Question

A solution contains 1.10x10-2 M potassium phosphate and 5.69x10-3 M ammonium cyanide. Solid zinc acetate is added slowly to this mixture. A. What is the formula of the substance that precipitates first? formula = formula = B. What is the concentration of zinc ion when this precipitation first begins? [Zn2+] = C M

Answer 1

Q1. potassium phosphate and ammonium cyanide

(A.) formula = Zn3(PO4)2

(B.) [Zn2+] = 4.22 x 10-10 M

Q2. ammonium chloride and potassium sulfite

(A.) formula = AgCl

(B.) [Ag+] = 1.96 x 10-8 M

Explanation

Q1. Given : concentration of potassium phosphate = [K3PO4] = 1.10 x 10-2 M

[PO4 3-] = [K3PO4] = 1.10 x 10-2 M

Ksp Zn3(PO4)2 = [Zn2+]3[PO4 3-]2

9.1 x 10-33 = [Zn2+]3 * (1.10 x 10-2 M)2

[Zn2+]3 = (9.1 x 10-33) / (1.10 x 10-2 M)2

[Zn2+]3 = 7.52 x 10-29

[Zn2+] = (7.52 x 10-29)1/3

[Zn2+] = 4.22 x 10-10 M

Given : concentration of ammonium cyanide = [NH4CN] = 5.69 x 10-3 M

[CN-] = [NH4CN] = 5.69 x 10-3 M

Ksp Zn(CN)2 = [Zn2+][CN-]2

8.0 x 10-12 = [Zn2+] * (5.69 x 10-3 M)2

[Zn2+] = (8.0 x 10-12) / (5.69 x 10-3 M)2

[Zn2+] = 2.47 x 10-7 M

Since less concentration of Zn2+ is required to precipitate phosphate, therefore, Zn3(PO4)2 will precipitate first.