Question

You have prepared 20.00 mL of a buffer, using 4.00 mL 0.010 M HA (weak acid) and 3.00 mL 0.010 M NaA (provides A-, the conjugate base of HA). pKa for HA is 6.25. How many moles of HA will be present in the mixture after addition of 0.20 mL 0.010 M NaOH?

Answer 1

The buffer starts with 4 mL x 0.01 = 0.04 mllimols HA and
3 mL x 0.01 = 0.03 millimoles A^-

........HA + OH^= --> A^- + H2O
I......0.04..0.......0.03.....
add........0.002--------------
C....-0.002.-0.002.+0.002.............
E...0.038....0.....0.032..........
So you have 0.038 millimols of the acid. Change that to mols.