Question

Part A

Determine the [OH−] of a solution that is 0.110 M in CO32−.

Express your answer using two significant figures.

Part B

Determine the pH of a solution that is 0.110 M in CO32−.

Express your answer to two decimal places.

Part C

Determine the pOH of a solution that is 0.110 M in CO32−.

Express your answer to two decimal places.

Please work out all algebra when using the quadratic equation, thanks!

Answer 1

Part 1)

CO₃^2-(aq) + H₂O(l)      HCO₃^-(aq) + OH^-(aq)      K_b = 1.8 x 10^-4

HCO₃^-(aq) + H₂O(l)       H₂CO₃(aq) + OH^-(aq)     K_b = 2.5 x 10^-8

Since the second kb is much smaller than the first, the hydroxide ions generated by the second equillibrium may be neglected.

In the first dissociation, we can see that the concentrations of both HCO₃^-(aq) and OH^-(aq) ions are the same

Therefore,

Kb = [HCO₃^-][OH^-]/[CO₃^2-]

1.8 x 10^-4 = [x][x]/[0.110]

(1.8 x 10^-4)[0.110] = x²

0.198 x 10^-4 = x²

x = 0.0044

[OH^-] = 0.0044 M

Part 2)

pOH = - log [OH^-]

pOH = - log [0.0044] = 2.35

pOH = 2.35

Part 3)

We have,

pH + pOH = 14

Therefore,

pH = 14 - pOH

pH = 14.00 - 2.35 = 11.65

pH = 11.65