Why is a buffer solution most effective when concentrations of salt and acid are the same?
Using the Henderson-Hasselbalch-Equation:
$$\mathrm{p}\ce{H}=\mathrm{p}{K_a}+\log\frac{\left[\ce{salt}\right]}{\left[\ce{acid}\right]}$$
and buffer capacity:
$$\phi=\frac{\text{number of moles of acid/base added to 1L solution}}{\Delta\mathrm{p}\ce{H}}$$
where:
$$\Delta \mathrm{p}\ce{H} = \left|\log\frac{\left[\ce{salt}\right]}{\left[\ce{acid}\right]}\right|$$
it is minimum when $$[\ce{salt}]=[\ce{acid}]$$