Buffer Question

[emitpeels]

Hey guys, I have a question. I'm struggling with buffers and I was wondering if someone could explain the following problem in the simplest way possible, showing each and every step. Thanks.

What is the pH of a solution that contains 0.003M HF and 0.02M NaF (Ka, HF=6.8x10-4)?

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[datthirty]

If you ignore the buffer, you will know it is a easy question.

HF -> H+ + F-
0.003 x 0.02 (x is too small compare with 0.02)

Ka = 6.8*10^(-4)
Ka = x*0.02/0.003

x = 10^(-4)

PH = -log(10^(-4)) = 4

MY EXPERIENCE IS: For buffer question, forget it is a BUFFER, then you will resolve it very quickly.