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Offline jdurg

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Re:Acid-base properties of salts
« Reply #15 on: December 15, 2004, 08:42:53 AM »
Is H3PO4 a strong acid? (my text book says "there are relatively few strong acids: HCl, HBr, H2SO4m HNO3m H3PO4 are the most familiar...", it is but it's not in your list...) Also, is H2SO3 a weak acid?

For (NH4)2SO4, how can I get Kb for HSO4-? Which one should I use from the following values?

Table
Acid   Conjugate Base  Ka1        Ka2           Ka3
H2SO4   HSO4-          very large  1.0x10-2


Is this explanation of Na2SO4 salt wrong?
"Na2SO4 is an acid salt. The first ionization of H2SO4 is strong and NaOH is strong. =>neutral"

Also, would Na2CO3 be a basic salt? (there is no answer in my text :()
(i)because HCO3- is weak
(ii)because H2CO3 is weak
Why (i) and (ii) both work as an explanation? Is there a case that (ii) will not work?

And would (NH3)2PO4 be a basic salt?

THANK YOU FOR YOUR TIME!

Hmmm.  I have NEVER heard of H3PO4 being considered a strong acid.  Every book I've ever seen has put it in the same class as acetic acid.  If it was a strong acid, it would cause all sorts of problems in its use as a food additive.  I also believe that H2SO3 is a "weak" acid.  

1):  You would use the Ka2 for HSO4-.  Looking at the chart, it's giving you the Ka of all parts of sulfuric acid.  Ka1 is VERY large because the H2SO4 easily gives up its first proton.  (That's why it's a strong acid).  HSO4- doesn't give it up nearly as easily so it's a weak acid with a the Ka being 1.0x10-2 (The Ka2 for H2SO4 is the Ka for the ion HSO4-).  So if you go by my method of acid/base determination, you would look at the Ka of HSO4- and compare it to the Kb of NH4OH.  If the Ka of HSO4- is greater than the Kb of NH4OH, your solution will be acidic.  For your explanation, I'd go by something like this:  "Na2SO4 is a basic salt because the SO4(2-) ion is the conjugate base of a weak acid (HSO4-), and the Na+ ion is a conjugate acid of a strong base (NaOH).  Therefore in solution, at equillibrium, some of the SO4(2-) will pull a proton off of water which would form the stable HSO4- ion.  The OH- ions will remain in solution, thus making it basic".

2):  Na2CO3 would be a basic salt.  For your reason you have to remember that when dealing with a multi-protic acid you only go back ONE step.  So for the PO4(3-) ion, you would only go back to HPO4(-2), not H2PO4-, or H3PO4.  For ANY conjugate base, when you are determining if it will form a basic or acidic solution you only go back one step.  So the answer to this question would be '(i)because HCO3- is weak' since HCO3- is one step back from CO3(2-).  

3):  (NH3)2PO4.  Hmmm.  I did not know that salt existed.  NH3 is a neutral molecule, and PO4 has a -3 charge.  So something is definitely not right there.
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Offline jdurg

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Re:Acid-base properties of salts
« Reply #16 on: December 15, 2004, 08:44:15 AM »
aproximately NEUTRAL

If those are your only two choices, I also concurr with AWK.  If you can give any response, the accurate one would be 'slightly basic'.
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777888

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Re:Acid-base properties of salts
« Reply #17 on: December 15, 2004, 10:38:02 AM »
Hmmm.  I have NEVER heard of H3PO4 being considered a strong acid.  Every book I've ever seen has put it in the same class as acetic acid.  If it was a strong acid, it would cause all sorts of problems in its use as a food additive.  I also believe that H2SO3 is a "weak" acid.  

1):  You would use the Ka2 for HSO4-.  Looking at the chart, it's giving you the Ka of all parts of sulfuric acid.  Ka1 is VERY large because the H2SO4 easily gives up its first proton.  (That's why it's a strong acid).  HSO4- doesn't give it up nearly as easily so it's a weak acid with a the Ka being 1.0x10-2 (The Ka2 for H2SO4 is the Ka for the ion HSO4-).  So if you go by my method of acid/base determination, you would look at the Ka of HSO4- and compare it to the Kb of NH4OH.  If the Ka of HSO4- is greater than the Kb of NH4OH, your solution will be acidic.  For your explanation, I'd go by something like this:  "Na2SO4 is a basic salt because the SO4(2-) ion is the conjugate base of a weak acid (HSO4-), and the Na+ ion is a conjugate acid of a strong base (NaOH).  Therefore in solution, at equillibrium, some of the SO4(2-) will pull a proton off of water which would form the stable HSO4- ion.  The OH- ions will remain in solution, thus making it basic".

2):  Na2CO3 would be a basic salt.  For your reason you have to remember that when dealing with a multi-protic acid you only go back ONE step.  So for the PO4(3-) ion, you would only go back to HPO4(-2), not H2PO4-, or H3PO4.  For ANY conjugate base, when you are determining if it will form a basic or acidic solution you only go back one step.  So the answer to this question would be '(i)because HCO3- is weak' since HCO3- is one step back from CO3(2-).  

