Shameful problem

[PineMangoes]

Hi people,

The adding of ammonium (NH4+) to water causes the pH to drop, right? I've been searching for a way to find the pH of a solution which contains respectively 1.8E-5, 7.1E-6, and 3.6E-6 mole/L ammonia, and found that the pH's were respectively 7.001; 7.19 and 7.35... There must be something wrong with my method, since even if it would cause the pH to rise, then it would rise together with the concentration, and not rise while concentration drops. Could anyone propose a correct solution to this problem?

A very confused (and not half ashamed) student