A solution is prepared by mixing 50.0 mL of 0.50 M Cu(NO₃)₂ with 50.0 mL of 0.50 M Co(NO₃)₂. Sodium hydroxide is slowly added to the mixture. Which precipitates first? [K_sp(Cu(OH)₂) = 2.2 × 10^-20, K_sp(Co(OH)₂) = 1.3 × 10^-15]

Question

Quizplus · Accepted Answer

The precipitation reaction occurs when the product of the ion concentration ([Cu2+] [OH-]2 or [Co2+] [OH-]2) exceeds the solubility product constant (Ksp) for each respective compound.

Calculating the initial ion concentration of Cu2+ and Co2+ in the mixture:
Cu2+: (0.50 mol/L) (0.0500 L) = 0.025 mol
Co2+: (0.50 mol/L) (0.0500 L) = 0.025 mol

As sodium hydroxide is slowly added to the mixture, OH- ions are gradually increasing in concentration. Cu(OH)2 has a smaller Ksp than Co(OH)2 (due to its larger Ksp value) and therefore will reach the saturation point and precipitate out first.

A Solution Is Prepared by Mixing 50