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Author Topic: Acid-base properties of salts  (Read 29894 times)

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777888

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Acid-base properties of salts
« on: December 13, 2004, 05:09:56 AM »

1. Does a strong acid/base have a Ka or Kb value? Or only a weak acid/base has one?

2. How can you predict if a salt dissolves, they will produce acidic, basic, or netural solution?

For example, why NaCl produces a neutral solution?
Why NH4Cl is an acidic salt?
Why NaC2H3O2 is a basic salt?
Why NH4CN forms basic solution?

I am totally confused of predicting whether a salt will produce neutral, acidic or basic solution. How can you know which type it will form?

Can someone teach me please? I would appreciate!
« Last Edit: December 13, 2004, 05:11:24 AM by 777888 »
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Re:Acid-base properties of salts
« Reply #1 on: December 13, 2004, 05:29:13 AM »

1.)  ALL acids and bases have Ka/Kb values.  It's just that for strong acids and bases the equillibrium is so far to the right that the Ka/Kb value is extraordinarily high.  (It's like with equillibrium constants.  All reactions have an equillibrium constant.  It's just that some reactions are so heavily favored towards the products side that we make the assumption that at equillibrium all the reactants have reacted).

2.)  This is pretty easy.  A salt will form a basic solution if the anion is that of a weak acid.  It will form an acidic solution if the cation is that of a weak base.  

a):  NaCl is a neutral salt because the sodium ion is the cation of a strong base (NaOH) and the chloride ion is the anion of a strong acid (HCl).  So when it goes into solution, the sodium is not going to pull an OH- off of water and form sodium hydroxide, thus leaving H+ ions in solution and making an acid, and the chlorine is not going to pull an H+ off of water and form HCl, thus leaving a bunch of OH- ions in solution and making a base.  Therefore, the solution will be neutral.

b):  NH4Cl is an acidic salt because when it goes into solution, the NH4 is going to pull some OH- ions from water and form the weak base NH4OH.  Since NH4OH is a weak base, it will remain in solution mostly as NH4OH, thus leaving some H+ ions floating around.  The presence of the H+ ions makes the solution acidic.

c):  NaC2H3O2 is a basic salt because the acetate ion will pull an H+ ion from water to form the weak acid CH3COOH (Acetic acid).  Since acetic acid is weak, it will mostly remain in water as is which will leave some OH- ions floating around.  The OH- ions floating around makes the solution basic.

d):  With NH4CN you have the cation from a weak base (NH4OH) and the anion from a weak acid (HCN).  However, HCN is a weaker acid than NH4OH is a weak base.  So the CN- ion will pull more H+ ions from water than NH4+ will pull OH- ions.  As a result, there will be more OH- ions floating around at equillibrium which makes the solution basic.  

So when you have the conjugate acids and bases of a weak acid and a weak base involved in a salt, the weaker of the two will predominate when determining if a solution is acidic or basic.  (In the NH4CN example, HCN is a weaker acid than NH4OH is a weak base, so when determining if the solution will be basic or acidic, you disregard the 'stronger' acid/base and make your determination based upon the ion that is leftover.
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777888

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Re:Acid-base properties of salts
« Reply #2 on: December 13, 2004, 06:42:53 AM »

Thank you very much for your help :)

My textbook says:
-If the cation is the conjugate acid of a weak base, it will make the solution more acidic.
-If the anion is the conjugate base of a weak acid, it will make the solution more basic.

WHY?

According to the concept:
"The stronger an acid, the weaker its conjugate base, and conversely, the weaker an acid, the stronger its conjugate base."

Then, would the conjugate acid of a weak base be a strong acid?

Also, how can you determine if an acid/base is strong or weak? Is there a gerneral rule to do that?

Is it true that the smaller the Ka or Kb value, the weaker the acid/base?

Thank you again!
« Last Edit: December 13, 2004, 06:45:07 AM by 777888 »
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Re:Acid-base properties of salts
« Reply #3 on: December 13, 2004, 07:31:42 AM »

Correct.  The smaller the Ka of an acid, the weaker the acid is.  The smaller the Kb of a base, the weaker the base is.  

Now for what your textbook says.  When you look at a Ka or Kb value, it's really just an equillibrium constant stating what the concentration of the undissociated and dissociated acid/base will be at equillibrium.  If you have a weak acid, like acetic acid, the small Ka means that at equillibrium, you will have more undissociated acetic acid in solution than you will acetate and hydronium ion.  Now look at the salt sodium acetate.  It is composed of the conjugate acid of a VERY strong base (NaOH), and the conjugate base of a pretty weak acid (Acetic acid).  So when it goes into solution, the equillibrium between NaOH and (OH- + Na+) will be favored towards the Na+ and OH-.  There will be no formation of NaOH.  For the acetate part of it, the equillibrium constant will favor the formation of acetic acid and not acetate and hydronium ion.  (I.E. there will be more CH3COOH in solution than there will be H+ and CH3COO-).  So when it's all at equillibrium, the acetate ion would rather be in the form of acetic acid.  Therefore, it will pull an H+ off of water leaving both items in their equillibrium favored state.  (The CH3COO- as CH3COOH and the Na+ as Na+ and OH-).  Therefore, the presence of the OH- ions makes the solution basic.  The reverse can be applied for bases.  If you have NH4Cl, you have the conjugate acid of a weak base (NH4OH) and the conjugate base of a strong acid (HCl).  Therefore, at equillibrium the conjugate acid would rather exist as the undissociated base (NH4OH) than it would as the conjugate acid (NH4+).  The conjugate base would rather exist as the conjugate base than it would the undissociated acid (HCl).  Therefore, it will remain in solution as the H+ and Cl- ions.  As a result, the solution is acidic.

