A new potential heart medicine, code-named X-281, is being tested by a pharmaceu

Question

A new potential heart medicine, code-named X-281, is being tested by a pharmaceutical company, Pharma-pill. As a research technician at Pharma-pill, you are told that X-281 is a monoprotic weak acid, but because of security concerns, the actual chemical formula must remain top secret. The company is interested in the drug's K_a value because only the dissociated form of the chemical is active in preventing cholesterol buildup in arteries. To find the pK_a of X-281, you prepare a 0.087 M test solution of X-281 at 25.0 degree C. The pH of the solution is determined to be 2.40. What is the pK_a of X-281? Express your answer numerically.

Explanation / Answer

recall that

HA <-> H+ +A-

Ka = [H+][A-]/[HA]

so

[H+] = [A-] = x

[HA] = M-x

from pH, get x

pH = -log(H+)

[H+] = 10^-pH = 10^-2.4 = 0.00398107

so

[A-] = 0.00398107

[HA] = M-x = 0.087-0.00398107 = 0.0830

calculate Ka

Ka = (0.00398107)(0.00398107)/(0.0830)

Ka = 0.00019095082

pKA = -log(Ka) = -log(0.00019095082 = 3.7190

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