Calculate the pH of a buffer solution #1?

Question

A buffer solution is made with:

40.0 mLs of 0.100 M acetic acid

20.0 mLs of 0.100 M NaOH

20.0 mLs of Deionized water

Use the Henderson-Hasselbalch equation to calculate the pH of the buffer solution

** Im having trouble with this kind of problem so the more detail provided the better, but I'm happy with all help.

Answer 1

Ka = 1.8 e-5 . pKa = 4.74

0.0400Litres @ 0.100 mol/litre acetic = 0.004 moles acetic acid

0.0200 litres @ 0.100mol/litre NaOH = 0.002 moles NaOH

these react & after words you have 0.002 moles of acid left over in 0.080 litres = 0.025 molar acid

but you have produced 0.002 moles of sodium acetate in 0.080 litres = 0.025 molar acetate

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Henderson-Hasselbalch

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

pH = pKa + log ( [0.025] / [0.025])

pH = pKa + log 1

pH = pKa + 0

pH = pKa

your answer: pH = 4.74