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Topic: Acid/Base Problem  (Read 11882 times)

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bewitchedsw

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Acid/Base Problem
« on: April 13, 2004, 01:47:03 PM »
We were asked to predict whether a list of salts were acidic or basic given their parent acids and bases.  I predicted that HCl + NH4OH ---> NH4Cl + H2O would yeild a ACIDIC salt, since HCl is a strong acid and NH4OH is a very weak base.  We tested the NH4OH salt solution (diluted into water) and the solution was neutral.

Why was the solution neutral and not acidic?

My guesses:

The HCl was dated 1987...Can it weaken over time?
The water somehow weakened the HCl?
Anytime a salt is produced, it is neural?

The assignment is due tomorrow and I only have to guess at the cause.  These are my guesses, I am looking for ways to prove them to myself.  I am going further in chemistry (dental school candidate) so I really WANT to understant this.  Am I on the right track with any of my guesses?

Sam
« Last Edit: April 24, 2004, 07:01:47 PM by hmx9123 »

Offline Mitch

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Re:weak base + strong acid = neutral salt?
« Reply #1 on: April 13, 2004, 02:56:15 PM »
No doubt it was suppose to make an acidic salt. Did you use enough HCl in the reaction? What was the molarity of HCl and NH4OH?
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bewitchedsw

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Re:weak base + strong acid = neutral salt?
« Reply #2 on: April 13, 2004, 03:20:16 PM »
Our instructions in our lab book said "a 1.00g sample of the salt in 5.00 mL of distilled water." However, we were instructed in the lab to use a "scoop" (not very scientific, but this is an introductory class) of a NH4Cl salt and fully dissolve it in distilled water.  We used ~1tbsp and it took a good 1/2 to 3/4 cup of water to fully dissolve it.  So, I don't know how many moles we used or what the mass was.  Do you think it had to do with the concentration of the NH4Cl in the water?

Thanks for your *delete me*

Offline Donaldson Tan

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Re:weak base + strong acid = neutral salt?
« Reply #3 on: April 13, 2004, 03:47:38 PM »
We were asked to predict whether a list of salts were acidic or basic given their parent acids and bases.  I predicted that HCl + NH4OH ---> NH4Cl + H2O would yeild a ACIDIC salt, since HCl is a strong acid and NH4OH is a very weak base.  We tested the NH4OH salt solution (diluted into water) and the solution was neutral.
U were instructed to add aq HCl to aq NH3 i suppose. aq NH3 is a weak alkali and aq HCl is a strong acid. Such combination should yield an acidic salt NH4Cl. That suggests that the final solution should be slightly acidic (after-all NH4+ isnt a strong acid).

The HCl was dated 1987...Can it weaken over time?
The water somehow weakened the HCl?
The strength of an acid is dependent on the extent of its dissociation. A strong acid dissociates completely, eg. HCl dissociates completely to yield H3O+ in solution.
« Last Edit: April 13, 2004, 03:48:12 PM by geodome »
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Offline Mitch

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Re:weak base + strong acid = neutral salt?
« Reply #4 on: April 13, 2004, 04:14:02 PM »
Our instructions in our lab book said "a 1.00g sample of the salt in 5.00 mL of distilled water." However, we were instructed in the lab to use a "scoop" (not very scientific, but this is an introductory class) of a NH4Cl salt and fully dissolve it in distilled water.  We used ~1tbsp and it took a good 1/2 to 3/4 cup of water to fully dissolve it.  So, I don't know how many moles we used or what the mass was.  Do you think it had to do with the concentration of the NH4Cl in the water?

Thanks for your *delete me*

I think you used too much water. And whatever acididity you should of seen got very diluted with 3/4 cup of water.
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bewitchedsw

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Re:weak base + strong acid = neutral salt?
« Reply #5 on: April 13, 2004, 04:38:36 PM »
Thanks Mitch!

Just to make sure that I understand, if there was just enough water to dissolve the NH4Cl, the solution should have been acidic, right?  And the reason the solution was neutral was because the H2O (neutral) diluted or neutralized the acid?

Just one more thing, the reaction of the acid + the base is a "neutralization" reaction because it produced a salt and H2O.  The salt can be acidic or basic or neutral, right?  Why is it called a neutralization reaction?



Offline Mitch

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Re:weak base + strong acid = neutral salt?
« Reply #6 on: April 13, 2004, 05:30:27 PM »
Your understanding is correct.

The simple-simple answer is, it's called a Neutralization because you made water.
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