The conjugate acid of a weak base really wouldn't be considered a strong acid since when put into solution, it doesn't completely form the weak base and H+ ions.  A small bit of it does, but not a large amount.  Therefore, it would be considered an average acid.  (Please correct me someone if I'm wrong).  What determines if an acid/base is strong or weak is the completion of its dissociation.  An acid/base that completely dissociates into its ions when placed in an aqueous solution is considered a strong acid/base.  Therefore, you can go by it's Ka/Kb value.  Sodium hydroxide, when placed in water, is pretty much 100% dissociated.  Therefore, it's considered a strong base.  The same can be said for hydrochloric acid.  There is an exact number for the Ka/Kb that can be used, but I can't recall it off the top of my head.  (Also, please note that for sulfuric acid the acid is considered a strong acid but only for the first hydrogen.  That is, when placed in solution H2SO4 will completely dissociate into HSO4- and H+.  But the HSO4- is a weak acid and only partially dissociates into H+ and SO4-).  
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777888

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Re:Acid-base properties of salts
« Reply #4 on: December 13, 2004, 09:31:26 AM »

Oh I see! Thanks!!

How can you know if an acid/base is strong or weak? Is there a gerneral rule to do that? (The teacher told us to memorize the strong and weak acids/bases. But I don't know how...there is an infinite number of compounds...)
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Re:Acid-base properties of salts
« Reply #5 on: December 13, 2004, 10:57:53 AM »

Basically you just need to memorize the strong acids and bases, and consider everything else weak.  It's like memorizing polyatomic ions.  There are far fewer strong acids/bases, so it's pretty easy to memorize them.

Strong acids:  Sulfuric acid, Nitric acid, Hydrohalic acids (Except HF), Halic acids (HClO3, HBrO3, HIO3), Perhalic acids (HClO4, HBrO4, HIO4).  Everything else is weak.  (Though that's kind of a lie.  There are other strong acids, but they are generally only encountered/used is specialist work.  If you were going to come across those acids, you'd already know everything we're going over here.  So just consider everything that's not listed above to be a weak acid).

Strong bases:  LiOH, NaOH, KOH, RbOH, CsOH, Mg(OH)2, Ca(OH)2, Sr(OH)2, Ba(OH)2, Group 1 and Group 2 amides (I.E. KNH2).  Now, the group 2 hydroxides listed above are considered strong bases because the teency bit of them that dissolve will completely dissociate.  

The definition of a strong acid or strong base is a compound that 100%, completely dissociates in water forming the H+ and OH- ions depending on whether it's an acid or a base.  All the strong bases and acids are listed above.  If it's not mentioned, then it's not considered as a strong acid/base.  

The best place to look this stuff up is your textbook.  Different textbooks may have slightly different lists, so if you're going to be tested on something I'd go by what's in your textbook.
« Last Edit: December 13, 2004, 03:31:19 PM by jdurg »
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777888

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Re:Acid-base properties of salts
« Reply #6 on: December 13, 2004, 02:10:23 PM »

Thanks!
For NH4C2H3O2,
Since NH4+ is conjugate acid of weak base(NH3) =>acidic
Since C2H3O2- is conjugate base of weak acid(Hc2H3O2) => basic

NH4+ + H2O <-> NH3 + H3O+        (Ka=5.8x10-10)
C2H3O2- + H2O <-> HC2H3O2 + OH- (Kb=5.6x10-10)(calculated)

Since Ka>Kb, it will form acidic soltuion. AM I RIGHT?



I don't know how to predict these ones:
(NH4)2SO4, MgO, Na2SO3

For (NH4)2SO4->2NH4+ + SO4 2-
I tried to explain:
Since NH4+ is conjugate acid of weak base(NH3) =>acidic
Since SO4 2- is conjugate base of HSO4- (WEAK ACID ?!?? OR do we look at H2SO4?)...Then what solution would the salt form?

For MgO->Mg2+ + O2-
Since Mg2+ is conjugate acid of strong base(Mg(OH)2) =>no effect on pH
How about O2-?

