Hi Ultima,
https://www.dropbox.com/s/2i79w0ow9hq7bsp/1111.PNG?dl=0
for part ii) , why is this thought process wrong : i want find
[H+] . i use ka formula. tried, but didnt get the correct
answer.
(p.s. , i've internalised the 2 methods/approach that was taught
to me , but want to know why above concept wrong. )
is it because, inside the solution, its not just the weak acid
inside, there's the cj base too? Help my concept please
T-T
thanks ultima
Being a buffer soln, you *can* indeed use Ka, or even Kb. It's just
that the Henderson-Hasselbalch equation saves you time as you can
work in moles instead of molarities. If you didn't get the correct
answer, it means your molarities (ie. moles / new total volume of
soln) of both HA and A- at that point (double-equivalence) are
wrong. Obviously you need to plug in the molarities of both
conjugate acid and conjugate base into the Ka (or Kb) equation, and
not all questions will be as kind as this question to let you have
maximum buffer capacity, so you better start practicing tougher
questions in which the moles or molarities of both conjugate acid
and conjugate base are different. Then try using all 3 formulae :
Ka, Kb, and Henderson-Hasselbalch equation, to prove *to yourself*
that all 3 formulae will give you the same final answer of the
correct pH.