Aqueous solutions of ammonia, ethylamine and phenylamine are prepared - AQA - A-Level Chemistry - Question 24 - 2018 - Paper 3

To determine the correct order of pH values for ammonia, ethylamine, and phenylamine, we must consider their basicity.

1. Ammonia (NH₃) is a well-known weak base and has a higher tendency to accept protons (H⁺) compared to ethylamine and phenylamine, resulting in a higher pH value.
2. Ethylamine is a stronger base than phenylamine due to the electron-donating effect of the ethyl group, making it more capable of attracting protons, but it is still weaker than ammonia.
3. Phenylamine is the weakest base among these three as the presence of the benzene ring reduces its basicity by delocalizing the lone pair of electrons on the nitrogen, leading to a lower pH.

Thus, the hierarchy in terms of pH values is:

Ammonia > Ethylamine > Phenylamine

The correct answer is therefore C.