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 0.089M test solution of X-281. The pH of the solution is determined to be 3.00. What is the pKa of X-281?

Express your answer numerically.

Explanation / Answer

Answer:

Given:

Prepared concentration of the X-281 solution = 0.089 M

Let the monoprotic acid be= HA

Given pH of the acid = 3

H+ ion concentration in acid is:

pH = -log[H+]

3 = -log[H+]

[H+] = Anti log [-3]

= 0.001

HA ?H+ + A-

Ka of the acid:

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

[H+] = [A-] = 0.001 M

[HA] = 0.089 M

Ka = [0.001][0.001]/[0.089]

= (1*10-6)/(8.9*10-2)

= 0.1123*10-4

Thus: pKa = -log(1.123*10-5)

= -[log(1.123)-5log10]

=-[0.0503-5]

= -(-4.94)

pKa = 4.94

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