Michael Week 3 Problem

[Garmani Kyaw]

For part B of this problem to get the ratio do we do C6H5CH2CH(CH3)NH2 to C6H5CH2CH(CH3)NH3+ or C6H5CH2CH(CH3)NH3+ to C6H5CH2CH(CH3)NH2? Does it matter because the answer for the question was when the ratio is calculated with the former one (not the latter).

The drug amphetamine, C6H5CH2CH(CH3)NH2 (pKb = 3.11), is usually marketed as the hydrogen bromide salt, C6H5CH2CH(CH3)NH3+Br-, because it is much more stable in this solid form. When the drug is ingested as a salt (solid form), it enters the stomach, which contains digestive fluids at pH = 1.7.
A) Will the drug remain as C6H5CH2CH(CH3)NH3+ or be converted to C6H5CH2CH(CH3)NH2 in the stomach?
B) What is the ratio of the concentrations of the two species at this pH?
Solve this problem by considering the salt in water.

[Sarah_Hoffman_2H]

Use the pH to solve for the pOH and then concentration of hydroxide in solution. Then set up and ice table, using that initial concentration of OH- and solve for the change in concentrations of the of the salt and its conjugate acid.

[edward_brodell_2I]

Considering the conjugate seesaw in which the stronger the acid the weaker the conjugate base, etc, the answer should also make intuitive sense once you do the table and for the conjugate acid.