Clarifying Achieve Hmk Acids and Bases Q #5

[Ivy Zhang 2A]

I wasn't really sure how to approach this question on the homework and would just like to clarify a few questions. What is the amine and how would it be used in the Kb dissociation constant equation? Would the amine represent [B] in the equation?

Thanks!

[Francis Le 2K]

It is not necessary to know what the specific amine is to answer the question. Since Kb is small, you can just treat the amine as a weak base and set up the equilibrium constant expression. Then, with the given pH, use the fact that pH + pOH = 14 to find pOH and then [OH-]. Use the concentration of hydroxide in the Kb expression to solve for the concentration of the amine. To find the percent protonation, divide the concentration of the conjugate acid (which is the same as the concentration of hydroxide) by the concentration of the amine before it is protonated.

[Harun Momin 1L]

Hello,

An amine is an organic compound that contains a nitrogen atom with lone pairs. Per the lecture and strong acid/base list, we know that amines act as weak bases due to the lone pair on the nitrogen atom. With this knowledge, we can set up the following chemical reaction, B(aq)+H2O(l) <-> BH+(aq) + OH-(aq) and use the Kb dissociation constant for the amine, B. This could be written as Kb = [BH+][OH-]/[B]. From there, we can set up the ice table to calculate the initial molarity of B, the amine. We know that the equilibrium concentrations of [BH+] and [OH-] will be the same due to their mole ratios. We can find their concentrations with the pOH (14-pH) by calculating 10^-pOH. We can then use the ice table to solve backwards for the initial concentration of [B]. Then, we would set the equilibrium concentration of [BH+] over the initial concentration of [B] to find percentage protonated.

Hope that helped!