Question

A new potential cure for the common cold, code-named Z-123, is being tested by a pharmaceutical company. As a research technician, you are told that Z-123 is a monoprotic weak acid, and the company is interested in the drug's Ka value. To find the pKa of Z-123, you prepare a 0.091 mol/L test solution of the drug. The pH of the solution is determined to be 2.77. What is the pKa of Z-123?

Answer 1

To determine the pKa of Z-123, we first convert the pH to hydronium ion concentration, then use that to calculate Ka, and finally, determine pKa by taking the negative logarithm of Ka. The pKa of Z-123 is found to be 4.49.

Step-by-step explanation:

The pKa of a monoprotic weak acid can be determined using the pH of its solution and its concentration. Given that the pH of the Z-123 solution is 2.77 and its concentration is 0.091 M, we can calculate the concentration of hydronium ions ([H3O+]) using the pH value.

First, we convert the pH to [H3O+] concentration:

[H3O+] = 10-pH = 10-2.77 = 1.7 × 10-3 M

Assuming that the concentration of [Z-123-] is equal to [H3O+] due to the 1:1 stoichiometry of a monoprotic acid dissociation, we then use the expression for the acid dissociation constant (Ka):

Ka = [H3O+]2 / [Z-123]

Substitute the found values: Ka = (1.7 × 10-3)2 / 0.091 M

Ka = 3.21 × 10-5

Now, calculate the pKa:

pKa = -log(Ka) = -log(3.21 × 10-5) = 4.49

Therefore, the pKa of Z-123 is 4.49.