What is the original molarity of a solution of a weak acid, given Ka and pH?

Ka=3.5⋅10−5
pH=5.25
@25 degrees Celsius

1 Answer
Dani W.
Mar 8, 2018

6.5⋅10−6 M

Explanation:

Construct an ICE table using the following reaction equation:
H2O + HA ⇌ A− + H3O+

Use the pH to calculate [H3O+] at equilibrium, which is also the change in concentration for the table.

Equilibrium Concentrations:
[HA]=x−5.6⋅10−6 M
[A−]=5.6⋅10−6 M
[H3O+]=5.6⋅10−6 M

Set up an equilibrium expression using Ka:
3.5⋅10−5=(5.6⋅10−6)2x−5.6⋅10−6
x=9.0⋅10−7

[HA]=9.0⋅10−7 M −5.6⋅10−6 M =6.5⋅10−6 M

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