If a buffer solution is 0.110 M in a weak base ( 𝐾b=8.4×10−5) and 0.490 M in its conjugate acid, what is the pH?

Answer 1

To solve this problem, use the Henderson-Hasselbalch equation.

Henderson-Hasselbalch equation is used to calculate the pH of a buffer.

Henderson-Hasselbalch equation is pH = pka + log[conjugate base/conjugate acid]

pka = -log(Ka)

To solve for Ka use Ka x Kb = Kw (1.0 x10^-14 at 25C). Since there is no temperature given we assume that temperature is at 25C.

Ka = 1.0 x10^-14

Kb

Ka = 1.0 x 10^-14

8.4 x10^-5

Ka = 1.19 x 10^-10

pka = -log( 1.19 x 10^-10)

pka = 9.92

pH = 9.92 + log [0.110M / 0.490M]

pH = 9.27