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 0.081M test solution of X-281. The pH of the solution is determined to be 2.70. What is the pKa of X-281?

Express your answer numerically.

Explanation / Answer

Given data

Concentration of the X-281 = 0.081 M

pH= 2.70

pka = ?

we are given with the pH from that calculate the concentration of the H3O+

pH = -log [H3O+]

therefore

[H3O+] = antilog [-pH]

             = antilog [-2.70]

             = 0.001995 M

[H3O+] = [A-] =0.001995 M

Using this [H3O+] concentration we can calculate ka

Assume X-281 as HA

Then reaction of HA with water is as follows

HA + H2O -----> H3O+ + A-

Write the ka equation for this reaction

Ka = [H3O+]] [A-] / [HA]

lets put the values in the formula

Ka = [0.001995] [0.001995] / [0.081 ]

Ka = 4.91*10-5

From the ka we can find pka as

pKa= -log ka

pKa = - log [4.91*10-5]

pKa = 4.31

there fore ka of the X-281 = 4.91*10-5and pKa = 4.31

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