Drug Diffusion Across a Semi-permeable Membrane Questions and Answers

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Drug Z is a weak acid whose pKa is 5.0. The volume of compartment A = volume of compartment B = 1 liter. The pH of the buffer in compartment A is 5.0, while the pH of the buffer in B is 6.0. The membrane between A and B only allows the passage of non-ionized drug Z. At the start of the experiment, the total (ionized plus non-ionized) concentration of drug Z is 100 X 10-9 M in A and 550 X 10-9 M in B.

a. At the start of the experiment, what is the concentration of non-ionized (diffusible) drug Z in A? in B?

b. Write an equation relating the concentration of non-ionized drug Z to total drug Z in compartment A? in compartment B?

c. Write an equation relating the concentration of non-ionized drug Z in compartment A to that in compartment B at equilibrium.

d. At equilibrium, what is the concentration of total (ionized + non-ionized) drug Z in compartment A? in compartment B?

e. At equilibrium, which side will have the greater concentration of non-ionized (diffusible) drug Z?