Chemistry Question

Question

What is the concentration of an HF solution if its pH is 1.7?

 a. 0.57 b. 0.027 C. 0.020 d. 7.2 x 10^-4 e. 5.0 x 10^-13

Answer 1

Hi Gabriella,

When I looked up the Ka for the weak acid HF, I found it is 6.3 x 10^-4. The equation for its dissolution is:

HF_(aq) + H₂O_(l) <-----> H₃O⁺_(aq) + F^-_(aq).

Now we can write an equlibrium constant expression:

Ka = [H₃O⁺] _* [F^-] / [HF]

Since the pH is 1.7, we know the concentration of H₃O⁺ 1 / 10^1.7, which is 0.02 M. This is also the concentration of F^-, since they are formed at the same rate in the same process (and the initial concentration of H₃O^{+ i}is negligible). So now let's substitute the known values into the equlibrium expression:

6.3 x 10^-4 = [0.02] _* [0.02] / [HF]

Rearranging we get :

[HF] = [0.02] _* [0.02] / 6.3 x 10^-4

so [HF] = 0.63 M. You really should only have one significant figure in this answer (since the pH value given only has one significant figure) so let's report the answer as 0.6 M.

I'd love to help you with more problems like this in a Wyzant session, Gabriella. Just let me know when I might be able to help you out.