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Topic: MCAT Acid/Base Question  (Read 3483 times)

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Offline mrlucky0

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MCAT Acid/Base Question
« on: June 10, 2009, 02:56:20 AM »
Hydrochloric acid is a strong acid. A solution of sodium chloride should have a pH of:

A. less than seven
B. seven
C. more than seven but less than fourteen
D. more than fourteen

The answer is B, but I choose C because:

The conjugate base of a strong acid is a weak base. It follows then, that a solution of chloride ions should be slightly basic: Cl- + H2O = HCl + OH-. Although the equilibrium lies far left, the [OH-] in this situation is still slightly greater than that off 10^-7 M.

Why am I wrong here?
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Offline Borek

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Re: MCAT Acid/Base Question
« Reply #1 on: June 10, 2009, 03:32:59 AM »
Quote from: mrlucky0 on June 10, 2009, 02:56:20 AM
The conjugate base of a strong acid is a weak base.

VERY weak base. Too weak for observable effects.
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Offline mrlucky0

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Re: MCAT Acid/Base Question
« Reply #2 on: June 10, 2009, 03:40:03 AM »
Quote from: Borek on June 10, 2009, 03:32:59 AM
Quote from: mrlucky0 on June 10, 2009, 02:56:20 AM
The conjugate base of a strong acid is a weak base.

VERY weak base. Too weak for observable effects.

 Good to know that my intuition of G-chem wasn't flawed. I guess that makes sense now since H2O is a very weak acid relative to Cl-. Still, I hope I don't get a question like this on my actual MCAT.   Thanks.
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