Question

Determine the [OH−] of a solution that is 0.150 M in CO32−.

Express your answer using two significant figures.

[OH] =		M

Part B

Determine the pH of a solution that is 0.150 M in CO32−.

Express your answer to two decimal places.

pH =

Part C

Determine the pOH of a solution that is 0.150 M in CO32−.

Express your answer to two decimal places.

pOH =

Answer 1

Part.A :-

ICE table is :

.............................CO₃^2- (aq) .............+................H₂O (l) <-----------------> HCO₃^- (aq) .............+....................OH^- (aq)

Initial (I)................0.150 M...............................................................................0.0 M...........................................0.0 M

Change (C)..........-α..........................................................................................+α................................................+α

Equilibrium (E)..(0.150-α) M..............................................................................α M.............................................α M

Where,

α = Degree of dissociation

Expression of K_b is :

K_b = [HCO₃^-].[OH^-] / [CO₃^2-]

2.1 x 10^-4 = α² / (0.150-α)

α² + 2.1 x 10^-4α - 3.15 x 10^-5 = 0

On solving

α = 0.0055

Hence, [OH^-] = α = 5.5 x 10^-3 M

--------------------

Part.C :-

As, pOH = - log [OH^-]

= - log 5.5 x 10^-3 M

= 2.26

Hence, pOH = 2.26

-------------------------

Part.B :-

As, pH + pOH = 14

So,

pH = 14 - pOH

pH = 14 - 2.26

pH = 11.74