CR Acid-base question

alexpie
Posts: 17
Joined: Sat May 06, 2017 7:27 pm

CR Acid-base question

Postby alexpie » Sat May 13, 2017 10:21 am

When doing calculations for Ka and such, I am confused as to when I am supposed to take into account the hydronium ions in water? I know you are supposed to do it when there are small concentrations of weak acid, but how much is "too small"? Also, if you have very small amounts of strong acid, does this rule apply as well?
NS_Tutor_Andrew
Site Admin
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Joined: Mon May 23, 2016 1:47 pm

Re: CR Acid-base question

Postby NS_Tutor_Andrew » Tue May 16, 2017 3:58 pm

Hi alexpie,

A similar topic came up in an earlier thread that might be helpful -- check that out and let me know if it helps.

The short version is for the MCAT that you need to worry about H+ from the autodissociation of water only when it will make a big enough difference to affect your answer choice. Generally a good rule of thumb is that you need to account for the autodissociation of water if the acid yields an amount of [H+] that is within about 2 orders of magnitude from the 10^-7 M that you'd get from water.

To answer your second question, it doesn't really matter whether you're talking H+ that's coming from a strong acid or a weak acid, although you'll usually see problems like this involving a weak acid. The reason is just that it's not very practical to use strong acids in such tiny amounts as to get H+ concentrations on the order of 10^-6 M -- basically, it's technically challenging to work w/ micromolar and sub-micromolar concentrations, and there's really no point when weak acids would do the job perfectly well. But H+ is H+.

Hope this is helpful!
Andrew D.
Content Manager, Next Step Test Prep.
alexpie
Posts: 17
Joined: Sat May 06, 2017 7:27 pm

Re: CR Acid-base question

Postby alexpie » Tue May 23, 2017 11:15 pm

Thank you!!

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