In: Chemistry
Estimate the pH of the resulting solution prepared by mixing 1.0 mole of solid disodium phosphate and 1.25 mole of hydrochloric acid. The pKas of phosphoric acid are 2.1,7.2,12.4. The answer is 2.1<pH<7.2. Please explain.
first equilibrium
Na2HPO4 + HCl -----------------------------> NaH2PO4 + NaCl
1.0 1.25 0 0
0.0 0.25 1.0 1.0
second equilibrium
NaH2PO4 + HCl --------------------> H3PO4 + NaCl
1.0 0.25 0 0
0.75 0 0.25 0.25
pH = pKa1 + log [NaH2PO4 / H3PO4 ]
pH = 2.1 + log (0.75 / 0.25)
pH = 2.58
2.58 is in between 2.1 to 7.2 . so answer is satisfied