A 7.50 x 10^-3 M solution of a weak base has a pH of 11.3.

Determine the K_b for this weak base.

Question

A 7.50 x 10^-3 M solution of a weak base has a pH of 11.3.

Determine the K_b for this weak base.

| General Chemistry 3

Accepted Answer

A^- + H₂O $⇌$ AH + HO^- [K_b]

A t = 0:

[A^-] = 7.50 x 10^-3 M and [AH] = [HO^-] = 0 M

At the equilibrium:

[A^-] = 7.50 x 10^-3 - X M and [AH] = [HO^-] = X M

pH = 14 – pOH = 14 + log [HO^-]

⇒ [HO^-] = X = 10^{pH – 14} = 10^-2.7

⇒ [HO^-] = 2.00 x 10^-3 M

K_b = $\frac{[AH] [{HO}^{-}]}{[A^{-}]}$

K_b= $\frac{{(2.00 \times 10^{- 3})}^{2}}{7.50 \times 10^{- 3} - 2.00 \times 10^{- 3}}$

K_b = 7.23 x 10^-4

Midterm 1 - Question 4 | General Chemistry 3