a. `[NaOH] = 2.5 "mmol", [Hac] = 5m` moles,
[HAc] left = 2.5 mmol, [salt] formed = 2.5 mmol
`pH = pK_(a) +log ("salt/acid") = 4.7447 +log (2.5//2.5)`
`= 4.7447`
b. `[NH_(4)OH] = 5 "mmol", [Hci] = 2.5 "mmol"`
`[NH_(4)OH]` left = 2.5 mmol
`[NH_(4)CI]` formed = 2.5 mmol
`pOH = pK_(b) +log ("salt/base") = 4.7447 +log (2.5//2.5)`
`= 4.7447`
`pH = 14 - 4.7447 = 9.2553`