3):  (NH3)2PO4.  Hmmm.  I did not know that salt existed.  NH3 is a neutral molecule, and PO4 has a -3 charge.  So something is definitely not right there.

Sorry, I mistyped the salt. It should be (NH3)3PO4.

(NH4)2SO4 <-> 2NH4+ +SO4(2-)
Can I compare the Ka for NH4+ to the Kb for HSO4-?

Kb HSO4- =1.0x10-14 / 1.0x10-2
If Ka NH4+ > Kb HDO4- , acidic solution , and vice versa

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Re:Acid-base properties of salts
« Reply #18 on: December 15, 2004, 10:39:58 AM »
If those are your only two choices, I also concurr with AWK.  If you can give any response, the accurate one would be 'slightly basic'.
WHY basic ???
Ka NH4+ > Kb C2H3O2 - , so it should be acidic, right?
NH4C2H3O2 -> NH4+ + C2H3O2 -

NH4+ + H2O <-> NH3 + H3O+
C2H3O2 - + H2O <-> HC2H3O2 + OH-

Ka NH4+ =5.8x10-10
Kb C2H3O2 - =1.0x10-14 / 1.8x10-5=5.6x10-10

Offline jdurg

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Re:Acid-base properties of salts
« Reply #19 on: December 15, 2004, 11:05:38 AM »
Okay.  Here's the explanation.  The way I was always taught to determine the acidity/baseness of a salt solution is to look not at the Ka or Kb of the salt itself, but at the Ka/Kb of the parent acid/base.  So for an acetate ion you wouldn't look at the Kb for CH3COO-, you'd look at the Ka of CH3COOH.  For the NH4+ ion, you'd look at the Kb of NH4OH and not the Ka of NH4+.  Now you just need to find the Ka and Kb.  From a source I found, the Ka of CH3COOH is 1.77x10^-5.  The Kb of NH4OH is 1.78x10^-5.  So the Kb of NH4OH is larger than the Ka of acetic acid.  Therefore to determine the acidity/baseness of the salt, you would only look at the CH3COO- ion.  Therefore the salt would be INCREDIBLY slightly basic.  Though this determination can change depending on your sources.  So 'Neutral' would be the proper response if your Ka/Kb values were only out to one decimal place.  (Since they'd then both be 1.8x10^-5).
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Offline jdurg

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Re:Acid-base properties of salts
« Reply #20 on: December 15, 2004, 11:12:28 AM »
I still think that the (NH3)3PO4 should be (NH4)3PO4 because NH3 is a neutral compound.  It has no charge.  So I'd triple check on that.   ;D

So assuming that it's (NH4)3PO4, you'd take a look at the Ka of HPO4(2-) and the Kb of NH4OH.  If the Ka of HPO4(2-) is larger than the Kb of NH4OH then your solution would be acidic.  If the Ka of HPO4(2-) is smaller than the Kb of NH4OH, then your solution would be basic.
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Re:Acid-base properties of salts
« Reply #21 on: December 15, 2004, 05:26:03 PM »
YES IT's (NH4)3PO4   :P

I searched the internet and most of them says Na2SO4 is a neutral salt... how come?

Offline jdurg

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Re:Acid-base properties of salts
« Reply #22 on: December 15, 2004, 05:33:59 PM »
YES IT's (NH4)3PO4   :P

I searched the internet and most of them says Na2SO4 is a neutral salt... how come?

They're saying that because the pH of a 5% sodium sulfate solution at 25 degrees Celcius is 7.2.  So in reality it is basic, but depending on how anal you want to get it's basically neutral.   ;D
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Re:Acid-base properties of salts
« Reply #23 on: December 17, 2004, 03:26:13 AM »
Okay.  Here's the explanation.  The way I was always taught to determine the acidity/baseness of a salt solution is to look not at the Ka or Kb of the salt itself, but at the Ka/Kb of the parent acid/base.  So for an acetate ion you wouldn't look at the Kb for CH3COO-, you'd look at the Ka of CH3COOH.  For the NH4+ ion, you'd look at the Kb of NH4OH and not the Ka of NH4+.  Now you just need to find the Ka and Kb.  From a source I found, the Ka of CH3COOH is 1.77x10^-5.  The Kb of NH4OH is 1.78x10^-5.  So the Kb of NH4OH is larger than the Ka of acetic acid.  Therefore to determine the acidity/baseness of the salt, you would only look at the CH3COO- ion.  Therefore the salt would be INCREDIBLY slightly basic.  Though this determination can change depending on your sources.  So 'Neutral' would be the proper response if your Ka/Kb values were only out to one decimal place.  (Since they'd then both be 1.8x10^-5).
But my text book says that
"If the salt has a cation and an anion that can both hydrolyze, compare the Ka and Kb values of the cation and anion. If Ka>Kb, then the solution will become more acidic. If Ka<Kb, the solution will become more basic. If Ka=Kb, the solution will be neutral"

And I also works for this one:
NH4CN-> NH4+ +CN-
Ka NH4+ =5.8x10^-10
Kb CN-=1.6x10^-5
Kb>Ka
Therefore basic! (and this is true!)

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