Ka Calculator

Finding the Unknown Ka

In a standard chemistry laboratory experiment, scientists are often asked to determine the unknown Acid Dissociation Constant (Ka) of a weak acid solution.

Because we cannot physically "see" the Ka value, we must calculate it by measuring the physical properties of the solution at equilibrium.

The Experimental Approach

To find the true Ka, you need two pieces of information:

1. The initial concentration of the acid you mixed into the water (M).
2. The pH of the resulting solution (measured using a pH meter).

From the pH, we can instantly calculate the equilibrium concentration of hydrogen ions ([H⁺]). Using an ICE table, we know that for a monoprotic weak acid, every hydrogen ion produced also produces one conjugate base ion, so $[H⁺] = [A⁻]$.

The Exact Formula

By substituting these values into the equilibrium expression $K_a = \frac{[H⁺][A⁻]}{[HA]}$, we get the exact formula:

Example Calculation

You dissolve an unknown weak acid to a concentration of 0.1 M. You measure the pH and find it is exactly 2.87.

1. Find [H⁺]: $10^{-2.87} = 1.35 \times 10^{-3}$ M
2. Square the [H⁺]: $(1.35 \times 10^{-3})^2 = 1.82 \times 10^{-6}$
3. Find Equilibrium [HA]: $0.1 - 0.00135 = 0.09865$ M
4. Divide: $\frac{1.82 \times 10^{-6}}{0.09865}$
5. Result: $1.84 \times 10^{-5}$

The Ka of your unknown acid is $1.84 \times 10^{-5}$, which matches the known Ka of acetic acid!