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Stuck on this problem

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tkle28:
Hello, I ran across this problem and was wondering if my answer is correct.
The question is: What is the pH of a 0.065 M solution of HF? Ka(HF) = 6.8 x 10^-4. For this one I got 2.26 and apparently the answer was 2.18. Can someone tell me what I did wrong?

Borek:
Tell us how you got 2.26.

(Actually 2.18 doesn't look exactly correct either).

tkle28:

--- Quote from: Borek on May 11, 2023, 06:35:06 AM ---Tell us how you got 2.26.

(Actually 2.18 doesn't look exactly correct either).

--- End quote ---

To be quite Frank with you, my calculations itself were all over the place. I somehow ended with 2.77 but since 2.26 was the closest answer I picked that. I found H+ by doing root symbol(0.00068) x (0.065) then got my answer of 0.00169, then I just put in -log(0.00169) in got an answer of 2.77. I’m trying to figure out what exactly I’m doing wrong.

Borek:
Putting numbers into random formulas is never going to be a valid strategy :/

$[H^+] = \sqrt{K_a\times C_a}$ is a formula that is valid only under a specific assumption about the acid dissociation. Do you recall what this assumption is?

tkle28:

--- Quote from: Borek on May 11, 2023, 07:02:10 AM ---Putting numbers into random formulas is never going to be a valid strategy :/

$[H^+] = \sqrt{K_a\times C_a}$ is a formula that is valid only under a specific assumption about the acid dissociation. Do you recall what this assumption is?

--- End quote ---

Is it whenever it’s weak?