Hi guys, i have a chemistry question that has to do with making a buffer solution.

the question tells me that i need to make a 200 ml buffer solution with a pH of 4.25. the buffer solution must consist of 0.1M HF and 0.15M NaF.

your help will be greatly appreciated
thanks :)

what is the pKa of HF = 3.2, Ka = 6.3x10^-4
in a buffer system, the NaF will completely dissociate further limiting the dissociation of the HF, a weak acid. to calculate the [H+] or acid concentration, we need to include the F- contributed by NaF

with 0.15M F- from NaF
ICE chart
.....HF.....H+.....F-
.....0.1....0........0.15
....-x......+x......+x
..0.1-x.....x....0.15+x
6.3x10^-4 = [H+][F-] = (x)(0.15 + x) = 0.15x + x^2
x^2 + 0.15x - 6.3x10^-4 = 0
x = [H+] = 0.0041M

pH = pKa + log[salt]/[acid]
4.25 = 3.2 + log[salt/0.0041M]
10^1.05 = ([salt]/0.0041M)
[salt] = 0.046M

0.2L x 0.0041M = 0.00082moles HF required = 0.0164g HF
0.2L x 0.046M = 0.0092moles NaF required = 0.3864g NaF

The answer is obvious...

π

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