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Topic: Buffer capacity in regards to strong acids and weak base?  (Read 2839 times)

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Offline Bruhmoment

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Buffer capacity in regards to strong acids and weak base?
« on: June 23, 2021, 02:25:20 AM »
For context I’ve got this problem here as part of a worksheet


“ consider a buffer solution prepared by adding .100 M HCl to 183 mL of .200 M ammonia. at what volume in mL of hydrochloride acid added will the buffer subsequently achieve its highest buffer capacity”


I’m completely lost as to how to solve this. I understand buffer capacity is higher when the ration of the reactants is close to 1:1? But how would this apply here?

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Re: Buffer capacity in regards to strong acids and weak base?
« Reply #1 on: June 23, 2021, 02:38:21 AM »
I understand buffer capacity is higher when the ration of the reactants is close to 1:1?

What are these "reactants" you mentioned?
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Offline Bruhmoment

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Re: Buffer capacity in regards to strong acids and weak base?
« Reply #2 on: June 23, 2021, 09:14:09 AM »
I understand buffer capacity is higher when the ration of the reactants is close to 1:1?

Alright so I was wrong, turns out it’s when the weak acid and it’s salt are in proportion that the buffer is at its highest. However, there is only HCl, a strong acid here.

What are these "reactants" you mentioned?

Offline Babcock_Hall

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Re: Buffer capacity in regards to strong acids and weak base?
« Reply #3 on: June 23, 2021, 09:26:42 AM »
It is never a bad idea to write a chemical equation.

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Re: Buffer capacity in regards to strong acids and weak base?
« Reply #4 on: June 23, 2021, 09:28:21 AM »
it’s when the weak acid and it’s salt are in proportion that the buffer is at its highest.

This is a common misconception: buffer is not something consisting of an acid and its salt. Buffer is a mixture of an acid and its conjugate base, no matter where they come from.

Can you try to apply it here?
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Offline Bruhmoment

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Re: Buffer capacity in regards to strong acids and weak base?
« Reply #5 on: June 23, 2021, 11:06:15 AM »
it’s when the weak acid and it’s salt are in proportion that the buffer is at its highest.

This is a common misconception: buffer is not something consisting of an acid and its salt. Buffer is a mixture of an acid and its conjugate base, no matter where they come from.

Can you try to apply it here?


So if I were to add NH3 to HCl, the resulting products would be Nh4 and Cl-

NH3 + HCl = NH4 + Cl-

So NH4 is going to be the weak acid, and NH3 is going to be the salt/conjugate base?

So adding HCl would increase the amount of NH4+ and Cl- 

Am I simply trying to find the volume at which the .0366 Mol of NH3 is 1:1 with the HCl? That would give an ander answer of 366 Ml of HCl, but I have no idea if that’s how to solve it.

Offline Bruhmoment

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Re: Buffer capacity in regards to strong acids and weak base?
« Reply #6 on: June 23, 2021, 11:46:32 AM »
it’s when the weak acid and it’s salt are in proportion that the buffer is at its highest.

This is a common misconception: buffer is not something consisting of an acid and its salt. Buffer is a mixture of an acid and its conjugate base, no matter where they come from.

Can you try to apply it here?


Wait hold on a minute


Is possible that all I have to do is

M1 V1 = M2 V2

So therefore my answer is 366 Ml?

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Re: Buffer capacity in regards to strong acids and weak base?
« Reply #7 on: June 23, 2021, 01:56:27 PM »
Cl- is NOT the conjugate base of NH4+.
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Offline Bruhmoment

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Re: Buffer capacity in regards to strong acids and weak base?
« Reply #8 on: June 23, 2021, 02:04:00 PM »
Cl- is NOT the conjugate base of NH4+.


Yeah I’m sorry but I’ve been at this problem all day trying to study for an and I’m unable to start it. I would appreciate a walkthrough but the furthest I’ve gotten is realizing that NH3 is the conjugate base of NH4+. I’m not sure how to find how much HCl I need for buffer capacity from there

Offline Bruhmoment

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Re: Buffer capacity in regards to strong acids and weak base?
« Reply #9 on: June 23, 2021, 02:09:24 PM »
Cl- is NOT the conjugate base of NH4+.


