Acid/Base buffer problem?

Question

How do i go about solving this Henderson Hasselback equation?

You are preparing an acetic acid/sodium acetate buffer with a pH of 4.20. What is the ratio of acid to conjugate base? Ka acetic acid = 1.8 x 10-5

Answer 1

Ok, the H H equation is like so...

pH = pKa + log ( [base]/[acid])

pKa = log(1/Ka) = log (1/1.8x10-5)

so...

4.20 = log (1/1.8x10-5) + log ( [base]/[acid])


4.20 - log (1/1.8x10-5) = log ( [base]/[acid])

4.20 - 4.07 = .13 = log ( [base]/[acid])

so

.13 = log ( [base]/[acid])

remember the inverse of log is the exponent of the base, in this case, 10

so

10 ^.13 = 10^log ( [base]/[acid]) = [base]/[acid] = 1.34 ratio

hows that?