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Question
At 25oC, will a precipitate of Mg(OH)2 form, in a 10−4M solution of Mg(NO3)2 if pH of the solution is adjusted to 9.0?Ksp[Mg(OH)2])=10−11M3 . At what minimum value of pH will precipitation start?
At 25oC, will a precipitate of Mg(OH)2 form, in a 10−4M solution of Mg(NO3)2 if pH of the solution is adjusted to 9.0?Ksp[Mg(OH)2])=10−11M3 . At what minimum value of pH will precipitation start?
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Solution
The correct option is B No, 10.5
Given, [Mg(NO3)2]=[Mg2+]=10−4 M
For, pH =9, [H+]=10−9M and [OH]−1=10−5M
Mg(OH)2⇌Mg2++2OH−
The ionic product, Qsp=[Mg2+][OH−]2=10−4×(10−5)2=10−14
The ionic product is much less than Ksp(10−11M)
So, no precipitation will occur.
Precipitation will start forming when Qsp=Ksp
Or, when [Mg2+][OH−]2=Ksp
[10−4][OH−]2=10−11
[OH−]=√10−7
[H+]=10−14[OH−]
pH=−log[H+]=−log(10−14)+log(10−72)
=14−3.5=10.5
Hence, (b) is correct.
Given, [Mg(NO3)2]=[Mg2+]=10−4 M
For, pH =9, [H+]=10−9M and [OH]−1=10−5M
Mg(OH)2⇌Mg2++2OH−
The ionic product, Qsp=[Mg2+][OH−]2=10−4×(10−5)2=10−14
The ionic product is much less than Ksp(10−11M)
So, no precipitation will occur.
Precipitation will start forming when Qsp=Ksp
Or, when [Mg2+][OH−]2=Ksp
[10−4][OH−]2=10−11
[OH−]=√10−7
[H+]=10−14[OH−]
pH=−log[H+]=−log(10−14)+log(10−72)
=14−3.5=10.5
Hence, (b) is correct.
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