Exemplar Problems
Problem 7 : A weak base (BOH) with a dissociation constant Kb of $$(2.0 \times 10^{-5}) M$$ ionizes partially in water to give OH- and B+ ions. Calculate the pH of a 0.02 M solution of (BOH).
Solution :
Given Kw at 25°C, we can use it to find the concentration of H+ ions [H+] in the solution.
$$(K_w = [H^+][OH^-] = 1.0 \times 10^{-14})$$
We are told that the solution is neutral, so [H+] = [OH-]. Therefore,
$$([H^+][H^+] = 1.0 \times 10^{-14})$$
$$([H^+]^2 = 1.0 \times 10^{-14})$$
Take the square root of both sides:
$$[H^+] = \sqrt{1.0 \times 10^{-14}} = 1.0 \times 10^{-7} \text{ M}]$$
Now, we can calculate the pH:
$$[pH = -\log[H^+] = -\log(1.0 \times 10^{-7}) = 7]$$
So, the pH of the solution is 7.
BETA
