homework question 5

[605815546]

I've been having a bit of trouble with this problem and was trying to understand the hint given-- why is the percent ionization equal to [BH+]/[B]formal? What is "formal concentration" and why we do we not just use the initial concentration of the amine? It was stated that [B]formal = [B]+[BH+], but how do we know/derive this information from the problem given? Is this specific to amines?
Apologies for the surplus of questions.

[Rohan Ghosh Choudhuri]

This problem starts off by giving us the pH, so we use this to find the pOH and then use that to find the equilibrium concentration of OH-. By using an ice table, we realize that BH+ and OH- have the same value of equilibrium, so then we use these values and the Kb value given to find the concentration of our base (B) at equilibrium. Once we find this, we then need to use the % protonated equation, so we add our x value we found from the ice table to the equilibrium value of the base (B) to find the original concentration of the base. Then, we use the equilibrium concentration of BH+ and original concentration of B to find the percent protonation.

I know this was wordy but I hope it helps.

[John_Sunthorn_3B]

We are using the initial concentration of the amine. "Formal concentration" is the concentration of the amine we are given (before it reaches equilibrium). We aren't given that in the question, but we can solve it by solving for the concentration of [B] (which is the concentration of the amine in equilibrium) and [BH+] (the amount of the amine that got protonated when the equation reaches equilibrium). You use the pH of the amine to calculate for pOH, use that to find the concentration of [OH-] (which helps us find out what [BH+] is, because there are equal moles of BH+ and OH- in the equation). Then using that, you can find out what [B] is.

We have [B] and [BH+], but what's important when thinking in terms of formal concentration is understanding what they mean. Both of these concentrations are when the system is in equilibrium, but for the formal concentration of B, we need to know what the initial concentration of [B] was. [BH+] is a part of the initial concentration of B that changes into [BH+] when reaching equilibrium, so in order to calculate [B]formal, you would simply have to add that to the [B] that was left over after the equation has left equilibrium.