Sapling week 2 #5

[StephanieIb]

The Kb for an amine is 8.672×10−5. What percentage of the amine is protonated if the pH of a solution of the amine is 9.715 ? Assume that all OH− came from the reaction of B with H2O.
can someone help me solve this? i did the whole process and got 167% but it's wrong

[Joyce Pang 2H]

I remember getting something similar and realized I had to add the value of x to amount of OH- protonated (your antilog of the pOH) to get the actual initial concentration, because the 'initial' value you solved for doesn't include the percent protonated.

[Saja Kamal 2L]

Hi!

Here's how I went about solving this problem!
1. Write out the chemical equation and the equilibrium concentration = Kb.
2. Find the pOH using the given pH.
3. Then use the equation [OH-] = 10^-pOH. The concentrations of both [OH-] and [BH+] from the products side of the chemical equation that you wrote out in step #1 will be found from doing so. (side note: that equation comes from pOH = -log(OH-).
4. Using the equilibrium expression you wrote out earlier, plug in your new value that you found for [OH-] and [BH+] into the numerator.
5. Since your value for Kb was given in the problem, and we have now found the value for both [OH-] [BH+], you can solve for [B].
5. Now that you have solved for [B], you can solve for the % protonation!

I hope this helped!