Question

Why, when an ammonia solution is titrated with a strong acid, the `pH` remains relatively constant up to the addition of a stoichiometric quantity of the acid?

Answer 1

Because you have described formation of a buffer solution.........

Explanation:

A `"buffer solution"` is a mixture of a weak acid and its conjugate base in APPRECIABLE quantities, and buffers act to resist GROSS changes in `pH`.

This is seen from the well-known buffer equation, the which we may represent as:

`pH=pK_a+log_10{[[A^-]]/[[HA]]}`

Where `[A^-]` is the conjugate base. And `[HA]` is its conjugate acid.

In this ammoniacal scenario, the free base is the uncharged species, the ammonia, and the ammonium acid is charged:

`pH=pK_a+log_10{[[NH_3]]/[[NH_4^+]]}`

Many students have difficulties with the logarithmic function. I do not suggest that you do. But I can encourage you and others not to be intimidated when you see a `log` function. When I write `log_ab=c`, I ask to what power I raise the base `a` to get `c`. Here, `a^c=b`. And thus `log_(10)10=1,` `log_(10)100=2,` `log_(10)10^(-1)=-1`. And `log_(10)1=0`.

Back in the day, before the advent of electronic calculators, students (and scientists, and engineers, statisticians, and economists) would routinely use log tables to do calculations that a calculator would these days render trivial.