Question

For a monoprotic acid, verify that:

a) At pH values above the pKa, the concentation of [HA] decreases by tenfold for each unit increase in pH (i.e. dpHA/dpH = 1)

b) At pH values below the pKa, the concentation of [A-] decreases by tenfold for each unit decrease in pH (i.e. dpA/dpH = -1)

Answer 1

a)

At pH values above the pKa,

Each unit increase in pH corresponds to tenfold decrease in as

Since constant as decreases tenfold,

increases tenfold. So if is kept constant, decreases tenfold.

b)

At pH values below the pKa,

Each unit decrease in pH corresponds to tenfold increase in as

Since constant as increases tenfold,

decreases tenfold. So if is kept constant, decreases tenfold.