium phosphate.
Solid chromium(III) acetate is added slowly to this mixture.

A. What is the formula of the substance that precipitates first?

formula =

B. What is the concentration of chromium(III) ion when this precipitation first begins?
[Cr³⁺] =___ M

show work

Answer 1

ok then we have NaOH and KH₂PO₄

We add Cr(C₂H₃O₂)₃

To know the substante that will precipate first we need to know wich rx occurs and the Kps of the compounds formed

NaOH + Cr(C₂H₃O₂)₃ Cr (OH)₃ + NaC₂H₃O₂ Kps Cr(OH)₃ = 6.3×10^–31

K₃PO₄ + Cr(C₂H₃O₂)₃ CrPO₄ + KC₂H₃O₂   Kps CrPO₄ =  2.4X10^-23

This means that the Chromium phosphate will precipate first because of the smaller Kps

B. [Cr³⁺]

CrPO_{4 (s)} Cr³⁺(aq) + PO₄^3- (aq)

Kps = [ Cr³⁺] * [PO₄^3- ]

Kps= X*X = X²

X = Kps

X = 2.4X10^-23

X = 4.89x10^-12 M = [ Cr³⁺]=[PO₄^3- ]