Approximating X

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Approximating X

Postby Celine_Ngo_3L » Sun Nov 27, 2016 12:35 pm

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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Re: Approximating X

Postby Amy_Bugwadia_3I » Sun Nov 27, 2016 12:53 pm

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.

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