erm, since molarity of acid need to be equal to the molarity of
conjugate base, and the number of moles of acid=2.5x10-3, the
number of moles of conjugate base should be (2 X 2.5x10-3?(since
its diprotic) H+=20H-
does the 'molarity' refers to concentration?
Molarity = molar concentration. It's better to use the more
unambiguous term "molarity" since concentration can be mol/dm3,
g/dm3, etc.
At initial, we have 2.5 x 10^-3 moles of the weak acid.
We must add 1.25 x 10^-3 moles of hydroxide ions, so that
upon acid-base neutralization, we end up with only 1.25 x 10^-3
moles of the weak acid and 1.25 x 10^-3 moles of its
conjugate base. This is point of maximum buffer capacity.
An ICF table (Initial, Change, Final) would show this clearly,
and its recommended that students use ICF tables together with ICE
tables (Initial, Change, Equilibrium).
ICF = human action; ICE = universe action.
ICF = single arow; ICE = double half-arrow.
ICF = moles, moles, moles; ICE = molarity, molarity,
molarity.
ICF = usually no algebra used; ICE = algebra usually used.
Some JCs teach only ICF, some teach only ICE, some do not teach
either, some teach both. Cambridge doesn't care whether you use
either, neither or both, as long as you get the correct answer.
These tables are simply optional tools to help you.
If you're interested to use them (eg. if you're struggling with
this topic), you can search them out on the internet, or look up
undergrad textbooks like Raymond Chang's "Chemistry", or come for
my tuition, etc.