pKa Calculator: Learn How to Calculate \({pk}_{a}\) from \(pH\) or \({k}_{a}\)

Understanding \({pk}_{a}\): A Comprehensive Guide

\({pk}_{a}\) is a fundamental concept in chemistry, especially in acid-base reactions. It helps predict the strength of acids and bases, their behavior in solution, and the direction of chemical equilibria. Let’s explore what \({pk}_{a}\) means, how to calculate it, and how it relates to other key values like Ka and pH.

What is \({pk}_{a}\)?

\({pk}_{a}\) is the negative base-10 logarithm of the acid dissociation constant \((k_{a})\) of a solution:

\({pk}_{a}\) = -\(\log_{10}{(k_{a})}\)

It represents how easily an acid gives up its proton (H⁺). A lower \({pk}_{a}\) indicates a stronger acid. Strong acids dissociate completely and have very low \({pk}_{a}\) values, while weak acids have higher \({pk}_{a}\) values.

\({pk}_{a}\) Table

Here is a simplified table of common acids and their \({pk}_{a}\) values:

\({pk}_{a}\) and pH — How to Calculate \({pk}_{a}\) from pH?

You can calculate \({pk}_{a}\) using the Henderson-Hasselbalch equation:

\({pk}_{a}=pH-\log_{10}{\frac{[A^{-}]}{[HA]}}\)

• pH is the acidity of the solution
• [A⁻] is the concentration of the conjugate base
• [HA] is the concentration of the weak acid

This formula is commonly used in buffer solutions to find the \({pk}_{a}\) of an acid.

\({pk}_{a}\) and Ka — How to Calculate \({pk}_{a}\) from Ka?

If you know the Ka (acid dissociation constant), you can directly calculate \({pk}_{a}\) using the formula:

\({pk}_{a}=pH-\log_{10}{a}\)

This method is widely used in analytical and physical chemistry to quantify acid strength.

Examples of \({pk}_{a}\) Calculation

Example 1: From pH and concentrations

If pH = 5.0, [A⁻] = 0.1 M, and [HA] = 1.0 M:

\({pk}_{a}\) = 5.0 - \(\log_{10}{\big(\frac{0.1}{1.0}\big)}\)= 5.0 - (-1) = 6.0

Example 2: From Ka

If \(K_{a}\) = 1.8 × 10^-5:

\({pk}_{a}\) = -log10(1.8 × 10^-5) ≈ 4.74

FAQs

• Q: What does a high \({pk}_{a}\) mean?
  A: A high \({pk}_{a}\) means the substance is a weak acid and does not easily donate protons.
• Q: Can \({pk}_{a}\) be negative?
  A: Yes, strong acids like HCl have very low or negative \({pk}_{a}\) values.
• Q: What is the relationship between \({pk}_{a}\) and \({pk}_{b}\)?
  A: For a conjugate acid-base pair: \({pk}_{a}\) + \({pk}_{b}\) = 14.
• Q: Why is \({pk}_{a}\) important in biology?
  A: \({pk}_{a}\) values help determine the charge of biomolecules at physiological pH, crucial for enzyme function and drug design.