Sapling Number 5

Moderators: Chem_Mod, Chem_Admin

Ethan Goode 2H
Posts: 122
Joined: Wed Sep 30, 2020 9:51 pm

Sapling Number 5

Postby Ethan Goode 2H » Sun Jan 24, 2021 3:20 pm

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
Posts: 89
Joined: Wed Sep 30, 2020 9:40 pm

Re: Sapling Number 5

Postby Kaihan_Danesh_2J » Sun Jan 24, 2021 3:23 pm

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

Najia Saleem 2G
Posts: 36
Joined: Wed Nov 18, 2020 12:20 am

Re: Sapling Number 5

Postby Najia Saleem 2G » Sun Jan 24, 2021 3:32 pm

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
Posts: 31
Joined: Thu Dec 17, 2020 12:20 am

Re: Sapling Number 5

Postby Mitzy Garcia 1k » Sun Jan 24, 2021 3:36 pm

This is how I found B. Hope that helps
Attachments
IMG_79BE37DE931F-1.jpeg

Yeprem
Posts: 77
Joined: Wed Sep 30, 2020 9:40 pm

Re: Sapling Number 5

Postby Yeprem » Sun Jan 24, 2021 5:10 pm

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

AHUNT_1A
Posts: 110
Joined: Wed Sep 30, 2020 9:41 pm

Re: Sapling Number 5

Postby AHUNT_1A » Sun Jan 24, 2021 5:26 pm

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

Truman Chong Dis 3G
Posts: 83
Joined: Wed Sep 30, 2020 9:38 pm
Been upvoted: 1 time

Re: Sapling Number 5

Postby Truman Chong Dis 3G » Sun Jan 24, 2021 5:39 pm

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

Jose Miguel Conste 3H
Posts: 80
Joined: Sat Oct 03, 2020 12:15 am

Re: Sapling Number 5

Postby Jose Miguel Conste 3H » Sun Jan 24, 2021 10:07 pm

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
Posts: 86
Joined: Wed Sep 30, 2020 9:34 pm

Re: Sapling Number 5

Postby AlbertGu_2C » Sun Jan 24, 2021 10:09 pm

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
Posts: 85
Joined: Wed Sep 30, 2020 9:46 pm

Re: Sapling Number 5

Postby Kandyce Lance 3E » Sun Jan 24, 2021 11:03 pm

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

RylanPurificacion 3J
Posts: 36
Joined: Mon Jan 04, 2021 12:09 pm

Re: Sapling Number 5

Postby RylanPurificacion 3J » Sun Jan 24, 2021 11:05 pm

Kandyce Lance 3E wrote: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
Posts: 84
Joined: Wed Sep 30, 2020 9:50 pm

Re: Sapling Number 5

Postby Griffin G » Sun Jan 24, 2021 11:06 pm

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
Posts: 87
Joined: Wed Sep 30, 2020 9:40 pm

Re: Sapling Number 5

Postby Adrienne Chan 1G » Sun Jan 24, 2021 11:14 pm

Kandyce, yes for this problem the two are interchangeable.

Jaclyn Schwartz 1I
Posts: 94
Joined: Wed Sep 30, 2020 9:45 pm

Re: Sapling Number 5

Postby Jaclyn Schwartz 1I » Sun Jan 24, 2021 11:28 pm

Yeprem wrote: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
Posts: 87
Joined: Wed Sep 30, 2020 9:36 pm

Re: Sapling Number 5

Postby joshtully » Sun Jan 24, 2021 11:37 pm

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

MichaelRaad_1F
Posts: 86
Joined: Wed Sep 30, 2020 9:35 pm

Re: Sapling Number 5

Postby MichaelRaad_1F » Sun Jan 24, 2021 11:42 pm

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
Posts: 85
Joined: Wed Sep 30, 2020 9:50 pm

Re: Sapling Number 5

Postby Caelin Brenninkmeijer 1G » Sun Jan 24, 2021 11:48 pm

Kandyce Lance 3E wrote: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
Posts: 86
Joined: Wed Sep 30, 2020 9:42 pm

Re: Sapling Number 5

Postby Susan Chamling 1F » Sun Jan 24, 2021 11:50 pm

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

Zihan Liu 2K
Posts: 15
Joined: Tue Jan 14, 2020 12:15 am

Re: Sapling Number 5

Postby Zihan Liu 2K » Sun Jan 24, 2021 11:57 pm

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


Return to “Equilibrium Constants & Calculating Concentrations”

Who is online

Users browsing this forum: No registered users and 2 guests