percentage protonation

[Juliana Misirian]

For this problem:
The Kb for an amine is 4.706×10−5. What percentage of the amine is protonated if the pH of a solution of the amine is 9.753? Assume that all OH− came from the reaction of B with H2O.

After using the equation Kb=[BH][OH]/[B] to find B, how do you find the percent protonated? The equation is stated as [BH]/[B]formal x 100, so how do we find [B]formal?

[Divya Hebbar]

Use the pH to find the [OH-] which will be equal to the [BH]. You can do this by subtracting the pH from 14 to get the pOH. 10^-pOH will give you the OH concentration. Then to find [B] initial you need to set up the kB equation. [OH-] and [BH] are the same value so the equation will look like x^2/ [B] = kB. You already know the x value from finding the OH concentration and you know the kB, so you can solve for B. Then take the x concentration and divide by the B concentration to find the % protonated!

[Tyler_Kim_2G]

To find [B]formal, you add the concentrations of B and [BH+]. I got this feedback from Achieve, and I think it is because [B] is actually the concentration at equilibrium, not the initial concentration. If you were to set up an ICE table, a relationship you could come up with is [B]-x=[B]formal, so [B]formal=[B]-x.

[Zechuan Rao 1K]

B formal is [B equalibrium] + [BH+]. Recommend always draw an ICE table to make things clear.

[Maggie Messer 1A]

You can find B formal by adding B at equilibrium and [BH] together!

[Alekhya_Pantula_2E]

First I would use the pH to calculate the pOH, then using -log[OH-] equation to calculate the [OH-]. Then applying this to the Kb equation, working backwards you would place the [OH-] value wherever X would have been and solve for the B formal value from there. Then once the B formal value is found, apply to the percent protonation equation.