Question

Calculate the Ka of a 0.35M weak acid with a pH of 4.2.

Answer 1

Ka = 1.14x10⁻⁸

Step-by-step explanation:

First we calculate [H⁺] from the pH:

• pH = -log[H⁺]

• [H⁺] =
  `10^(-pH)`

• [H⁺] = 6.31x10⁻⁵ M

For a monoprotic weak acid, the molar concentration of H⁺ of a solution can be expressed as:

• [H⁺] = √(C*Ka)

Where C is the molar concentration of the weak acid solution.

• 6.31x10⁻⁵ M =
  `√(0.35M*Ka)`

• 1.14x10⁻⁸ = Ka