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Topic: pH of weak base + strong base  (Read 534 times)

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Offline sharbeldam

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pH of weak base + strong base
« on: March 26, 2021, 10:37:48 AM »
If we add 0.008 moles of strong base like NaOH to 1 liter of ammonia with concentration of 15.3M , what would the pH be?

Now in all my years of studying chem alone, i have never encountered such problem, usually we can ignore the OH- that comes from the weak base but not in this case since its very concentrated so what is the way?

NH3+H2O <----> NH4++OH-

i can do an ice table for the weak base alone, i'd get the concentration of OH- alone and then the moles, and then can i just add the moles of OH- from the strong base to that and calculate new concentration and then pOH and then pH?
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Offline chenbeier

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Re: pH of weak base + strong base
« Reply #1 on: March 26, 2021, 12:01:12 PM »
You need the equilibrium constant for the reaction NH3 + H2O =》NH4 + + OH-
If you add Hydroxide the equlibrium will pushed to the left side, ammonia gas will pushed out.
I think it works like an alkaline buffer. Probably can use HH equation. In the logarithmen you have decrease of NH4+ And increase of NH3

Offline sharbeldam

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Re: pH of weak base + strong base
« Reply #2 on: March 26, 2021, 04:10:34 PM »
i have the kb, can you tell me the values for the HH formula? since i dont have NH4+?
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Offline chenbeier

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Re: pH of weak base + strong base
« Reply #3 on: March 26, 2021, 04:20:58 PM »
You know the dissociation of the ammonia in water. You have 15.3 mol/ l  total.
pH = pka + log(cNH4+/cNH3). The change will be (cNH4+ - cOH-)/ (cNH3 + cOH-)

Offline sharbeldam

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Re: pH of weak base + strong base
« Reply #4 on: March 26, 2021, 04:24:40 PM »
so i calculate the concentration of NH4+ and NH3 in equilibrium first alone using the ice table of the weak base, and then i do what you said in HH equation?

also 1 more question, why isnt my method correct? to just add the moles of OH- together from weak base and addition of OH-?
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Online Orcio_Dojek

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Re: pH of weak base + strong base
« Reply #5 on: March 26, 2021, 04:40:26 PM »
NH3 + H2O ---> NH4+ + OH-

K = [NH4+] x [OH-] / [NH3] = 1,77 x 10^-5

Concentration of NH3 will not change, it will still be around 15,3 M.

Concentration of NH4+ is concentration of ammonia x it's dissociation degree.

Concentration of OH- is concentration of ammonia x it's dissociation degree plus concentration of NaOH (it is fully dissociated).

You need only to calculate quadratic equation (ax2 + bx + c = 0).

Offline sharbeldam

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Re: pH of weak base + strong base
« Reply #6 on: March 26, 2021, 04:52:45 PM »
Thank you first of all,
Is this also right in the picture? i can just find the initial concentration and hence moles of [OH]- and just simple add the moles of the hydroxide solid that i added and then calculate pH.

also you said NH3 doesnt change, but if we added OH- doesnt the reaction shift to the left and NH3 should increase a little?

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Online Orcio_Dojek

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Re: pH of weak base + strong base
« Reply #7 on: March 26, 2021, 05:11:56 PM »
Quote
Is this also right in the picture? i can just find the initial concentration and hence moles of [OH]- and just simple add the moles of the hydroxide solid that i added and then calculate pH.

No, because as you said:

Quote
if we added OH- doesnt the reaction shift to the left

(your result will differ from true)

Quote
also you said NH3 doesnt change, but if we added OH- doesnt the reaction shift to the left and NH3 should increase a little?

Yes, but it is very small change (less than 1 %) :)
« Last Edit: March 26, 2021, 05:24:54 PM by Orcio_Dojek »

Offline sharbeldam

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Re: pH of weak base + strong base
« Reply #8 on: March 26, 2021, 05:20:25 PM »
like this just to be sure yes?
ahhh no, it should shift to the left, im frickin confused
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Re: pH of weak base + strong base
« Reply #9 on: March 26, 2021, 05:23:43 PM »
Now it is correct.

Offline sharbeldam

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Re: pH of weak base + strong base
« Reply #10 on: March 26, 2021, 05:26:49 PM »
I really understand but its still a bit confusing because before the addition of OH-, what i have in the flask? NH3, NH4+ and OH-, NH3 Is 15.3 all the way, NH4+ and OH- are both x (that we can calculate).

after the addition that x should become smaller because the reaction goes to the left, so NH3 increase a tiny bit but we ignore that, but the concentration of NH4+ should decrease (x-y) and the OH- should also decrease so it would be like x+0.008-y, do you get my point?
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Re: pH of weak base + strong base
« Reply #11 on: March 26, 2021, 05:32:02 PM »
Maybe it isn't "0,008 mol of NaOH added to 15,3 M NH3" but rather a "0,008 M solution of NaOH and 15,3 M solution of NH3".

Maybe you should look at this from that point.

Offline sharbeldam

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Re: pH of weak base + strong base
« Reply #12 on: March 26, 2021, 05:37:00 PM »
it is 0.008 moles solid NaOH. but it's okay we can deal with moles, dont see the issue, especially that it's 1 liter solution of ammonia
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Re: pH of weak base + strong base
« Reply #13 on: March 26, 2021, 05:42:12 PM »
Maybe it is 1 liter and 0,008 moles, maybe 0,5 liter and 0,002 moles NaOH....

It is still question about pH, not about dissolving NaOH in ammonia solution.

Quote
and the OH- should also decrease so it would be like x+0.008-y, do you get my point?

I understand, it is wrong.

Maybe (?) it can be calculated like that but it will be more difficult.
 

Offline Borek

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Re: pH of weak base + strong base
« Reply #14 on: March 27, 2021, 05:43:43 AM »
the OH- should also decrease so it would be like x+0.008-y

0.008 is definitely larger than y, so the OH- increases.
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