Question

# For each of the following buffer problems, start by writing out the relevant salt dissolution equation...

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

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

K2HPO4 2K+ + HPO42-

HPO42- + H+ H2PO4-

According to Henderson-Hasselbalch equation, pH = pKa + Log(nHPO42- / nH2PO4-), where n = no. of moles

i.e. 7.2 = 7.2 + Log(nHPO42- / nH2PO4-)

i.e. Log(nHPO42- / nH2PO4-) = 0

i.e.nHPO42- = nH2PO4-

Here, HPO42- + nH2PO4 = 0.2

Therefore, nHPO42- = nH2PO4- = 0.1 mol

i.e. nHPO42- = 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 !!!!!

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