Question

A solution contains 0.05 (M) each of NaCl and Na₂CrO₄. Solid AgNO₃ is gradually added to it. K_sp (AgCl) = 1.7 × 10^–10 and K_sp (Ag₂CrO₄) =1.9 × 10^–12

Which of the following would occur ?

• A. Cl^– ions are precipitated first
• B. CrO₄^2– ions are precipitated first
• C. Both Cl^– and CrO₄^2– ions are precipitated together
• D. The second ion starts precipitating when half of the first ion is precipitated

Answer 1

Answer: (A)

Cl^– ions are precipitated first

Answer 2

To precipitate Cl^–, required [Ag⁺] = $\frac{(K_{sp})_{AgCl}}{0.05}$

To precipitate CrO₄^2– , required [Ag⁺] = $\left\{ \frac{(K_{sp})_{Ag_{2}CrO_{4}}}{0.05} \right\}^{1/2}$

Since for the precipitation of Ag_­2CrO₄ required concentration of Ag⁺ is greater, hence Cl^– ions precipitate first in the form of AgCl (s).