April 17, 2021, 11:45:07 PM
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Topic: Solvent Extraction - Theoretical Yield Calculation  (Read 579 times)

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Calcium1808

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Solvent Extraction - Theoretical Yield Calculation
« on: April 03, 2021, 12:14:34 PM »
I need to calculate the theoretical yield for a solvent extraction experiment. We've been given some correlations, however I don't understand from this data how I would get the theoretical yield. I calculated the theoretical yield as 0.725 (0.29 *2.5 for the 250cm3 of solution) however from the experimental data collected the actual is 0.748 therefore actual is greater than theoretical so it mustn't be right.

https://imgur.com/a/L1tNOOy
Any help and explanations as to why it must be calculated a different way are appreciated.

Thanks
« Last Edit: April 03, 2021, 01:36:30 PM by Borek »

Orcio_Dojek

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Re: Solvent Extraction - Theoretical Yield Calculation
« Reply #1 on: April 03, 2021, 12:58:42 PM »
0,5 % sodium hydroxide is 0,125 mol / l, not 0,0125 mol / l solution.

Aside from that..

Theoretical yield is 2 g of benzoic acid reduced by it is solubility, as you start from that amount (2 g), and have enough NaOH to extract it to water and then precitipate with acid.

However:

Quote
I calculated the theoretical yield as 0.725 (0.29 *2.5 for the 250cm3 of solution)

This is wrong - 250 ml conical flask means volume of flask rather than volume of solution.

Volume of solution is 50 ml + amount of added HCl.
« Last Edit: April 03, 2021, 01:15:41 PM by Orcio_Dojek »

Calcium1808

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Re: Solvent Extraction - Theoretical Yield Calculation
« Reply #2 on: April 03, 2021, 01:42:34 PM »

Theoretical yield is 2 g of benzoic acid reduced by it is solubility, as you start from that amount (2 g), and have enough NaOH to extract it to water and then precitipate with acid.

So is the theoretical yield just 2 g? Or do I need to use the solubility to adjust it? This is the part I am quite confused about.

Quote
I calculated the theoretical yield as 0.725 (0.29 *2.5 for the 250cm3 of solution)

This is wrong - 250 ml conical flask means volume of flask rather than volume of solution.

Volume of solution is 50 ml + amount of added HCl.

I was trying to refer to the fact you repeat the seperation 3 times, each time being 50ml resulting in 250(however I know this is wrong).

Orcio_Dojek

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Re: Solvent Extraction - Theoretical Yield Calculation
« Reply #3 on: April 03, 2021, 02:02:39 PM »
You should adjust solubility, as 0,29 g / 100 ml is a measurable amount. But you will unable to do it if you did not record volume of HCl added.

Calcium1808

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Re: Solvent Extraction - Theoretical Yield Calculation
« Reply #4 on: April 03, 2021, 03:57:21 PM »
This was a "online laboratory" so this was all the information we've been given unfortunately, there is no data for the amount of HCl used.

Borek

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Re: Solvent Extraction - Theoretical Yield Calculation
« Reply #5 on: April 03, 2021, 04:34:09 PM »
1% HCl is around 0.28 M, compared to 0.126 M for 0.5 NaOH. Neutralization requires about a bit less than half the volume of NaOH used, quite a lot.

Not clear to me why you are asked to collect each extract separately. Are you expected to weight recovered acid from each part separately, or are you just interested in the sum/total?
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Orcio_Dojek

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Re: Solvent Extraction - Theoretical Yield Calculation
« Reply #6 on: April 03, 2021, 04:46:05 PM »
This is very hard to calculate, as in first and second extract NaOH is utilised completely, but some benzoic acid can go to the aqueos solution.

We don't know how much without specific Nernst value (https://en.wikipedia.org/wiki/Distribution_law).

In third extract there is only sodium benzoate and NaOH, so there is no trouble.

Calcium1808

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Re: Solvent Extraction - Theoretical Yield Calculation
« Reply #7 on: April 03, 2021, 04:49:32 PM »
1% HCl is around 0.28 M, compared to 0.126 M for 0.5 NaOH. Neutralization requires about a bit less than half the volume of NaOH used, quite a lot.

Not clear to me why you are asked to collect each extract separately. Are you expected to weight recovered acid from each part separately, or are you just interested in the sum/total?

I expect it is just the total amount required.

Calcium1808

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Re: Solvent Extraction - Theoretical Yield Calculation
« Reply #8 on: April 03, 2021, 04:50:38 PM »
This is very hard to calculate, as in first and second extract NaOH is utilised completely, but some benzoic acid can go to the aqueos solution.

We don't know how much without specific Nernst value (https://en.wikipedia.org/wiki/Distribution_law).

In third extract there is only sodium benzoate and NaOH, so there is no trouble.

I imagine that this will form part of the error, so if we were to assume that there is only NaOH and Sodium Benzoate, how would I go about calculating the theoretical yield?

I don't think it will be as complicated as the Nernst value, I think it will more or less be using the 2g of Benzoic Acid and the ratios somehow to give a theoretical value. However I'm really struggling to see anything other than 2g is the theoretical and anything else is just a loss in the extraction processs. (Ultimately, if you've got 2g of Benzoic Acid, you want to separate out the entirety of that amount?) But those solubilities must be used otherwise they won't be provided!

Orcio_Dojek

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Re: Solvent Extraction - Theoretical Yield Calculation
« Reply #9 on: April 03, 2021, 05:00:34 PM »
@Calcium1808

You can assume that benzoic acid doesn't dissolve in water phase during first and second extraction (all acid remains in CH2Cl2) - it will not give you any big difference.

So - you have that much sodium benzoate how much NaOH you have added.

In third extract you have sodium benzoate and unused NaOH.

This doesn't change anything as HCl reacts with sodium benzoate and NaOH in the same way.

But - NaOH still must be neutralised ("until the solution is acid").

You can calculate final volume by stoichiometry.

Orcio_Dojek

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Re: Solvent Extraction - Theoretical Yield Calculation
« Reply #10 on: April 03, 2021, 05:11:47 PM »
@Calcium1808

Quote
(Ultimately, if you've got 2g of Benzoic Acid, you want to separate out the entirety of that amount?) But those solubilities must be used otherwise they won't be provided!

You're right.

I would write that 2 g = 100 %, then use those solubilities to calculate max. efficiency (for example - 60 - 70 %) which can be obtained, and at last - compare it to experimental data.

Calcium1808

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Re: Solvent Extraction - Theoretical Yield Calculation
« Reply #11 on: April 06, 2021, 10:59:34 AM »
@Calcium1808

Quote
(Ultimately, if you've got 2g of Benzoic Acid, you want to separate out the entirety of that amount?) But those solubilities must be used otherwise they won't be provided!

You're right.

I would write that 2 g = 100 %, then use those solubilities to calculate max. efficiency (for example - 60 - 70 %) which can be obtained, and at last - compare it to experimental data.

Thinking about it more, I think this approach is wrong. Is this not something to do with the partition coefficient?

Orcio_Dojek

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Re: Solvent Extraction - Theoretical Yield Calculation
« Reply #12 on: April 06, 2021, 12:04:54 PM »
Partition coefficient will lower maximum experimental efficiency as some sodium benzoate can go to the lower CH2Cl2 layer, but this is neglible change (I think less than 1 %).

Orcio_Dojek

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Re: Solvent Extraction - Theoretical Yield Calculation
« Reply #13 on: April 06, 2021, 01:37:28 PM »
@Calcium1808 I must apologize, as I writed some untrue:

Quote
This is very hard to calculate, as in first and second extract NaOH is utilised completely, but some benzoic acid can go to the aqueos solution.

We don't know how much without specific Nernst value (https://en.wikipedia.org/wiki/Distribution_law).

In third extract there is only sodium benzoate and NaOH, so there is no trouble.

OK. In 1 i 2 extraction some benzoic acid will go to aqeous layer, so - in 3 extraction there will be less of it as it results from stoichiometry, but..

That doesn't change anything as in 1 and 2 extractions there will be MORE acid as residue, and in 3 - LESS.

I mean - part of benzoic acid which dissolve in sodium benzoate solution doesn't change anything. Volume of HCl can be calculated normally on the basis of stoichiometry.

I don't know why did I think some other way.

That is just by the way.
« Last Edit: April 06, 2021, 03:13:37 PM by Orcio_Dojek »

Calcium1808

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Re: Solvent Extraction - Theoretical Yield Calculation
« Reply #14 on: April 06, 2021, 02:01:13 PM »
So working through this.

If some of the benzoic acid goes into the aqueous layer in extraction 1 then you just extract again in extraction 2. So by Stochiometry I get that by the end of extraction 2, all the Benzoic Acid should be precipitated, there is no more left over for extraction 3. So theoretical yield is just 2 g by this method.

I however think, that the question may be asking more so about the partition coefficient. However, I'm not sure I understand it fully. Do I have the necessary information to calculate the partition coefficient? I don't think I do as i have the solubilities however I do not have the solubilities in each of the different layers (only have solubilities in aqueous, not for organic layer).

Am I correct in thinking I cannot calculate the partition coefficient with the information given to me?