Kb Calculator

Identifying Weak Bases

Just as we can experimentally determine the Ka of an acid, we can also determine the Base Dissociation Constant (Kb) of an unknown weak base by analyzing its equilibrium state in water.

Weak bases (like amines and ammonia) react with water to produce hydroxide ions ([OH⁻]). By measuring the pH of a known concentration of the base, we can reverse-engineer the exact Kb.

The Experimental Math

Because a pH meter only measures hydrogen ions, we must first convert the measured pH into pOH to find the hydroxide concentration.

• pOH = 14.00 - measured pH
• [OH⁻] = $10^{-pOH}$

Using an ICE table, we know that for a standard weak base, $[OH⁻] = [BH⁺]$. We can plug these equilibrium concentrations back into the standard Kb expression.

The Exact Formula

Example Calculation

You prepare a 0.1 M solution of an unknown weak base. The pH meter reads 11.13.

1. Find pOH: $14.00 - 11.13 = 2.87$
2. Find [OH⁻]: $10^{-2.87} = 1.35 \times 10^{-3}$ M
3. Square the [OH⁻]: $(1.35 \times 10^{-3})^2 = 1.82 \times 10^{-6}$
4. Find Equilibrium Base: $0.1 - 0.00135 = 0.09865$ M
5. Divide: $\frac{1.82 \times 10^{-6}}{0.09865}$
6. Result: $1.84 \times 10^{-5}$

The calculated Kb is $1.84 \times 10^{-5}$, which matches the textbook value for ammonia!