What is the pH of a solution that consists of 0.50 M H₂C₆H₆O₆ (ascorbic acid) and 0.75 M NaHC₆H₆O₆(sodium ascorbate)? For ascorbic acid, K_a = 6.8 × 10⁻⁵ A)3.76 B)3.99 C)4.34 D)4.57 E)5.66

Question

Quizplus · Accepted Answer

To solve this problem, we need to set up an equilibrium expression for the dissociation of ascorbic acid:

HS1U1B12S1U1B0CS1U1B16S1U1B0HS1U1B16S1U1B0OS1U1B16S1U1B0 ⇌ H+ + S1U1B12S1U1B0C6Hs1U1B38S1U1B0O6-

The Ka for this reaction is given as 6.8 x 10^-5, which allows us to calculate the concentration of H+ ions at equilibrium:

Ka = ([H+][S1U1B12S1U1B0C6Hs1U1B38S1U1B0O6-])/[HS1U1B12S1U1B0CS1U1B16S1U1B0HS1U1B16S1U1B0OS1U1B16S1U1B0]

6.8 x 10^-5 = (x^2)/(0.50 - x)

Using the quadratic equation or an ICE table, we find that x = 1.03 x 10^-3 M.

The overall concentration of ascorbate is 0.75 M, so we can assume the concentration of HS1U1B12S1U1B0CS1U1B16S1U1B0HS1U1B16S1U1B0OS1U1B16S1U1B0 is essentially constant at 0.75 M (since it is a weak base and won't react substantially with water). Thus, the pH of the solution is:

pH = -log[H+] = -log(1.03 x 10^-3) = 3.99

Therefore, the answer is choice B.

What Is the PH of a Solution That Consists of 0.50