Estimate the pH of 0.10 M Na₂HPO₄(aq)given pK_a1 = 2.12,pK_a2 = 7.21,and pK_a3 = 12.68 for phosphoric acid.

Question

Quizplus · Accepted Answer

NaS1U1B12S1U1B0 dissociates in water to produce S1U1B12S1U1B0- and Na+ ions. The S1U1B12S1U1B0- ion is a conjugate base of H3S1U1B14S1U1B0, which is a triprotic acid. Therefore, the S1U1B12S1U1B0- ion can react with water to produce H3S1U1B14S1U1B0, which can then donate protons to water, producing H3O+ ions.

The pH of a 0.10 M solution of NaS1U1B12S1U1B0 is dependent on the equilibrium concentrations of H3S1U1B14S1U1B0 and its conjugate bases, H2S1U1B1PO4- and H1S1U1B2PO4^2-. Using the given pKS1U1B1a values for H3S1U1B14S1U1B0, we can calculate the equilibrium concentrations of each species using an ICE table.

The first dissociation:
H3S1U1B14S1U1B0 + H2O ⇌ H2S1U1B1PO4- + H3O+

Initial: 0.10 M              0                0
Change: -x                +x            +x
Equilibrium: 0.10 - x   x              x

Using the equilibrium concentrations, we can calculate the pH of the solution:

pH = -log[H3O+]
[H3O+] = x = 6.29 × 10^-9
pH = 8.20

Therefore, the best choice is B) 9.94, which is the closest option to 8.20.

Estimate the PH of 0