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Offline Rutherford

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Short yield issue
« on: August 20, 2012, 11:28:05 AM »
If i have the following reaction:
2liquidA :rarrow: 1liquidB -yield=30%
I started from 3000ml of liquid A, how many ml of B I can get (densities of both liquids are almost identical)?

I solved this way: 0.3*3000=900ml, as it is 2:1 in the reaction so I can get 450ml of liquid B, but in the answer is 900ml. Is mine solution correct?

I actually shortened the original problem, as this part confuses me.

Offline discodermolide

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Re: Short yield issue
« Reply #1 on: August 20, 2012, 11:49:44 AM »
If i have the following reaction:
2liquidA :rarrow: 1liquidB -yield=30%
I started from 3000ml of liquid A, how many ml of B I can get (densities of both liquids are almost identical)?

I solved this way: 0.3*3000=900ml, as it is 2:1 in the reaction so I can get 450ml of liquid B, but in the answer is 900ml. Is mine solution correct?

I actually shortened the original problem, as this part confuses me.

No it's not correct the yield is 900mL of B
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Offline Rutherford

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Re: Short yield issue
« Reply #2 on: August 20, 2012, 03:34:22 PM »
How so? The amount of liquid A that reacted was 900ml so the amount of liquid B produced should be two times less (from the reaction, from 2 mole of A I get 1 mole of B, so from 900ml I should get 450ml). Why isn't this true?

Offline discodermolide

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Re: Short yield issue
« Reply #3 on: August 21, 2012, 12:04:22 AM »
How so? The amount of liquid A that reacted was 900ml so the amount of liquid B produced should be two times less (from the reaction, from 2 mole of A I get 1 mole of B, so from 900ml I should get 450ml). Why isn't this true?

The reaction is 2 mol A gives 1 mol B. End of story.

Think about acetylene.
3 HCH≡CH  :rarrow: 1 x benzene.

3 moles acetylene give 1 mole benzene.

« Last Edit: August 21, 2012, 01:04:34 AM by discodermolide »
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Offline Rutherford

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Re: Short yield issue
« Reply #4 on: August 21, 2012, 04:42:54 AM »
"The reaction is 2 mol A gives 1 mol B."
I don't understand what you are saying. If from 2 mol A I get 1 mol B then from 900ml of A I can get only 450ml of B.
In the answer, when the yield is 30%, 900ml of B is produced. This means when the yield is 100%, 3000ml of B is produced. From 3000ml A they would get 3000ml B, but this is impossible because from 2 mole A should be get 1 mole B.

Looking at it from a different way: From 3000ml of A I could get most 1500ml of B. If the yield is 30%, then 0.3*1500=450ml.

Offline discodermolide

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Re: Short yield issue
« Reply #5 on: August 21, 2012, 04:59:43 AM »
"The reaction is 2 mol A gives 1 mol B."
I don't understand what you are saying. If from 2 mol A I get 1 mol B then from 900ml of A I can get only 450ml of B.
In the answer, when the yield is 30%, 900ml of B is produced. This means when the yield is 100%, 3000ml of B is produced. From 3000ml A they would get 3000ml B, but this is impossible because from 2 mole A should be get 1 mole B.

Looking at it from a different way: From 3000ml of A I could get most 1500ml of B. If the yield is 30%, then 0.3*1500=450ml.

Sorry I do not think this is correct.
you work out the yield in the normal way
2mol gives 1 mol
That's it.
No need to divide by 2 or anything else
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Offline Rutherford

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Re: Short yield issue
« Reply #6 on: August 21, 2012, 06:08:14 AM »
I think I know now what confused me. These are the volumes of two liquids, not gasses, so they can't be compared as molar ratios. If these were gasses then it had to be divided by 2 as it is said here http://www.wikihow.com/Calculate-Percent-Yield-in-Chemistry in step 5.

I will write what was in the original problem:
2acetone :rarrow: diacetone-alcohol
I have 3000ml of acetone, and it is assumed that the densities of both liquids are equal, I will write them as 1g/cm3 (it doesn't matter), so there are 3000g of acetone or 51.27 moles of acetone. From 51.27 moles of acetone I can get a maximum of 25.64mol of diacetone-alcohol, because the yield is 30% I actually get 25.64*0.3=7.69mole of the aldol. Its mass is 7.69*116=892g≈900g (I rounded the results too much). Using the density I get a volume of 900ml which is correct.

Like you knew that it was an organic chemistry problem  ;D. It is actually really long, explaining some complex synthesis (using Soxhlet apparatus), but this part was the one that confused me, so I thought of it as a general high school chemistry problem.

Offline Vidya

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Re: Short yield issue
« Reply #7 on: August 21, 2012, 06:56:53 AM »
this question needs to be solved on the basis of moles.
2A(liquid)----> B(liquid)
we don't know the molar mass and hence can not work in moles.We need to work in terms of volume here.Here volume should remain same during the reaction as they have the same density.So total volume before and after the reaction should be 3000ml of A.30% reaction means that out of 3000mL ,30% is converted into B and 70% is left as A
hence answer should be 30% of 3000= 900mL

Offline Rutherford

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Re: Short yield issue
« Reply #8 on: August 21, 2012, 07:44:13 AM »
The problem is that this question mustn't be solved with the basis of moles. The number of moles decreases two times, but the mass of B is two times higher than A so there is no change in the mass and because densities are equal there is no change in the volume, too.
"Here volume should remain same during the reaction as they have the same density." I think this isn't necessarily true, only if the masses are equal, too then it is true and this counts only for liquids. For gasses the change of moles would change the volume (so the gasses can't have same density).
I think that the conclusion is: the volume of liquids depends on their mass and density, and the volume of gasses depends on their quantity (moles), T and p. Density of gasses depends on the temperature, but this isn't the case for liquids.

Offline Rutherford

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Re: Short yield issue
« Reply #9 on: August 21, 2012, 09:20:19 AM »
I tried to help myself understand this by deriving the following equation from the general equation for density: V=n*M/ρ, for the problem I posted, the number of moles decreases 2 times the molar mass increases two times, so the volume doesn't change, so this can be applied for liquids with same density.
For the gasses it is the same situation, only that here the density can't be same, because from the gas law: ρ=M*p/(R*T), when I put this in the previous equation I will get M2, so the volume will drop two times. This is probably the correct reason.

EDIT: Mistake, I get the M neglected, so I am at the beginning again. This is really hard for me to understand, can someone explain it clearly, or at least post a link where this is explained (I mean the difference for the gasses).

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