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Topic: Which one is a base?  (Read 10271 times)

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Offline Jules18

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Which one is a base?
« on: September 01, 2009, 06:29:27 PM »
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Which of the following salts forms a basic solution when dissolved in water?
(A) NaCl
(B) (NH4)2SO4
(C) CuSO4
(D) K2CO3
(E) NH4NO3

The way I understand, they would all dissociate like this:
NaCl(s) --> Na(+) + Cl(-)

I'm pretty sure the Cl(-) is a Bronsted base, or anything with a -ve charge, but as for the solution as a whole being basic, I'm not sure what they're asking....

Basically I'm screwed. Is this something you have to just know already?
Can anyone help me figure this out or refer me to a helpful resource?

Offline renge ishyo

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Re: Which one is a base?
« Reply #1 on: September 01, 2009, 06:47:17 PM »
Your approach for NaCl is correct. Try writing the chemical reaction (specifically the net ionic equation) for each one. For example:

Na+ + Cl- + H2:rarrow: HCl + Na+ + OH-

Net Ionic (cancel spectator ions, i.e. sodium):

Cl- + H2:rarrow: HCl + OH-

Now look at the product of your equation. Notice that the charged species on the right side of the equation is an "OH-"? So NaCl will be basic when added to water (although very weakly due to the strength of HCl and OH that are formed). Try doing this with some of the others and see what you get (tip: use your memorized chart to figure out which ions are spectators and which ones aren't. For example, potassium is also a spectator ion as is nitrate...).

Offline Borek

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Re: Which one is a base?
« Reply #2 on: September 02, 2009, 02:39:25 AM »
Your approach for NaCl is correct. Try writing the chemical reaction (specifically the net ionic equation) for each one. For example:

Na+ + Cl- + H2:rarrow: HCl + Na+ + OH-

Very dangerous approach. This is at the same time wrong and right in a very confusing way.

1. It is half way correct when compared with Brønsted-Lowry theory of acids and bases.

2. It is wrong, as Cl- is so weak a base that its reaction with should be neglected. If anything reacts wth water in this case, it is Na+, that shifts pH down - but even that effect is difficult to measure.

Jules: conjugate base of weak acid is usually realtively strong, while conjugate base of strong acid is very weak. Does it help?
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Offline renge ishyo

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Re: Which one is a base?
« Reply #3 on: September 02, 2009, 01:15:38 PM »
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1. It is half way correct when compared with Brønsted-Lowry theory of acids and bases.

Yep, I think that is what she is being taught (she mentioned Bronsted in the first post), so I am falling in line with what the answer would be if you take that approach (even though I did add a disclaimer along the lines of the fact you mentioned, that this solution would only be VERY weakly basic).  I also left out the equilibrium in the equation on purpose as when you are taught bronsted theory you think that all chemical reactions are one way at that point. I do know one thing, that she is not supposed to know that water can complex with sodium ions yet ;)

From the way that the problem was asked I assume that this is a question from that section and they are supposed to predict the outcome based on finding whether or not the reacting species contains either a bronstead acid or bronstead base (each one can be answered this way if you ignore the spectator ions). I don't think that she can answer that NaCl is neither, can she? I could be wrong though...

Offline Borek

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Re: Which one is a base?
« Reply #4 on: September 02, 2009, 02:23:39 PM »
I do know one thing, that she is not supposed to know that water can complex with sodium ions yet ;)

It doesn't matter whether it is complex or not. There is no reason to treat Cl- and Na+ differently - they are both classified as Bronsted-Lowry acid/base (both very weak ones), your reaction equation suggest that one of them is different. That contradicts universality of the Bronsted-Lowry theory.
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Offline renge ishyo

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Re: Which one is a base?
« Reply #5 on: September 02, 2009, 04:34:49 PM »
Well, I am not going to argue this point (because I know you're correct ;)).  I think that, by even mentioning that sodium can react with water, we have long since scared off the student who started the thread anyhow.

Offline Jules18

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Re: Which one is a base?
« Reply #6 on: September 03, 2009, 11:07:12 AM »
If either of you are still looking at this thread, I thought I should mention that the answer key says the answer is K2CO3. 

So maybe that will help you figure out which approach the question wants me to take ... maybe it wants me to choose which one is the most basic? 

Offline renge ishyo

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Re: Which one is a base?
« Reply #7 on: September 03, 2009, 11:45:39 AM »
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If either of you are still looking at this thread, I thought I should mention that the answer key says the answer is K2CO3.

So maybe that will help you figure out which approach the question wants me to take ... maybe it wants me to choose which one is the most basic?

That would make more sense. Yeah, CO32- reacts to form bicarbonate ion (HCO3-) and will readily form a basic solution upon addition of the salt to water.  A lot of the others are somewhat ambiguous because the species involved are weak (and therefore have very strong conjugates).

Offline Jules18

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Re: Which one is a base?
« Reply #8 on: September 03, 2009, 12:09:45 PM »
Okay, I get why that would be very basic because of the HCO3-, but why wouldn't the sulfate compounds ionize to form HSO4-?

Offline Borek

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Re: Which one is a base?
« Reply #9 on: September 03, 2009, 12:18:52 PM »
If either of you are still looking at this thread, I thought I should mention that the answer key says the answer is K2CO3.

Please read last phrase of my first post in this thread.
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Offline renge ishyo

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Re: Which one is a base?
« Reply #10 on: September 03, 2009, 12:52:45 PM »
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Okay, I get why that would be very basic because of the HCO3-, but why wouldn't the sulfate compounds ionize to form HSO4-?

That's what Borek has been saying about certain things being too weak. CuSO4 is in the same category as NaCl and would probably best be categorized as neither if given the option (and from the answer key it looks like you are given that option). The other two solutions might best be categorized as weakly acidic leaving only D as a clear answer.

In the future, when you study acids and bases in more detail you will have better tools (Ka and Kb's!) to answer questions like this more confidently so don't worry ;)

Cheers.

Offline simpleton

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Re: Which one is a base?
« Reply #11 on: September 06, 2009, 09:20:59 AM »
you got to examine the basicity of the compounds. i agree that NaCl is not likely to form a basic solution because Cl is a very weak base. You need a relatively strong base to form a basic solution. What is a strong base? It is a base of weak acids...

For the case of HCl for example.. HCl being a very strong acid has a weak base (Cl-). While CH3COOH being a weak acid has a strong base (CH3COO-). So from here you can say that if you were to add NaCH3COO you will be able to obtain a basic solution while with NaCl, you can't.

Hope it helps. :)

Offline Borek

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Re: Which one is a base?
« Reply #12 on: September 06, 2009, 02:42:11 PM »
It is a base of weak acids...

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HCl being a very strong acid has a weak base (Cl-)

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While CH3COOH being a weak acid has a strong base (CH3COO-)

CONJUGATE base, not just base.
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Offline simpleton

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Re: Which one is a base?
« Reply #13 on: September 09, 2009, 01:45:51 AM »
ya. sorry missed out the conjugate.  :-X

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