The Ka of ammonia (NH3) is 1.8 x 10^-5 at 25°C. This means that ammonia is a weak base, and it does not completely dissociate in water. When ammonia dissolves in water, it reacts with water to form ammonium ions (NH4+) and hydroxide ions (OH-). The equilibrium constant for this reaction is the Ka value.

The Ka value of a base is a measure of its strength. The stronger the base, the higher the Ka value. For example, the Ka value of sodium hydroxide (NaOH) is 1.0 x 10^14, which means that it is a very strong base and it completely dissociates in water.

The Ka value of ammonia is relatively low, which means that it is a weak base. This means that it does not completely dissociate in water, and there is a significant concentration of undissociated ammonia molecules in solution.

The Ka value of a base can be used to calculate the pH of a solution of that base. The pH of a solution is a measure of its acidity or basicity. The lower the pH, the more acidic the solution. The higher the pH, the more basic the solution.

The pH of a solution of ammonia can be calculated using the following equation:

```

pH = 14 - log[OH-]

```

where [OH-] is the concentration of hydroxide ions in the solution.

The Ka value of ammonia can be used to calculate the concentration of hydroxide ions in a solution of ammonia. The concentration of hydroxide ions can then be used to calculate the pH of the solution.