Posted by **maura** on Thursday, March 29, 2012 at 12:12am.

I keep getting wrong answers for this question.

Determine the pH of an HF solution of each of the following concentrations.

a) .280 M

b) 5.3*10^-2 M

c) 2.50*10^-2 M

No Ka has been given. I haven't attempted b or c yet because I want to at least solve a first. So far I got a answer of 3.75 and 1.88 and both are incorrect. I think I know how to solve for this. Using Ka and approximations. I just don't understand where I am going wrong. Thanks in advance!

- chemistry -
**DrBob222**, Thursday, March 29, 2012 at 12:17am
These can't be solved with a Ka value. Why don't you post your work for a and let me look at it. I'm sure I'll see immediately what is going on.

- chemistry -
**maura**, Thursday, March 29, 2012 at 12:24am
Oh okay I thought you used Ka to solve these. That's what I'm doing wrong. I tried looking up the Ka for HF and multiplying it to .280 then take the -log of that number. The reason why I had to different numbers is because I found two different Ka's. If I can't use Ka do I just take the -log of .280?

- chemistry -
**DrBob222**, Thursday, March 29, 2012 at 7:46pm
First I made a typo with my response. You certainly DO need Ka and that's the only way you can do it. I just didn't type in with instead of without.

My text hqas 7.2E-4 for HF.

...........HF ==> H^+ + F^-

initial.0.280.....0.....0

change....-x......x.....x

equil....0.280-x...x....x

Ka = 7.2E-4 = (x)(x)/(0.280-x)

Solve for x and convert to pH.

Probably you can get by without a quadratic equation for the 0.280but it may take one for the others, especially the very weak one.

Using this value I get 1.85 for the pH of the 0.280 M soln. Your text may give a different value for Ka.

## Answer this Question

## Related Questions

- Algebra - - ok... we keep getting this one wrong: Solve the following equation ...
- Chemistry - Glutamic acid is triprotic amino acid with pKa values of 2.23, 4.42...
- Chemistry - Calculate the molarity of a solution prepared by dissolving 16 g of ...
- Chemistry - Help me set up this problem. I have been looking at a few examples ...
- Chemistry - Calculate the concentrations of all species in a 0.810 M Na2SO3 (...
- chemistry - I have this question: A 71.7-mL solution of 0.182 M NaOH is titrated...
- Chemistry - A 10.0-mL solution of 0.300 M NH3 is titrated with a 0.100 M HCl ...
- Physical Chemistry - Determine the temperature change when 18.0 g of the ...
- Math - Consider the following test scores obtained by a class. Test score 90 80 ...
- ALGBRA - Consider the following test scores obtained by a class. Test score 90 ...

More Related Questions