Percent Protonation for Weak Bases

[405513470]

In class we calculated the % protonation by doing [BH+]equillibrium/ [B]initial x100. However on Achieve it says to do ([BH+]/[B]formal)× 100. Does the BH+ on the second one mean at equilibrium or initial ? Also why is a different equation given and did we ever talk about the second one?

[Aaron Tang 2F]

This is the same formula, except in this problem, the value for pH at equilibrium was given, causing you to find the [BH+] concentration at equilibrium, then finding [B] at equilibrium. However, to calculate percent protonation, you can't just divide these two values, you need the initial [B] value in the denominator. The actual initial [B] is the sum of [B] and [BH+] at equilibrium, because all of the generated BH+ came from the initial B.

[Amy Huynh 1B]

I think they are the same formula. However, we use one or the other depending on the information we have. For example, the formula talked about in class is applicable when we know the [B]initial. The second one in achieve was used because we were not given [B]initial and had to calculate that number from the other information given in the problem.