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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