Problem: The pH of a 0.021 M solution of a weak acid is 3.95. What is the value of Ka for this acid.a. 5.3x10-3b. 5.9x107c. 6.0x10-7d. 1.7x107e. 6.0x10-8

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We are being asked to calculate the Ka of a weak acid if 0.021 M solution of the weak acid has a pH of 3.95. 

We're going to represent the weak acid as HA. Since we’re dealing with a weak acid and Ka is an equilibrium expression, we will have to create an ICE chart to determine the equilibrium concentration of each species:

• HA → weak acid → proton donor
• H2O → will act as the weak base → proton acceptor


Equilibrium reaction:        HA(aq) + H2O(l) ⇌ A-(aq) + H3O+(aq)

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Problem Details

The pH of a 0.021 M solution of a weak acid is 3.95. What is the value of Ka for this acid.

a. 5.3x10-3
b. 5.9x107
c. 6.0x10-7
d. 1.7x107
e. 6.0x10-8

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