### Why do this problem?

This problem involves the application of logarithms to pH. This
interesting application, in which the critical value of pH 7 is
familiar, will give a good motivation for engaging with the
logarithm.

### Possible approach

To do this question, students will need to understand the
precise mathematical meaning of both the dissociation constant and
the pH (both are clearly described in the question, so no
particular knowledge of chemistry is required). Initially students
might need to think on how they can connect the various pieces of
information.

### Key questions

What equations will you need to use to solve the
problem?

How can you use the table of data in the question?

Why is an approximation in the question needed? At what point
in a question will an approximation be made?

### Possible extension

Students will need to interpolate the data in the table to be
able to provide an approximation. Can they suggest
differentpossibilities for this interpolation? Which one is
best?

### Possible support

Ask the students to work out the pH at the different
temperatures given in the table.