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uzairhamid · Mar 20, 2009, 02:33 AM

What would be the pH measured for 0.1M H2S solution at 25 degrees Celsius?
Choices are: 1, 2, 4 or 6

When I calculated it, my answer came out to be 0.69.
My method:
H2S-> 2H + S
0.1M .2M .1M

[H]=0.2
pH=-log[H]
=0.69
Can you help me with this question?

Perito · Mar 20, 2009, 03:06 AM

H2S is not a strong acid. Therefore, it's not fully ionized. Since you aren't given the dissociation constant $pK_a$ , you either have to look it up, or figure out a guess. If you were to assume that only one of the hydrogens ionizes, you'd get

[H]=0.1

Right?

Then pH = 1

But, not even one hydrogen is fully ionized in H2S.

Acetic acid has only one proton available. Its pKa is about 4.76. Common solutions of this are in the range of pH=4.1 to 4.8. Is acetic acid a stronger acid than H2S, or a weaker acid?

uzairhamid · Mar 20, 2009, 03:10 AM

Originally Posted by Perito

H2S is not a strong acid. Therefore, it's not fully ionized. Since you aren't given the dissociation constant $pK_a$ , you either have to look it up, or figure out a guess. If you were to assume that only one of the hydrogens ionizes, you'd get

[H]=0.1

Right?

Then pH = 1

But, not even one hydrogen is fully ionized in H2S.

OK, but suppose pKa was given and the molarity, then how would you have solved it?

Perito · Mar 20, 2009, 03:28 AM

From the definition of pKa:

if $HA = H^+ + A^-$

$pKa = \frac{[H^+][A^-]}{[Acid]}$

You know pKa and the concentration of $[H^+]$ and $[A^-]$ are the same. You also know the concentration of the acid (and you can ignore changes in that concentration for very weak acids). Therefore, you can calculate $[H^+]$ and determine the pH.