5% approximation for weak acids/bases

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gwynlu1L
Posts: 62
Joined: Fri Sep 28, 2018 12:19 am

5% approximation for weak acids/bases

Postby gwynlu1L » Wed Jan 16, 2019 1:36 pm

hello,
I know today in lecture Lavelle talked about how in weak acid/base equilibrium questions we ca make the approximation that x is not significant enough so we can simplify the quadratic and make solving for x easier, as long as x is less than 5% of the initial. can we assume this pretty much for all weak acid/base equilibrium questions, and if not is there a way to check this approximation is valid before going through the calculations instead of checking the percent ionization after using the approximation?

Tarika Gujral 1K
Posts: 62
Joined: Fri Sep 28, 2018 12:27 am

Re: 5% approximation for weak acids/bases

Postby Tarika Gujral 1K » Wed Jan 16, 2019 1:49 pm

The 5% approximation can be used for values of K that are very large or very small.

If K is large — P+x = P
If K is small — R-x = R

But the verification must be computed after calculating equilibrium values.

Samantha Ito 2E
Posts: 64
Joined: Fri Sep 28, 2018 12:29 am

Re: 5% approximation for weak acids/bases

Postby Samantha Ito 2E » Wed Jan 16, 2019 2:07 pm

Lavelle said that if the K value is less than 10^-3, then typically we can use the approximation.


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