Achieve Week 2, #5

[LaurenAkason2A]

Hello! I'm super confused on how to find the answer to number 5. I found the OH- concentration but I don't know where to go from there. My numbers are Kb = 1.589×10^−5 and pH is 9.788. If anyone could help that would be great!

[Anthony Stone 1K]

I found it easiest to set up your chemical equation which would be B + H2O --> BH+ + OH-. Once you find the concentration of OH- molecules, use the equation of [products]/[reactants (x or B)] = Kb. The [products] would be the [OH-] squared. You already have the Kb too, so you are solving for x or the initial concentration. Once you find this, use the equation to find percent protonation. It is [OH-]/[B or x] x 100. Hope this makes sense.

[Michael Vigman 2D]

For #5,

Begin by setting up an ICE Table under the general reaction for a base, B + H2O <=> OH- + BH+.

Working backwards, you can find the concentration of OH using pH to pOH to [OH-]. After which, you can find the concentration of B at equilibrium by filling in the equation: Kb = ([OH]/[B]).

After this, you can find the initial concentration of B since you know the difference (x) from the reactants side.

From here, you can find the percent protonation using [BH+] (at equilibrium) divided by the sum of [BH+] + [B] and multiply by 100 for a %.

[Michelle Gong]

Hi! so what I would recommend to do is find the pOH using the pH by subtracting 14 by the pH. Then once you have this, take the antilog of the pOH to get the concentration of OH-, this will be your equilibrium concentration of OH- and because it is the same 1 to 1 ratio as the BH, then you can assume it is the same concentration at equilibrium. Then you just are trying to find the unknown x or B's concentration at equilibrium. You could do this by simply using the Kb and then once you have that you can figure out what the change in concentration was from initial to equilibrium and then once you find the initial concentration of B, you could then just divide the equilibrium concentration of B by the initial concentration of B and multiply by 100 to get the answer. I hope this helps!

[Brynn 2F]

I made the mistake of using the equation [OH-]/[B] x 100% when calculating the percent protected instead of [OH-]/ [B] + [BH+] x 100%. This may be the mistake you are making.