pKa to pH Calculator

We created this pKa to pH calculator to help compute the pH of a solution based on its pKa (acid dissociation constant) value and concentration.

Let’s consider acetic acid (CH3COOH), a weak acid with a pKa of 4.76. If we have a 0.1 M solution of acetic acid, a pKa to pH converter help us determine the pH of this solution. Using the Henderson-Hasselbalch equation, we can calculate that the pH of this solution would be approximately 2.87.

pKa to pH Conversion Chart

pKa to pH Formula

The formula used to convert pKa to pH is known as the Henderson-Hasselbalch equation:

pH = pKa + log([A-] / [HA])

Where:

• pH is the negative logarithm of the hydrogen ion concentration
• pKa is the negative logarithm of the acid dissociation constant
• [A-] is the concentration of the conjugate base
• [HA] is the concentration of the undissociated acid

For a weak acid HA that’s 50% dissociated, [A-] = [HA], and the equation simplifies to:

pH = pKa + log(1) = pKa

This means that when a weak acid is 50% dissociated, its pH equals its pKa.

How do you calculate pH from pKa?

To calculate pH from pKa, you need to know the concentration of the acid:

Suppose we have a 0.05 M solution of a weak acid with a pKa of 4.7. We want to find its pH.

First, we need to determine the extent of dissociation. For weak acids, we can often assume that the amount of dissociation is small compared to the initial concentration.

Let x be the concentration of H+ ions produced by dissociation. Then: [H+] = [A-] = x
[HA] = 0.05 – x

Substitute these into the Henderson-Hasselbalch equation: pH = 4.7 + log(x / (0.05 – x))

Since x is small, we can approximate 0.05 – x as simply 0.05: pH ≈ 4.7 + log(x / 0.05)

We know that for acids, pH = -log[H+], so: -log(x) ≈ 4.7 + log(x / 0.05)

Solving this equation (which involves some algebra), we get: x ≈ 1.12 × 10^-5 M

Therefore, the pH = -log(1.12 × 10^-5) ≈ 4.95

This example demonstrates how to use the pKa value to determine the pH of a weak acid solution.