What is the pH of a 0.026 M Sr(OH)2 solution?

1 Answer
Truong-Son N.
May 27, 2018

Well, what's the pOH? I get pOH=1.28. What then is the pH at 25∘C?

We assume Sr(OH)2 is a strong base, such that

Sr(OH)2(s)H2O(l) −−−−−→Sr2+(aq)+2OH−(aq)

and thus, it supposedly gives rise to 0.026×2=0.052 M OH−. As a result,

pOH=−log[OH−]=−log(0.052)=1.28

But clearly, we have the pOH and not the pH. At any temperature,

pH+pOH=pKw,

and at 25∘C, pKw=14. Therefore:

pH=14−1.28=12.72

But as chemists, we must check the data... The Ksp of Sr(OH)2 is around 6.4×10−3. The ICE table gives:

Sr(OH)2(s)⇌Sr2+(aq)+2OH−(aq)

I − 0 0
C − +s +2s
E − s 2s

This is equal to the mass action expression:

Ksp=6.4×10−3=[Sr2+][OH−]2

=s(2s)2=4s3

=12(2s)3=12[OH−]3

And so, the maximum concentration of OH− at 25∘C is:

[OH−]=(2Ksp)1/3

=(2⋅6.4×10−3)1/3

= 0.234 M

And since 0.052 M < 0.234 M, the concentration given in the question is valid and physically realizable.

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