What is the pH of a 1.0 M solution of Na₃AsO₄?( Ka1 = 6 × 10^-3, Ka₂ = 1 × 10^-7, Ka₃ = 3 × 10^-12 ) A) 2.5 B) 7.0 C) 8.2 D) 11.5 E) 5.8

Question

Quizplus · Accepted Answer

This compound is a salt of a weak acid (H3AsO4) and a strong base (NaOH). The anion (AsO4 3-) is the conjugate base of the weak acid. The pH of a solution containing the salt of a weak acid and a strong base can be calculated using the following equation:
pH = pKa + log([base]/[acid])
where pKa is the dissociation constant of the weak acid (given in the problem), [base] is the concentration of the conjugate base (in this case, AsO4 3-), and [acid] is the concentration of the weak acid (H3AsO4).

Using the given equilibrium constants, we can calculate the pKa values for the three ionization steps of H3AsO4:
pKa1 = 3.22
pKa2 = 7.21
pKa3 = 11.50

The pH of the solution can be calculated using the equation above and assuming that the concentration of AsO4 3- is equal to the concentration of NaSAsO4:
pH = 8.21 + log([NaSAsO4]/[H3AsO4])
pH = 8.21 + log(1/1)
pH = 8.21

Therefore, the pH of the solution is 8.2, which is answer choice C.

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