A handbook states that to prepare a particular buffer solution mix 39.0 mL of 0.20 M NaH₂PO₄with 61.0 mL of 0.20 M Na₂HPO₄. What will be the pH of this buffer? A) 7.2 B) 7.4 C) 7.0 D) 6.8 E) 6.6

Question

Quizplus · Accepted Answer

This is a buffer solution made from the conjugate acid-base pair, NaHSO4/Na2SO4. The pH of this buffer can be calculated using the Henderson-Hasselbalch equation:

pH = pKa + log([base]/[acid])

where pKa is the dissociation constant of the acid, and [base] and [acid] are the concentrations of the conjugate base and acid, respectively.

The pKa of HSO4- is 1.99, so:

pH = 1.99 + log([Na2SO4]/[NaHSO4])

We can calculate the concentrations of Na2SO4 and NaHSO4 using the given volumes and molarities:

[NaHSO4] = (0.20 M) x (39.0 mL/1000 mL) = 0.0078 M
[Na2SO4] = (0.20 M) x (61.0 mL/1000 mL) = 0.0122 M

Substituting these values into the equation above:

pH = 1.99 + log(0.0122/0.0078) = 7.4

Therefore, the answer is B.

A Handbook States That to Prepare a Particular Buffer Solution