Question

21. The Ksp of Ag, CrO4, AgCl, AgBr and Agl are
respectively, 1.1 x 10-12, 1.8 x 10-10, 5.0 x 10-13,
8.3 x 10-17. Which one of the following salts will
precipitate last if AgNO3 .solution is added to the
solution containing equal moles of NaCl, NaBr, Nal
and Na, Cro ?
[AIPMT-2015]
(1) Ag, Cro4
(2) Agl
(3) AgCI
(4) AgBr​

Answer 1

Answer:

1 is the correct answer as the common ion effects

Answer 2

Ag₂CrO₄ will precipitate last if AgNO₃ solution is added to the solution containing equal moles of NaCl, NaBr, NaI and Na₂CrO₄.

Given-

• Ksp of Ag₂CrO₄ = 1.1 × 10⁻¹²
• Ksp of AgCl = 1.8 × 10⁻¹⁰
• Ksp of AgBr = 5.0 × 10⁻¹³
• Ksp of AgI = 8.3 × 10⁻¹⁷

Those salt which have the greater solubility will precipitate last.

From the given value of Ksp we can calculate the value of solubility.

For Ag₂CrO₄

Solubility = (ksp/4)^1/3 = 0.65 × 10⁻⁴

For AgCl

Solubility = $\sqrt{Ksp}$ = 1.34 × 10⁻⁵

For AgBr

Solubility = $\sqrt{Ksp}$ = 0.71 × 10⁻⁶

For AgI

Solubility = $\sqrt{Ksp}$ = 0.9 × 10⁻⁸

For the above data of solubility it is very clear that solubility of Ag₂CrO₄  is maximum that's why it will precipitate last.