Question

For each of the following buffer problems, start by writing out the relevant salt dissolution equation and then relevant acid/conjugate base equation. Then do the appropriate calculations to determine the amounts of each material that will be needed to make the solution.

1. 1.00 L of 0.20 M Phosphate buffer, pH=7.20, from K2HPO4 (FW=174.18) and either 6.0M HCl or 6.0M NaOH

2. 1.00 L of 0.15 M carbonate buffer, pH 10.00, from the salt NaHCO3 (FW=84.01) and either 8.0M HCl or 8.0M NaOH (pKa of carbonic acid= 3.77)

3. 1.00L of 0.70M TRIS buffer, pH 7.60, from Trizma base (FW=121.1) and 12.0M HCl. pKa=8.08

Answer 1

1. The no. of moles of phosphate buffer = 1 L * 0.2 M, i.e. 0.2 mol

K₂HPO₄ 2K⁺ + HPO₄^2-

HPO₄^2- + H⁺ H₂PO₄^-

According to Henderson-Hasselbalch equation, pH = pKa + Log(n_HPO4^2- / n_H2PO4^-), where n = no. of moles

i.e. 7.2 = 7.2 + Log(n_HPO4^2- / n_H2PO4^-)

i.e. Log(n_HPO4^2- / n_H2PO4^-) = 0

i.e.n_HPO4^2- = n_H2PO4^-​

Here, _HPO4^2- + n_H2PO4 = 0.2

Therefore, n_HPO4^2- = n_H2PO4^- = 0.1 mol

i.e. n_HPO4^2- = 0.1 mol * 174.18 g/mol, i.e. 17.418 g

Similarly, you can calculate for problems 2 and 3. All the very best !!!!!