Determining Ka Values for Weak Acids Based on pH Measurements

How can we determine the Ka values for weak acids based on pH measurements?

Given the pH values of different aqueous solutions of weak acids, how can we calculate the Ka values for these acids?

Answer:

In order to determine the Ka values for weak acids based on pH measurements, we can follow a specific procedure. Let's take a look at the examples provided to understand this process in depth:

A. Determination of Ka for Acetic Acid:

To determine the Ka for acetic acid, we first need to understand the dissociation process of the acid in water. Acetic acid (CH₃COOH) partially dissociates into hydrogen ions (H⁺) and acetate ions (CH₃COO⁻) when dissolved in water.

The equilibrium reaction can be represented as: CH₃COOH ⇌ H⁺ + CH₃COO⁻

The Ka expression for this reaction is: Ka = [H⁺][CH₃COO⁻] / [CH₃COOH]

From the pH value of 2.618 obtained for the acetic acid solution, we can determine the concentration of hydrogen ions (H⁺). The pH is calculated as the negative logarithm of the hydrogen ion concentration.

Substituting the pH value into the formula, we get the concentration of hydrogen ions: [H⁺] = 10^(-2.618).

Given that acetic acid is a monoprotic acid, the concentration of hydrogen ions is equal to the concentration of acetic acid in the solution, which is 0.350 M. By substituting these values into the Ka expression, we arrive at the Ka value for acetic acid, approximately 1.75 x 10^(-5).