Based on the achieve problems #2 and #5, can someone clarify this for me?
When calculating % ionized for question 2, I got the question right the first time by using [HA] initial, as I had written in my notes from Lavelle's lecture. This question started off by giving the value for Ka.
However, for question 5, when finding % protonated, I used the [B] initial and this ended up being wrong. After looking at the hints, it said to use [B] formal ([B] + [BH+]), which I don't think was mentioned in the lecture and is not in my notes. Lavelle instead used [B] initial, so I'm a bit confused by this. Can someone explain what this is, and why we don't use [B] initial? Thank you!
Achieve #5, [B] formal
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Re: Achieve #5, [B] formal
Hi,
I think you are always supposed to divide by the initial concentration when trying to find percentage protonated. In number 5, earlier in their solution, they used the equation Kb = [BH+][OH-]/[B] to find [B]. The equilibrium constant, Kb, is always calculated using equilibrium concentrations of the reactants and products which means that the [B] that they are referring to is the equilibrium concentration of the base. To find the initial concentration, you would have to add [BH+] because in an ice table, [BH+] is equal to the change that was subtracted from the initial concentration to get to equilibrium. So, they are still dividing by the initial concentration. I hope that makes sense!
I think you are always supposed to divide by the initial concentration when trying to find percentage protonated. In number 5, earlier in their solution, they used the equation Kb = [BH+][OH-]/[B] to find [B]. The equilibrium constant, Kb, is always calculated using equilibrium concentrations of the reactants and products which means that the [B] that they are referring to is the equilibrium concentration of the base. To find the initial concentration, you would have to add [BH+] because in an ice table, [BH+] is equal to the change that was subtracted from the initial concentration to get to equilibrium. So, they are still dividing by the initial concentration. I hope that makes sense!
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Re: Achieve #5, [B] formal
I believe in the Achieve, their [B] formal equals to the [B] at equilibrium plus the [BH+], because that was the change that was subtracted from the [B]initial. Therefore, you are correct! To find the percent protonated, you do the equilibrium concentration [BH+] / [B]initial, which just happens to be [B] at equilibrium + [BH+] (AKA the [B] formal), then times 100. I believe it's the same thing, just worded differently!
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