- #1
Qube
Gold Member
- 468
- 1
Homework Statement
Find the pH of [itex]1.0 x 10^{-7} M [/itex] [itex]H_{2}S[/itex] solution.
Homework Equations
We can take three approaches to this problem, one which is mathematically and chemically correct, one mathematically correct but not necessarily chemically correct, and one which is unequivolcally wrong. All involve the [itex]K_{a}[/itex] of [itex]H_{2}S[/itex]:
[itex]K_{a}= 1.1x10^{-7} = [H_{3}O^{+}][[HS^{-}]/[H_{2}S][/itex]
Approach 1:
Use charge balance and mass balance. Start from the below relationships and substitute into the Ka expression.
[itex][H_{3}O^{+}] = [HO^{-}] + [HS^{-}][/itex]
[itex][H_{2}S] + [HS^{-}] = 1.1x10^{-7} M[/itex]
Approach 2:
Realize there are two significant sources of hydronium ion - one source being water since the solution is so dilute - the acid ionization constant approaches the initial molarity of the acid - and the actual acid itself.
Use both the acid ionization constant and the auto ionization of water (Kw) to find pH. Use the acid ionization constant below with x = not to hydronium ion molarity but instead hydronium ion contribution from the acid.
Approach 3 (wrong but what a typical chemistry student would do):
Follow through on the below formula, with [itex]x = [H_{3}O^{+}][/itex].
[itex]K_{a} = x^{2}/M_{i}-x [/itex]
The Attempt at a Solution
Approach 2:
Realize there are two significant sources of hydronium ion - one source being water since the solution is so dilute - the acid ionization constant approaches the initial molarity of the acid - and the actual acid itself.
Use both the acid ionization constant and the auto ionization of water (Kw) to find pH. Use the acid ionization constant below with x = not to hydronium ion molarity but instead hydronium ion contribution from the acid.
Now, I just showed my chem teacher this method, and although it yields the right answer, he told me it was unequivocally wrong.
I see what he's saying. We should only plug in the equilibrium molarities into our Ka expression. We can think what we want, but ignore the chemistry, and we run the peril of being wrong. We can delineate between hydronium ion contribution between the acid and water all we want, but the one thing we should be plugging into the Ka expression is neither of the aforementioned but rather the sum or equilibrium concentration of the aforementioned.
What do you think?
Last edited: