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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Messi on October 13, 2012, 08:58:58 PM

Title: Species in Solution
Post by: Messi on October 13, 2012, 08:58:58 PM: If I have a solution of Sodium Acetate (CH₃OH), what species would I expect in solution in order of decreasing concentration?

I said:
CH₃COONa
H₂O
CH₃COOH
NaOH
H⁺ & OH^-

I wasn't sure where I would put Na⁺ and CH3COO^- as I guess some of these ions would be expected in the solution, no?
Title: Re: Species in Solution
Post by: Dan on October 14, 2012, 05:43:52 AM: Quote
I have a solution of Sodium Acetate (CH₃OH)

A methanolic solution of sodium acetate? Can you explain how you propose to form water from sodium acetate and methanol?
Title: Re: Species in Solution
Post by: Messi on October 14, 2012, 09:26:59 AM: Sorry, I meant Sodium Acetate (CH₃COONa)! I had a brain fart when I wrote the molecular formula!
Title: Re: Species in Solution
Post by: Arkcon on October 14, 2012, 09:41:23 AM: Quote from: Messi on October 13, 2012, 08:58:58 PM
I wasn't sure where I would put Na⁺ and CH3COO^- as I guess some of these ions would be expected in the solution, no?

Sure, can you work on the rest? How much [NaAcetate] molecular do you expect to exist an aqueous solution?
Title: Re: Species in Solution
Post by: Messi on October 14, 2012, 10:15:24 AM: Ok I guess I'll put this as my list in order of decreasing concentration:

I said:
CH3COONa
H2O
CH3COO^- & Na⁺
CH3COOH
NaOH
H+ & OH-

I think the above list is correct? It makes sense to me in a conceptual manner.
Title: Re: Species in Solution
Post by: Borek on October 14, 2012, 10:21:49 AM: Acetic acid is a weak one, so you should expect quite strong hydrolysis.
Title: Re: Species in Solution
Post by: Arkcon on October 14, 2012, 10:54:49 AM: So you expect sodium acetate to exist in solution mostly as a molecular species? So, comparing a solution of sodium acetate in water, you think the properties are more like sucrose in water, and less like sodium chloride in water? That is to say, sodium acetate isn't very ionic in character? A solution won't conduct electricity, for example?
Title: Re: Species in Solution
Post by: fledarmus on October 14, 2012, 11:39:34 AM: Why would you expect less water than sodium acetate? What is the solubility of sodium acetate in water?
Title: Re: Species in Solution
Post by: Messi on October 14, 2012, 03:43:45 PM: Well here is my rational:

There are two equilibriums going on... one of them is:
H₂O :rarrow: OH + H
The other one is:
CH3COONa + H20 :rarrow: CH3COOH + NaOH

H2O rarely breaks down into OH and H so these should be in the lowest concentration. NaOH is a strong base so dissociates almost completely so there should not be much NaOH left. If NaOH reacts with CH3COOH then there should not be much CH3COOH either?

So my list is:
CH3COONa
H2O
CH3COO- & Na+
CH3COOH
NaOH
H+ & OH-
Title: Re: Species in Solution
Post by: Borek on October 14, 2012, 03:50:23 PM: You can safely assume acetate is dissociated 100%. You can also assume there is almost no NaOH in the solution, as a strong base it is 100% dissociated as well.

You are still missing the hydrolysis part which is what the question is most likely about.
Title: Re: Species in Solution
Post by: Messi on October 14, 2012, 06:49:31 PM: Hydrolysis part? Didn't I show the Sodium acetate being hydrolysed into NaOH and acetic acid?
Title: Re: Species in Solution
Post by: Borek on October 15, 2012, 04:03:27 AM: You listed them, but you ignored other hydrolysis products.
Title: Re: Species in Solution
Post by: Messi on October 15, 2012, 06:16:13 AM: Do you mean... H2O + H2O :rarrow: H3O + OH ?
Title: Re: Species in Solution
Post by: Borek on October 15, 2012, 06:24:53 AM: No.

Write reaction equation of the CH₃COO^- hydrolysis. What are the products?
Title: Re: Species in Solution
Post by: Messi on October 15, 2012, 06:27:52 AM: Did I not already write them? ???

CH3COONa + H20 :rarrow: CH3COOH + NaOH

or

CH3COO + H20 :rarrow: CH3COOH + OH
Title: Re: Species in Solution
Post by: AWK on October 15, 2012, 06:45:46 AM: Quote
what species would I expect in solution in order of decreasing concentration?
In water solution of sodium acetate you have a few moles of sodium acetate in 1 liter and ~50 moles of water.
Title: Re: Species in Solution
Post by: Messi on October 15, 2012, 07:43:55 AM: Quote from: AWK on October 15, 2012, 06:45:46 AM
Quote
what species would I expect in solution in order of decreasing concentration?
In water solution of sodium acetate you have a few moles of sodium acetate in 1 liter and ~50 moles of water.

Ok, yes, I agree with that. So to reformat my table, I would put:

H2O
CH3COONa
CH3COO- & Na+
CH3COOH
NaOH
H+ & OH-
Title: Re: Species in Solution
Post by: Borek on October 15, 2012, 08:17:14 AM: Quote from: Messi on October 15, 2012, 06:27:52 AM
CH3COO + H20 :rarrow: CH3COOH + OH

Stop omitting charges.

If there is an OH^- produced per each CH₃COOH produced, is it possible that the concentration of OH^- is much lower than the concentration of CH₃COOH?
Title: Re: Species in Solution
Post by: Messi on October 15, 2012, 09:50:56 AM: Good point Borek. Sorry for omitting charges.

I guess my final table in order of decreasing concentrations should read:

H₂O
CH₃COONa
CH₃COO^- & Na⁺
H⁺ & OH^- & NaOH & CH₃COOH

I am not sure if there would be the same amount concentrations of CH₃COONa and CH₃COO^- & Na⁺.
Title: Re: Species in Solution
Post by: Borek on October 15, 2012, 09:56:10 AM: You were told more than once that CH₃COONa is dissociated 100%, yet you still list it as the second most important species.
Title: Re: Species in Solution
Post by: Messi on October 15, 2012, 10:22:32 AM: My apologies.

H₂O
CH₃COO^- & Na⁺
H⁺ & OH^- & NaOH & CH₃COOH

Seems correct I guess.

Thank you for correcting me Borek. As you can tell, I am not very brilliant.
Title: Re: Species in Solution
Post by: Messi on October 15, 2012, 11:56:24 AM: Quote from: Messi on October 15, 2012, 10:22:32 AM
My apologies.

H₂O
CH₃COO^- & Na⁺
H⁺ & OH^- & NaOH & CH₃COOH

Seems correct I guess.

Thank you for correcting me Borek. As you can tell, I am not very brilliant.

After thinking for a long time, I would probably change my answer to:

H₂O
CH₃COO^- & Na⁺
OH^-
CH₃COOH
H⁺

What do you guys think?
Title: Re: Species in Solution
Post by: Borek on October 15, 2012, 12:07:00 PM: Much better.
Title: Re: Species in Solution
Post by: Messi on October 15, 2012, 01:03:35 PM: Quote from: Borek on October 15, 2012, 12:07:00 PM
Much better.

Is "much better" 100% correct? :)
Title: Re: Species in Solution
Post by: Borek on October 15, 2012, 01:32:18 PM: Exact answer depends on how much you are expected to know and what level of details you were already taught. At some level this is a 100% correct answer, at other levels it is incomplete.
Title: Re: Species in Solution
Post by: Messi on October 15, 2012, 08:00:10 PM: And at some levels it's wrong! ;)