Ammonia, NH3, is a weak base with a Kb value of 1.8×105. A.) What is the pH of a

Question

Ammonia, NH3, is a weak base with a Kb value of 1.8×105.

A.) What is the pH of a 0.100 M ammonia solution? Express your answer numerically to two decimal places.

B.) What is the percent ionization of ammonia at this concentration? Express your answer with the appropriate units.

Another way to express acid strength is by using pKa: pKa=logKa

Another way to express base strength is by using pKb: pKb=logKb

C.) 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 at 25.0 C. The pH of the solution is determined to be 2.40. What is the pKa of X-281?

Explanation / Answer

A) Given that Kb = 1.8×105 and C =  0.100 M

pOH = [-logKa -logC]/2

=  [-log( 1.8×105) -log(0.1)]/2

= 2.87

Hence, pH = 14- 2.87 = 11.13

B) Kb = ca2 , a = percent ionization

a = (Kb/c)1/2

= ( 1.8×105/0.1)1/2

= 0.013

= 1.3%

Therefore, percent ionization of ammonia = 1.3 %

C) Given that pH = 2.4 , C = 0.093 M

pKa = ?

pH = [pKa - logC]/2

2.4 = [pKa - log 0.093 ]/2

pKa = 3.76

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