Acid-Base and pH
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01-10-2016, 02:18 PM
Acid-Base and pH
If 500.0 mL of a 0.20 M solution of ammonia is mixed with 500.0 mL of a 0.20 M solution of HBr, what is the pH of the resulting solution of NH4Br ?

The problem listed above is really getting to me. Frusty

I'm in Chem 2 and have been doing well so far. When it comes to acid-base interpretation I find myself overthinking it. The book hasn't really been very useful on this. Any help or suggestions guys? Please!!! I want to have a solid grasp on this for exams.
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01-10-2016, 03:09 PM
RE: Acid-Base and pH
500 mL combined 500 mL is 1 L, so the volume of the mixture is 1 L.

To find the number of moles and thereby the concentrations of each: n=CV

HBr n= (.20 M)(1 L)
HBr n= .20 moles

NH3 n= (.20 M)(1 L)
NH3 n= .20 moles

So the starting concentrations of each would have to be

[HBr] = .20 mol/L
[NH3] = .20 mol/L

right?
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01-10-2016, 03:46 PM
RE: Acid-Base and pH
(01-10-2016 03:09 PM)Grimm Wrote:  500 mL combined 500 mL is 1 L, so the volume of the mixture is 1 L.

To find the number of moles and thereby the concentrations of each: n=CV

HBr n= (.20 M)(1 L)
HBr n= .20 moles

NH3 n= (.20 M)(1 L)
NH3 n= .20 moles

So the starting concentrations of each would have to be

[HBr] = .20 mol/L
[NH3] = .20 mol/L

right?

That's the right track, but don't use the diluted volume (1.00 L) unless you're also going to use the diluted concentrations.

Easier to just use the original molarities and volumes.

Not that you really need to. pH is dependant on concentration, not moles or volume. Since n is constant, n = C1V1 = C2V2, which can be easily rearranged to find C2.

Now look up the dissociation constant for ammonium bromide. It looks like a weak acid so [H3O+] won't be the same as [NH4Br].

---
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01-10-2016, 04:44 PM (This post was last modified: 01-10-2016 04:53 PM by Grimm.)
RE: Acid-Base and pH
(01-10-2016 03:46 PM)Paleophyte Wrote:  That's the right track, but don't use the diluted volume (1.00 L) unless you're also going to use the diluted concentrations.

Easier to just use the original molarities and volumes.

Not that you really need to. pH is dependant on concentration, not moles or volume. Since n is constant, n = C1V1 = C2V2, which can be easily rearranged to find C2.

Now look up the dissociation constant for ammonium bromide. It looks like a weak acid so [H3O+] won't be the same as [NH4Br].

Ok so

n=M1V1=M2V2

[Hbr] = [NH3] = 0.20 M / .500 L = 0.40 M I believe.

The Ka for NH+ is 5.6 x 10^-10

What I'm also looking for is the reasoning behind the correct approach so that I can learn from this and take the correct action next time.

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

HBr + H2O ---> H3O+ + Br-

I know that Br- is a neutral salt ion and will not affect pH.
HBr is a strong acid and reacts to completion so there won't be any of it left to affect pH. NH4+ is an acidic ion and should affect pH. But shouldn't OH- and H3O+ affect pH as well? Or is it an instance of finding one will give the the answer to the other since all the coefficients in the balanced equation are 1.

I'm having a hard time knowing what to focus on and what to exclude.
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01-10-2016, 05:59 PM
RE: Acid-Base and pH
(01-10-2016 04:44 PM)Grimm Wrote:  
(01-10-2016 03:46 PM)Paleophyte Wrote:  That's the right track, but don't use the diluted volume (1.00 L) unless you're also going to use the diluted concentrations.

Easier to just use the original molarities and volumes.

Not that you really need to. pH is dependant on concentration, not moles or volume. Since n is constant, n = C1V1 = C2V2, which can be easily rearranged to find C2.

Now look up the dissociation constant for ammonium bromide. It looks like a weak acid so [H3O+] won't be the same as [NH4Br].

Ok so

n=M1V1=M2V2

Yup. Rearrange to solve for M2.

Quote:[Hbr] = [NH3] = 0.20 M / .500 L = 0.40 M I believe.

No...

If you include your units then you can use their cancellation to check your work. At least in part. In your equation above, 0.20 mol/L / 0.500 L gives you 0.40 mol/L/L, which shows that something has gone wrong.

Quote:The Ka for NH+ is 5.6 x 10^-10

What I'm also looking for is the reasoning behind the correct approach so that I can learn from this and take the correct action next time.

