[ilikecats]

I was wondering if someone could help me.


I'm really confused as to why when I calculate pH, sometimes you subtract the amount of the the base with the acid in the denominator and sometimes you dont.

For example, here are 2 very similar problems

Calculate pH of a solution with 0.01 M sodium hydroxide when added to 0.03 M acetic acid.
pka = 4.76

pH = pka + log([base]/[acid])
pka = 4.76
[base] = 0.01 M
[acid] = 0.03 M - 0.01 M = 0.02 M

pH = 4.76 + log([0.01 M]/[0.02 M])
pH = 4.46




Calculate the pH of a solution containing 0.1 M sodium acetate and 0.2 M acetic acid
pka = 4.7
[base] = 0.1 M
[acid] = 0.2 M

pH = pka + log([base]/[acid])
pH = 4.7 + log([0.1 M]/[0.2 M])
pH = 4.7 - 0.3
pH = 4.4




How come in the first problem for the denominator, i had to subtract 0.03 M of acetic acid with 0.01 M of sodium hydroxide, but for the second problem I didn't have to subtract 0.2 M of acetic acid with 0.1 M of sodium acetate?

[Borek]

What is the reaction taking place in the first problem? Does it take place in the second problem?

More general question, which probably points to the basic problem with your understanding of this type of questions: what is the base, and what is the acid when calculating pH of the buffer?