Problem+Set+3

Here is the ProblemSet #3 file. The on-line due day is 07-02-2010. Please downnload it, answer the questions assigned to your LG (LG-WX, LG from Worcester; LG-MX, LG from Manchester), and upload your answer file back accordingly (below). If you have any problems to upload your file back, please send your file directly to me at jonathan.sheng@mcphs.edu and l will do it for you. Each question will be answered by two LGs independently.

Question A LG-W18: A. Questions from Pandit’s //Introduction to the Pharmaceutical Sciences// Cimetidine (Tagamet) is a drug used to reduce excessive secretion of gastric acid for patients with duodenal ulcers. It has a pKa = 6.8 and forms hydrochloride salt. 1. Is this drug an acid or base? Weak or strong?

Weak base

2. Calculate the percentage of the un-ionized drug molecules in stomach (pH=2), intestines (pH=8) and blood (pH=7.4).

pH= 2: 0.01% unionized pH= 7.4: 80% unionized pH= 8: 94% unionized

3. When cimetidine hydrochloride is added to water, will the pH of the solution go up or down? Why?

The pH of the solution will go up thus becoming more basic. Water will react with the cation of the salt to reform the weak base. The reaction will favor the reactants.

LG-W19:

1. Tagamet is a weak base 2. Using the equation on page 12 of Handout #2: % ionization= 100/ (1 +10^(pH-pKa)) at pH 2, percent ionized=99.9%, thus 100-99.9= **%unionized of 0.01%**at pH 7.4, percent ionized=20%, thus 100-20%= **%unionized of 80%**at pH 8.0, percent ionized=5.9%, thus 100-5.9%= **%unionized of 94%** 3. We think that the molecule will become more acidic in water because it will donate the hydrogen becoming the salt form and thus make the water more acidic.

LG-W20:

A. Questions from Pandit’s //Introduction to the Pharmaceutical Sciences// Cimetidine (Tagamet) is a drug used to reduce excessive secretion of gastric acid for patients with duodenal ulcers. It has a pKa = 6.8 and forms hydrochloride salt. 1. Is this drug an acid or base? Weak or strong?

Weak base

2. Calculate the percentage of the un-ionized drug molecules in stomach (pH=2), intestines (pH=8) and blood (pH=7.4).

pH 2 = 99.99% pH 8 = 5.94% pH 7.4 = 20.08%

3. When cimetidine hydrochloride is added to water, will the pH of the solution go up or down? Why? The pH will increase because Cimetidine is a base (pH>7). The -OH from the base will decrease the hydronium ion concentration (H3O+) of the water therefore raising the pH.

The pH of the solution will go up because cimetidine hydrochloride is a conjugated acid with the pka less than pH of water (7). The solution will have less [H+].