Question

Consider the compounds with the generic formulas listed below and their corresponding molar solubilities in pure water. AX2 ;molarsolubility= 1.75×10−4 M A2X ;molarsolubility= 2.25×10−4 M AX ;molarsolubility= 1.25×10−6 M

Part A Which compound will have the smallest value of Ksp? a. AX b. AX2 c. A2X

Answer 1

a) AX

Explanation

i) Ksp of AX2

Solubility equilibrium of AX2 is

AX2(s) <--------> A²⁺(aq) + 2X^-(aq)

Ksp = [A²⁺][X^-]²

If molar solubility of AX2 is represented by S

at saturated solution

[A²⁺] = 1S = 1.75 ×10^-4 M

[X^-] = 2S = 2× 1.75 ×10^-4M = 3.5 × 10^-4M

Ksp = (1.75 ×10^-4)(3.5 ×10^-4)²

Ksp = 2.14 × 10^-11

ii) Ksp of A2X

Solubility equilibrium of A2X is

A2X(s) <---------> 2A⁺(aq) + X^-(aq)

Ksp = [A⁺]²[X^-]

at saturated solution

[A⁺] = 2S = 2 × 2.25×10^-4M = 4.5 ×10^-4M

[X^-] = 1S = 2.25 ×10^-4M

Ksp = (4.5 × 10^-4M)²(2.25 ×10^-4M)

Ksp = 4.56 × 10^-11

iii) Ksp of AX

Solubility of equilibrium of AX

AX(s) <--------> A⁺(aq) + X^-(aq)

Ksp = [A⁺][X^-]

At saturated solution

[A⁺] = 1S = 1.25 ×10^-6M

[X^-] = 1S = 1.25 × 10^-6M

Ksp = 1.25 ×10^-6M × 1.25 ×10^-6M

Ksp = 1.56 ×10^-12

Therefore,

Compound which have lowest Ksp value is AX