Asked by maura

pH of HF
5.3*10^-2 M

Since HF is a weak acid, I'm not sure which formula to use.
Answered by DrBob222
Which formula to use for what?
Answered by maura
to find pH
Answered by DrBob222
Go through the Ka for a weak acid, solve for H^+, then convert to pH.
Answered by maura
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
Answered by DrBob222
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.
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