Small amount of freshly precipitated magnesium hydroxides are stirred vigorously in a buffer solutio

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Small amount of freshly precipitated magnesium hydroxides are stirred vigorously in a buffer solution containing 0.25 M of NH4Cl and 0.05 M of NH4OH. [Mg2+] in the resulting solution is (Kb for NH4OH = 2.0 × 10–5 and Ksp of Mg(OH)2 = 8.0 × 10–12)

Accepted Answer

$\left\lbrack OH^{-} ightbrack = K_{b}.\frac{\left\lbrack NH_{4}OH ightbrack}{\left\lbrack NH_{4}^{+} ightbrack} = 2 	imes 10^{- 5} 	imes \frac{0.05}{0.25} = 4 	imes 10^{- 6}	ext{ M}$$	herefore\left\lbrack Mg^{2 +} ightbrack = \frac{K_{sp}}{\left\lbrack OH^{-} ightbrack^{2}} = \frac{8 	imes 10^{- 12}}{\left( 4 	imes 10^{- 6} ight)^{2}} = 0.5	ext{ M}$

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