## Approximating X

Celine_Ngo_3L
Posts: 19
Joined: Sat Jul 09, 2016 3:00 am

### Approximating X

In the course reader when it gives an example of calculating pH (p. 156), it says, "if X is less than 5% of the initial then approximation is okay" and we can approximate since Ka is small (in this case, Ka = 1.8 x 10^-5).

What is considered a "small" Ka value? And when do we calculate if x is less than 5%?

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Joined: Wed Sep 21, 2016 2:56 pm
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### Re: Approximating X

A "small" Ka value is when Ka = 10^-5 or smaller. However, as good practice, it's advised to calculate the percent error, and if it's less than 5% then using the approximation is okay. The approximation is essentially when we disregard the " - x" in the denominator, so we are left with Ka = X^2 divided by the initial molar concentration (instead of the initial minus X), thus making the algebra easier.