Gen Chem Buffer pH Problem?

[MAK1186]

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I was hoping someone could help me with this problem. Thanks in advance!

A liter of solution containing 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 forms a buffer solution of pH = 4.74. Calculate the pH of this solution after 0.020 mol NaOH is added (ignore volume changes.)

 

[Tina324]

crap this is hard...someone?

 

[joonkimdds]

pH of buffer = pKa + log [base/acid]
since base and acid ratio is 1:1, we get log of 1 and that's 0
so pH = pKa = 4.74
using this information, we add 0.02M of base.
at this point, without doing any calculation, we use our brain and it should be
around 4.74 + about 0.5 and the answer is about 5.

What's the answer?