The pKₐs of phosphoric acid are 2.1,7.2,and 12.4.Which of the following values is the correct estimate of the pH of the resulting solution prepared by mixing 1.0 mole of solid disodium phosphate (Na₂HPO₄)and 1.25 moles of hydrochloric acid?
A)pH

Question

The pKₐs of phosphoric acid are 2.1,7.2,and 12.4.Which of the following values is the correct estimate of the pH of the resulting solution prepared by mixing 1.0 mole of solid disodium phosphate (Na₂HPO₄)and 1.25 moles of hydrochloric acid?
A)pH < 2.1
B)pH = 2.1
C)2.1 < pH < 7.2
D)pH = 7.2

Quizplus · Accepted Answer

When disodium phosphate is added to hydrochloric acid, the following reaction takes place:

Na₂HPO₄ + 2HCl → 2NaCl + H₂O + HPO₄²⁻

The pH of the resulting solution depends on the relative concentrations of the acid and its conjugate base (in this case, HPO₄²⁻), as well as the concentrations of H+ and OH- ions. Since the pKa of HPO₄²⁻ is 12.4, it acts as a weak base and will partially neutralize the H+ ions from the hydrochloric acid. This means that the resulting solution will have a pH greater than 2.1 (the pKa of the strongest acid, H₃PO₄) but less than 7.2 (the pKa of the second-strongest acid, H₂PO₄⁻). Therefore, the best choice is C - 2.1 < pH < 7.2.

The PKₐs of Phosphoric Acid Are 2