Sapling Number 5

[Ethan Goode 2H]

The Kb for an amine is 5.136×10−5. What percentage of the amine is protonated if the pH of a solution of the amine is 9.376 ? Assume that all OH− came from the reaction of B with H2O.

How do I find the initial B concentration in this problem? I have been getting answers that have been off.

[Kaihan_Danesh_2J]

To find the initial B concentration, you have to add the concentration of BH+ to B.

[Najia Saleem 2G]

Hello, here is how I would approach this problem:

1) List out what is given first, which is the Kb and pH.
2) Use the pH to find pOH.
3) Then, you should use the pOH to find the initial concentration of products by taking the inverse of a logarithm. This operation is 10 to the power of the negative pOH.
4) You should set up a Kb concentration with products/reactants with the initial value that you are looking for as X.

Then, it is as simple as plugging in the numbers and solving. I hope this was helpful!

[Mitzy Garcia 1k]

This is how I found B. Hope that helps

[Yeprem]

I completely forgot to add [B] with [BH+] to find the [B] formal:(
Thank you

[AHUNT_1A]

Using this as reference because I am stuck on 5 as well.

[Truman Chong Dis 3G]

I was stuck on this question for so long and then I realized I used e instead of 10... :/

[Jose Miguel Conste 3H]

I think you have been getting the correct answer you just forget to add [OH-] with [B], I was doing that and getting wrong answer

[AlbertGu_2C]

This question was pretty confusing to me as well, but as stated above, adding [BH+] to [B] should net you the right answer

[Kandyce Lance 3E]

For this problem, are [OH-] and [BH+] interchangeable ?

[RylanPurificacion 3J]

For this problem, are [OH-] and [BH+] interchangeable ?

Yes, since it's a monoprotic reaction, both values taken in this problem are the same. Although it would probably be good form to use [OH-] and [BH+] properly, just to lessen the chances of messing up in the future when they're not.

[Griffin G]

This one is tricky because you have a lot of moving parts. But basically, if you can lockdown the BH+ value, you just add that to B.

[Adrienne Chan 1G]

Kandyce, yes for this problem the two are interchangeable.

[Jaclyn Schwartz 1I]

I completely forgot to add [B] with [BH+] to find the [B] formal:(
Thank you

Thanks for clarifying this! I have been stuck on this for so long. This helped make it clear what B formal is!

[joshtully]

Find the initial B value by adding the values of BH+ and B at equilibrium.

[MichaelRaad_1F]

I first calculate concentration of OH and BH using the given pH and inverse log. I set these values as final concentrations for OH and BH on my ice chart and B reactant is x-the final concentration of OH. I then would solve for x after setting up my expression for kb.

[Caelin Brenninkmeijer 1G]

For this problem, are [OH-] and [BH+] interchangeable ?

I wouldn't describe them as interchangeable. You can use either value because for every mole of OH- produced, there's exactly one mole of BH+ produced, making them equal to each other.

[Susan Chamling 1F]

To find the initial [B], you can add the equilibrium [B] with the x you used for the ICE table.

[Zihan Liu 2K]

I think it's just the inverse of an ICE table.
I would asume that you've already found the concentration of OH-, let's say it's 10. Since for one mole of OH-, BH+ is produced, BH+ also has a concentration of 10.

Therefore, for the ICE table, you'll get:
BOH OH- BH+
I ? 0 0
C ? ? ?
E ? 10 10

Now since you know that since the initial concentration of OH- and BH- is 10, you would get that the concentration of OH- and BH+ as added 10. This means that BOH's concentration would decrease by 10. Consequently, the end result of BOH would be it's initial value minus 10.

Now the ICE table looks like this:
BOH OH- BH+
I ? 0 0
C -10 10 10
E ?-10 10 10

Now that you've got all the end values and the K, you can list an equation and solve out what ? is