Question

Phosphoric acid is a triprotic acid with the pk, values pha=2.148, p 2 7.198, and pks = 12.375. You wish to prepare 1.000 L of a 0.0600 M phosphate buffer at pH 7.780. To do this, you choose to mix the two salt forms involved in the second ionization. Nah,PO, and Na,HPO,, in a 1.000 L volumetric flask and add water to the mark. What mass of each salt will you add to the mixture? mass NaH,PO, 1.87 mass Na, HPO,= 4.88 What other combination of phosphoric acid and its salts could be mixed to prepare this buffer? H,PO, and NaH,PO, Na, HPO, and Na PO, Nah,PO, and Na PO, H.PO, and Na, HPO H,PO, and Na, PO

Answer 1

СА-1 Acid must be in second ionization for having a buffer at 7.780. So, use pka2= 7.198 According to Henderson - Hasselbalch equation; pH = pka + log [A-] THA 7.780 - 7.198+ log in loga 72 = 0.582 [A- [HA] [A-] = 3.819x[HA] Also ; [A-]+[HA] - 0.0600 3.819[HA] + [HA] = 0.0600 4.819* [HA] - 0.0600 [HA] = 0.01245M [A-] = 0.0600 -0.01245 [A-] = 0.04755 M Tani = 3.819

NaH₂PO4 + Na HPO4 HP02- + H₂PO4 + 3 Not Second ionization is; H,POA = HPOA+H . Stoichiometry is 1:2 for both. Number of moles of NaH₂POA = 0.01245M X 1 L -0.01245 mol Number of moles of Na, HPOR = 0.04755M X 2L =0.04755 mol Molar mass of Natt, Pon = 119.98 g/mol Molar mass of Nag HPO = 141.96 g/mol Mass of NaHz POH = 0.01215 x 119.98 = 1.49g Mass of Na, H POH = 0.04755 x 141.96 = 6.75g For preparing this buffer, it is necessary to have in solution the species H₂PO4 and HPOR. So oter combination of phosphoric acid and its saltmixed to prepare this buffer is; H₂PO4 and Na₂HPOA