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 Ka value because only the dissociated form of the chemical is active in preventing cholesterol buildup in arteries.

To find the pKa of X-281, you prepare a 0.093 M test solution of X-281. The pH of the solution is determined to be 2.40. What is the pKa of X-281?

Express your answer numerically.

Explanation / Answer

By definition, in equilibrium:

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

Then

Assume that [H+] = [A-] = x … due to stoichiometry (i.e. 1 mol of H+ per each mol of A-)

Then, in equilibrium [HA] = M – x (i.e. the original concentration minus the acid in equilibrium)

Substitute

[H+] = 10^-pH = 10^-2.4 = 0.0039810

[A--] = 0.0039810

[HA] = 0.093- 0.0039810 = 0.089019

Ka = (0.0039810)(0.0039810)/(0.089019) = 0.00017803346

Ka = 1.78*10^-4

pKa0 -log(Ka) = -log(1.78*10^-4)

pKa = 3.749579

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