[klausina mausina]

I've been tutoring chemistry and I think something has gone horribly wrong in my brain - I was working with someone today and we came across the question 'calculate the pH of the mixture formed when 25 cm3 of 2 mol dm–3 sodium hydroxide solution is added to 50 cm3 of 2 mol dm–3 ethanoic acid, for which Ka = 1.7 × 10–5 mol dm–3.'

My answer kept coming out at above 13 but the answers given in the question were all between 2 and 4 - am I doing something wrong? My working is:
moles OH-
moles H+ using Ka
Subtract moles H+ from OH-
Divide Kw by OH- moles and then -log the answer.

What am I doing wrong?

[Babcock_Hall]

A good place to start is to write out a chemical equation.  Do you know anything about buffers?

[klausina mausina]

Yeah, but isn't a buffer a weak acid + salt of the weak acid? Here we've got strong base + weak acid which I thought was basically just an uneven neutralisation.

My equation would be

NaOH + CH3COOH -> H2O + NaCH3COO

[chenbeier]

Yes it is, but here you produce the salt if you add the hydroxide to the weak acid, so how much salt and how much acid is left.

[Corribus]

Just going by pure intuition here, without doing any calculations: you are mixing approximately equal volumes (same order of magnitude) of a equally concentrated weak acid and strong base. A pH of 2-4 for the resulting solution seems way off. I would expect alkaline solution as well.

[Borek]

Pure intuition (plus some mental math) tells me pH=pKa here

[Corribus]

Oh wait, my intuition was stupid. Forgot about the whole equilibrium thing. Brain fart, need more coffee, haha

[Babcock_Hall]

Chocolate has a stimulant too, does it not?