Question

Calculate the `pH` of the following mixtures given `K_(a) = 1.8 xx 10^(-5)` and `K_(b) = 1.8 xx 10^(-5) (pK_(a) = pK_(b) = 4.7447)`.
a. `50mL 0.05 M NaOH +50mL` of `0.1 M CH_(3)COOH`
b. `50mL 0.1 M NH_(4)OH +50 mL` of `0.05 MHCI`

Answer 1

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`