The pKₐs of phosphoric acid are 2.1,7.2,and 12.4.Which of the following values is the resulting pH if equal amounts of Na₂HPO₄ and NaH₂PO₄ are mixed in water?
A)2.1
B)4.65
C)7.2
D)9.8

Question

Quizplus · Accepted Answer

When equal amounts of Na₂HPO₄ and NaH₂PO₄ are mixed in water, they react to form HPO₄²⁻ and H₂PO₄⁻. The pH of the resulting solution can be calculated using the Henderson-Hasselbalch equation:

pH = pKa + log([HPO₄²⁻]/[H₂PO₄⁻])
    
At halfway point, [HPO₄²⁻] = [H₂PO₄⁻]. Therefore,

pH = pKa + log(1) = pKa
    
Thus, the resulting pH will be equal to the pKa of the acid that is present in equal amounts as its conjugate base. In this case, the pKa of H₂PO₄⁻ is 7.2, so the resulting pH will be 7.2.

The PKₐs of Phosphoric Acid Are 2