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

HBr + H2O ---> H3O+ + Br-

I know that Br- is a neutral salt ion and will not affect pH.
HBr is a strong acid and reacts to completion so there won't be any of it left to affect pH. NH4+ is an acidic ion and should affect pH. But shouldn't OH- and H3O+ affect pH as well? Or is it an instance of finding one will give the the answer to the other since all the coefficients in the balanced equation are 1.

I'm having a hard time knowing what to focus on and what to exclude.

You're essential reactions are:

NH4Br(s) <---> NH4+(aq) + Br-(aq)

Since ammonium bromide dissolves pretty much completely in water, at least at these concentrations, you can ignore the back reaction so it's really,

NH4Br(s) ---> NH4+(aq) + Br-(aq)

Now check your two ions. If either forms a strong conjugate acid or base then you can ignore it.

H3O+(aq) + Br-(aq) <---> HBr(aq) + H2O(l)

Technically,that happens, but HBr is a strong acid. It dissociates completely, so any HBr that actually forms is present in absurdly minute quantities and dissociates before it can be measured. So the equation is really

H3O+(aq) + Br-(aq) <--- HBr(aq) + H2O(l)

The bromide won't sequester hyronium as undissociated HBr, so you can ignore it. If you were working with NH4F, you'd have to include the fluoride ion because HF is a weak acid so it will affect the equilibrium. Fortunately, you don't have to deal with that in this case.

Ammonium on the other hand...

NH4+(aq) + H2O(l) <---> NH3(aq) + H3O+(aq)

and according to your information that has a Ka of 5.6 x 10^-10

That means that both reactions in the equilibrium are significant and the concentration of H3O+ at equilibrium will be important to the pH of the solution. Since we've already determined that the Br- won't affect the pH, it's the only reaction that matters.

To solve, write out the equation for your equilibrium constant (Ka), plug in your knowns ([NH4+]) and solve for [H3O+]. There are a couple of short-cuts that you may be able to use. Check the chapter(s) on weak acids/bases, especially the section on determining Ka/Kb and pH to see if they're applicable to this case.

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01-10-2016, 08:02 PM
RE: Acid-Base and pH
I would like to announce that I have solved the problem. The only thing I did wrong was the beginning concentrations! Facepalm

I feel so stupid. I think I was remembering earlier problems that always tried throwing in different amounts of molarities and container sizes to make sure you were paying attention.

M1V1 = M2V2

(.20 mol/L)(.5 L) = M2(1.0 L)

.1 mol = M2(1.0 L)

.1 mol/L = M2

ka = [H3O+][NH3] / [NH4+]


5.6 x 10^-10 = [x^2] / [.10 - x] With such a small k value I can use the 5% rule.

5.6 x 10^-10 = [x^2] / [.10]

5.6 x 10^-11 = [x^2]

7.48 x 10^-6 = x = [H3O+]


-log(7.48 x 10^-6) = 5.13 = pH

If I use this in my ICE table, I get the right answer. pH=5.13

Thank you Paleophyte!! BowingBowingBowing I'm not worthy!
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01-10-2016, 08:42 PM
RE: Acid-Base and pH
(01-10-2016 08:02 PM)Grimm Wrote:  I would like to announce that I have solved the problem. The only thing I did wrong was the beginning concentrations! Facepalm

Sounds like the sort of mistake I'm infamous for. I'd always have one mistake on the final where I'd divided by 2 instead of multiplying or something equally ingenious. Facepalm

The rest looks great. Don't you love it when you don't have to solve a quadratic for weak acids/bases? Looks like you have it down pretty well, just be careful with those initial calculations.

---
Flesh and blood of a dead star, slain in the apocalypse of supernova, resurrected by four billion years of continuous autocatalytic reaction and crowned with the emergent property of sentience in the dream that the universe might one day understand itself.
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01-10-2016, 08:46 PM
RE: Acid-Base and pH
My knowledge of chemistry:

If I take one Raro sachet..
[Image: RARO%20-%203%20PACK%20-%20ISLAND%20GROOVE.jpg]
And add it to water and stir it in I end up with flavored water.

[Image: oscar.png]
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01-10-2016, 09:28 PM
RE: Acid-Base and pH
Paleophyte, yes I love that little rule that saves me from twice the work. I definitely have to watch my concentrations in the future as you've said. By the way what is your field of study or area of interest?

earmuffs, so that's some sort kool aid? (or better?)

Never had a Raro sachet or any particular sachet. Where can I procure some?
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02-10-2016, 12:54 AM
RE: Acid-Base and pH
Geology, geochem and geochron.

It's been a while since I last used that but analytical chem pounded it pretty thoroughly into my skull bones.

---
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