posted by maura .
pH of HF
Since HF is a weak acid, I'm not sure which formula to use.
Which formula to use for what?
to find pH
Go through the Ka for a weak acid, solve for H^+, then convert to pH.
There is no Ka value that I see, so I looked it up hopefully on a reliable website. Could you make sure this is correct? 7.2*10^-4
so would this problem be:
7.4*10^-4= (x^2)/ (5.3*10^-2-x)
On a calculator I get this number 0.006262587 then -log to get 2.20
It is 7.2E-4 = Ka in my text (about 15 years old). Yes, that's what you do but I didn't end up with your numbers. I solved the quadratic and obtained 0.00583M for x and the pH = 2.23
If you neglect x (which is what you did) you should obtain 6.18E-3. Your value is a little higher than that because you punched in 7.4 and not 7.2.