Question

Determine the [OH?] of a solution that is 0.115M in CO32-.

The kb value for CO32- is 2.1e-4.

Determine the pH and pOH of a solution that is .115 M in CO32-

Answer 1

1. ICE table of CO₃^2- is :

.........................CO₃^2- ..............+...................H₂O -----------------------> HCO₃^- .............+.................OH^-

Initial (I)............0.115 M............................................................................0.0 M..................................0.0 M

Change (C)........- y .....................................................................................+y ...................................y

Equilibrium (E)...(0.115 - y ) M......................................................................y M.................................y M

Expression of K_b is :

K_b = [HCO₃^-].[OH^-] / [CO₃^2-]

K_b = y² / (0.115 - y)

2.1 x 10^-4 = y² (0.115- y)

0.00002415 - 0.00021 y = y²

y² + 0.00021 y - 0.00002415 = 0

On solving, we have

y = 0.00481

Therefore, Concentration of OH^- = [OH^-] = y = 0.00481 M

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We know ,

pOH = - log[OH^-]

pOH = - log 0.00481

pOH = 2.32

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Also,

pH = 14 - pOH

pH = 14 - 2.32

pH = 11.68

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