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Question: What is the pH at which Mg \(( \mathrm { OH } ) _ { 2 }\) begins to precipitate from a solution cont...

What is the pH at which Mg (OH)2( \mathrm { OH } ) _ { 2 } begins to precipitate from a solution containing MMg2+\mathrm { M } \mathrm { Mg } ^ { 2 + } ions?

A

4

B

6

C

9

D

7

Answer

9

Explanation

Solution

: Ksp\mathrm { K } _ { \mathrm { sp } } for

[Mg2+][OH−]2=(0.1)×[OH−]2=1.0×10−11\left[ \mathrm { Mg } ^ { 2 + } \right] \left[ \mathrm { OH } ^ { - } \right] ^ { 2 } = ( 0.1 ) \times \left[ \mathrm { OH } ^ { - } \right] ^ { 2 } = 1.0 \times 10 ^ { - 11 }

[OH−]2=1×10−110.1=10−10\left[ \mathrm { OH } ^ { - } \right] ^ { 2 } = \frac { 1 \times 10 ^ { - 11 } } { 0.1 } = 10 ^ { - 10 }

[OH−]=10−5;[H+]=10−1410−5=10−9\left[ \mathrm { OH } ^ { - } \right] = 10 ^ { - 5 } ; \left[ \mathrm { H } ^ { + } \right] = \frac { 10 ^ { - 14 } } { 10 ^ { - 5 } } = 10 ^ { - 9 }

pH = 9