Question
The solubility products of Mg(OH)2, Cd(OH)2, Al(OH)3 and Zn(OH)2 are 4 × 10–11, 8 × 10–6, 8.5 × 10–23 and 1.8 × 10–14, respectively. The cation that will precipitate first as hydroxide, on adding limited quantity of NH4OH in a solution containing equimolar amount of metal cations, is
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Solution
[OH– ]min to start ppt. of Mg(OH)2 = $\sqrt{\frac{K_{sp}}{\left\lbrack Mg^{2 +} \right\rbrack}} = \sqrt{\frac{4 \times 10^{- 11}}{x}}$
For Cd(OH)2, [OH– ]min = $\sqrt{\frac{8 \times 10^{- 6}}{x}}$
For Al(OH)3, [OH– ]min = $\left( \frac{8.5 \times 10^{- 23}}{x} \right)^{1/3}$
For Zn(OH)2, [OH– ]min = $\sqrt{\left( \frac{1.8 \times 10^{- 14}}{x} \right)}$
Considering significant concentration of metal ion (x is not extremely small), [OH–] needed is minimum for Al3+ and hence, it will precipitate first.
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