chemistry
posted by maura .
pH of HF
5.3*10^2 M
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*10^2x)
On a calculator I get this number 0.006262587 then log to get 2.20 
It is 7.2E4 = 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 6.18E3. Your value is a little higher than that because you punched in 7.4 and not 7.2.