For Na2SO3, is SO3(2-) conjugate base of weak acid(HSO3- OR do we look at  H2SO3?)?? If so, basic solution will be formed, right?
« Last Edit: December 13, 2004, 02:42:52 PM by 777888 »
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Re:Acid-base properties of salts
« Reply #7 on: December 13, 2004, 03:30:53 PM »

1).  When deciding if a salt is acidic or basic, you have to look at the Ka/Kb of the related acid/base.  NH4+ is not the conjugate acid of NH3.  It is the conjugate acid of NH4OH, so you'd have to look up the Kb of NH4OH.  For the acetate portion, you'd look up the Ka of acetic acid.  If acetic acid has a larger Ka than NH4OH has Kb, then the salt will form an acidic solution since the NH4+ will take over and form NH4OH which will leave a bunch of H+ ions in solution.  So since acetic acid is a stronger acid than NH4OH is a weak base, you would disregard the acetate ion and make your decision solely on the NH4+ ion.  Therefore, the solution would be acidic.

(Ka Acetic Acid = 9.5x10^-5.  Kb NH4OH = 1.8x10^-5).  So your end result is correct.   ;D  (Though the method of getting there may not have been).   :P


For (NH4)2SO4, the conjugate acid is the NH4+ ion which is the conjugate acid of the weak base NH4OH.  The conjugate base is the SO4(-2) ion which is the conjugate base of a weak acid (HSO4-).  Therefore, you just need to look up the Ka of HSO4-, and the Kb of NH4OH.  If the Ka value you find is greater than the Kb value you find, you would make your assumption based upon the NH4+ ion.  If you find that the Ka is smaller than the Kb, you would make your decision based upon the SO4(-2) ion.   :)

For MgO, you need to remember some rules about metal oxides.  All metal oxides form basic solutions.  Therefore, when the MgO moves into solution, it forms Mg(OH)2 which doesn't really dissolve all that much, but the portion that does goes to completion.  (I forgot about Mg(OH)2 in my earlier post.  It has now been edited).  So the solution will be basic.

For Na2SO3, you are correct in using HSO3- as the parent acid for the conjugate base SO3(-2).  So the solution will be basic.   :D
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Re:Acid-base properties of salts
« Reply #8 on: December 13, 2004, 07:04:16 PM »

Quote
Ka Acetic Acid = 9.5x10^-5

!?
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jdurg

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Re:Acid-base properties of salts
« Reply #9 on: December 14, 2004, 02:02:03 AM »

!?

That's the value I found by doing a quick google search.  Ahhhhh... but that value was not at STP.  Okay.  I made a boo-boo.   ;D
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Re:Acid-base properties of salts
« Reply #10 on: December 14, 2004, 04:07:45 AM »

I see :)
For (NH4)2SO4,
Since NH4+ is conjugate acid of weak base=>acidic
Since SO4(2-) is conjugate acid of strong base(H2SO4)=>no effect on pH

*H2SO4 is diprotic, so forms HSO4- and then SO4(2-)

Therefore the solution is acidic!? right?
« Last Edit: December 14, 2004, 04:15:20 AM by 777888 »
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Re:Acid-base properties of salts
« Reply #11 on: December 14, 2004, 04:40:36 AM »

I see :)
For (NH4)2SO4,
Since NH4+ is conjugate acid of weak base=>acidic
Since SO4(2-) is conjugate acid of strong base(H2SO4)=>no effect on pH

*H2SO4 is diprotic, so forms HSO4- and then SO4(2-)

Therefore the solution is acidic!? right?

Incorrect.  Only the first proton of sulfuric acid completely dissociates.  The second one does not fully dissociate, so it's considered a weak acid.  Sulfuric acid overall is a strong acid because the first proton completely dissociates when in solution.  The second one does not.  SO4(2-) is the conjugate base of a weak acid (HSO4-).  For multi-protic acids, when determining if a salt forms an acidic or basic solution, you only move back one place.  (I.E. the acid you look up has one more proton than the conjugate base).  If the conjugate base was the bisulfate ion (HSO4-), then you would use H2SO4.  But since in (NH4)2SO4 you only have the SO4(2-) ion in solution, you would have to look at HSO4- instead.  The only time you would look back to the fully undissociated acid is if ALL of the forms are considered to be strong acids.  So if H2SO4 AND HSO4- were strong acids, then you'd use H2SO4 when making your determination.  But for sulfuric acid, only the first proton is considered to be a strong acid.  The second one (HSO4-) is a weak acid.  (I actually do not know of any multi-protic acids which are considered 'strong' for all of the protons).  

So for this example, you need to find the Kb of NH4OH and the Ka of HSO4-.  Whichever one has a larger value would then be 'disregarded' when determining if the solution would be acidic or basic.  (So if the Ka of HSO4- was greater than the Kb of NH4OH, the solution would be acidic and vice-versa)
« Last Edit: December 14, 2004, 04:41:58 AM by jdurg »
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Re:Acid-base properties of salts
« Reply #12 on: December 14, 2004, 05:12:11 PM »

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)3PO4 be a basic salt?

THANK YOU FOR YOUR TIME!
« Last Edit: December 15, 2004, 04:33:41 AM by 777888 »
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Re:Acid-base properties of salts
« Reply #13 on: December 14, 2004, 08:17:34 PM »

One more question...
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

Then should I say the solution is acidic or approximately NETURAL?

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Re:Acid-base properties of salts
« Reply #14 on: December 14, 2004, 10:33:33 PM »

Quote
Then should I say the solution is acidic or approximately NETURAL?

aproximately NEUTRAL
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