Week 2, achieve question 5

[Nidhi Thati 3I]

The Kb for an amine is 3.559×10−5. What percentage of the amine is protonated if the pH of a solution of the amine is 9.253? Assume that all OH−
came from the reaction of B with H2O.

In the final step, I divided the concentration of OH- by the concentration of B to find the percent protonated. When I was calculating the concentration of B, I was originally getting a percent protonated higher than 100%. I realized that I needed to add the calculated number for B to the concentration of OH-, and then use that number as the denominator in my final step. Why did I have to add that other number to the calculated concentration of B?

[Anabelle Pham 2C]

You needed to use the other number for the final step because the formal concentration of B is B plus BH+.