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Question

Freshly precipitated aluminium and magnesium hydroxides are stirred vigorously in a buffer solution containing 0.05 mol L1 of NH4OH and 0.25 mol L1 of NH4Cl. Calculate the concentration of magnesium ions in solution.
Ksp Al(OH)3=6×1032
Kb NH4OH=1.8×105
Ksp Mg(OH)2=6×1010

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Solution

pOH=log10[Salt][Base]log Kb
log10[OH]=log100.25log101.8×1050.05
log10[OH]=log101.8×1055
[OH]=0.36×105 mol L1
[Al3+]=Ksp(Al(OH)3)[OH]3
=6×1032(0.36×105)3
=1.29×1010 mol L1
[Mg2+]=Ksp (Mg(OH)2)[OH]2
=6×1010(0.36×105)2=46.3 mol L1

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