Approximating X

Moderators: Chem_Mod, Chem_Admin

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

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%?

Amy_Bugwadia_3I
Posts: 37
Joined: Wed Sep 21, 2016 2:56 pm
Been upvoted: 1 time

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.


Return to “Calculating the pH of Salt Solutions”

Who is online

Users browsing this forum: No registered users and 1 guest