Yeah I’m sorry but I’ve been at this problem all day trying to study for an and I’m unable to start it. I would appreciate a walkthrough but the furthest I’ve gotten is realizing that NH3 is the conjugate base of NH4+. I’m not sure how to find how much HCl I need for buffer capacity from there



I know buffer capacity is at its highest when the weak acid and conjugate base are thus at nearly a 1:1 ration in concentration.

So adding HCl would end up treating more conjugate acid. So am I trying to find how much HCL would creat 0.0366 mole of NH4+?

Offline Babcock_Hall

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Re: Buffer capacity in regards to strong acids and weak base?
« Reply #10 on: June 23, 2021, 02:13:20 PM »

So if I were to add NH3 to HCl, the resulting products would be Nh4 and Cl-

NH3 + HCl = NH4 + Cl-

So NH4 is going to be the weak acid, and NH3 is going to be the salt/conjugate base?

So adding HCl would increase the amount of NH4+ and Cl- 

Am I simply trying to find the volume at which the .0366 Mol of NH3 is 1:1 with the HCl? That would give an ander answer of 366 Ml of HCl, but I have no idea if that’s how to solve it.
This is really more Borek's area than mine, but maybe writing an ICE table would help.  Minor point:  I would not call NH3 a salt; it is the conjugate base of the ammonium ion.

Offline Bruhmoment

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Re: Buffer capacity in regards to strong acids and weak base?
« Reply #11 on: June 23, 2021, 02:28:51 PM »

So if I were to add NH3 to HCl, the resulting products would be Nh4 and Cl-

NH3 + HCl = NH4 + Cl-

So NH4 is going to be the weak acid, and NH3 is going to be the salt/conjugate base?

So adding HCl would increase the amount of NH4+ and Cl- 

Am I simply trying to find the volume at which the .0366 Mol of NH3 is 1:1 with the HCl? That would give an ander answer of 366 Ml of HCl, but I have no idea if that’s how to solve it.
This is really more Borek's area than mine, but maybe writing an ICE table would help.  Minor point:  I would not call NH3 a salt; it is the conjugate base of the ammonium ion.

I see. For some reason my Professor calls it a salt so I’ve just been going with that.

. I’ve tried a few ice charts but at this point I’m starting to think either my answer key is wrong or I’m just not grasping something.

I’m starting charts with either NH3 + HCl or NH3 + H20 but I’m just not getting the right answer no matter what I do. I’ve been trying to solve this problem since about 8 am now so I’ve pretty much just given up.

Offline Babcock_Hall

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Re: Buffer capacity in regards to strong acids and weak base?
« Reply #12 on: June 23, 2021, 03:15:19 PM »
"I know buffer capacity is at its highest when the weak acid and conjugate base are thus at nearly a 1:1 ration in concentration."  This is correct.  If you added 366 mL of 0.1 M HCl, how much NH3 remains at this point?
« Last Edit: June 23, 2021, 03:25:40 PM by Babcock_Hall »

Offline Bruhmoment

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Re: Buffer capacity in regards to strong acids and weak base?
« Reply #13 on: June 23, 2021, 03:58:16 PM »
"I know buffer capacity is at its highest when the weak acid and conjugate base are thus at nearly a 1:1 ration in concentration."  This is correct.  If you added 366 mL of 0.1 M HCl, how much NH3 remains at this point?


Okay so from my understanding

We make an ice table


NH3 + HCl = NH3 + Cl-

This is jn equilibrium and we are putting Everything in concentrations, so molarity.

We have an initiation 0.2 Molarity NH3 and a 0.1 Molarity HCL

The HCl is used up entirely giving us 0.1 NH3 left over, and creating 0.1 M NH4+ and 0.1 M Cl-

This means that the NH3 and the NH4+ are in equal concentration, so buffer capacity is reached.


This means that with 0.1 M at 183 mL, we have 0.0183 Mol of NH3.

So we would need 0.0183 Mol of HCl, and at 0.1 M that is 183 ml.

Is that it?

Offline Babcock_Hall

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Re: Buffer capacity in regards to strong acids and weak base?
« Reply #14 on: June 23, 2021, 04:08:58 PM »
I don't see any errors.  I usually write my table in terms of moles or millimoles and only convert to molarity afterward, but other people may do it differently.
« Last Edit: June 23, 2021, 04:55:34 PM by Babcock_Hall »

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