Organic acids and bases

Question

Which weak acid would be best to use when preparing a buffer solution with a pH of 8.40 ?

1. an acid with p𝐾a=4.30
2. an acid with p𝐾a=9.60
3. an acid with p𝐾a=6.20
4. an acid with p𝐾a=3.20
5. an acid with p𝐾a=5.10
6. an acid with p𝐾a=8.38

Which pair of compounds will form a buffer in aqueous solution?

1. HCl and NaOH
2. HCl and NaCl
3. HCN and NaCN
4. HCN and HCl

Answer 1

When choosing a buffer, the rule of thumb is to choose one that has a pKa +/- 1 pH unit of the desired pH. In this case, for pH 8.40, the best choice would be (6) an acid with p𝐾a=8.38

A buffer is formed when you have a weak acid plus the conjugate base.

Any of the options with HCl will not form a buffer since HCl is NOT a weak acid

The correct choice is (3) HCN + NaCN. HCN is a weak acid and CN- is the conjugate base.