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Topic: Percentage yield above 100%?  (Read 383 times)

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Offline Tezz

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Percentage yield above 100%?
« on: August 25, 2020, 07:34:55 AM »
Hi all, when I do this question I get a percentage yield above 100% which makes me question whether I am right. Could someone check it over? Thanks

I currently have an answer of 116.86%
« Last Edit: August 25, 2020, 08:56:42 AM by Borek »

Online sjb

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Re: Percentage yield above 100%?
« Reply #1 on: August 25, 2020, 08:51:11 AM »
Can you show your working? 10 lines of text shouldn't be that large to type out (which will also help searches in the future, in case the image breaks)

Offline Tezz

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Re: Percentage yield above 100%?
« Reply #2 on: August 25, 2020, 08:54:34 AM »
1/28.1 = 0.035587... mols

Molecular mass of SiCl4 = 28.1+4(35.5)
=170.1

0.035587 x 170.1 = 6.05338g

6.05338/3.5 = 1.729537 g/cm^3

1.729537/1.48 = 1.1686

Therefore, 116.86%

Offline Borek

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Re: Percentage yield above 100%?
« Reply #3 on: August 25, 2020, 08:58:31 AM »
You got it reversed.

(not that it makes much sense comparing density with hypothetical density of the product, calculate mass of the product from the data given)
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Offline Tezz

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Re: Percentage yield above 100%?
« Reply #4 on: August 25, 2020, 09:00:21 AM »
Ah, I thought I might have done that. So it should be 1.48/the other value?

I thought that 1.48 was the theoretical yield of the reaction

Offline DrCMS

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Re: Percentage yield above 100%?
« Reply #5 on: August 25, 2020, 09:02:42 AM »
1.48g/ml is the density of the product.

Offline Tezz

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Re: Percentage yield above 100%?
« Reply #6 on: August 25, 2020, 09:04:19 AM »
1.48g/ml is the density of the product.
Alright thanks, I’ll go and modify my answer to whatever 1.48/other density comes out as. Appreciate the help

Offline Borek

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Re: Percentage yield above 100%?
« Reply #7 on: August 25, 2020, 12:00:42 PM »
Alright thanks, I’ll go and modify my answer to whatever 1.48/other density comes out as. Appreciate the help

You do understand it doesn't make physical sense (even if it produces a correct number)?

Density is not something that can randomly change, for a given substance (and PT) it is always the same.

What matters here is not the density, but mass of the product.
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Offline DrCMS

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Re: Percentage yield above 100%?
« Reply #8 on: August 25, 2020, 12:04:39 PM »
1.48g/ml is the density of the product.
Alright thanks, I’ll go and modify my answer to whatever 1.48/other density comes out as. Appreciate the help

NO there is no other density in the question and you can not calculate a density of anything from the information given. 

The correct way to solve the problem is:

  • Write out the balanced equation for the reaction and determine how many moles of product are formed per mole of starting material in this case it is 1:1 but in other cases it will be different.
  • Calculate how many moles of starting material were used.
  • Calculate how man moles of product were formed (for this you'll need to calculate the weight of product from the volume and density given)
  • Divide the moles of product formed by the moles of starting material used to give the % yield (if from the reaction equation they are not in a 1:1 ratio you need to account for that at this stage.

Offline Enthalpy

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Re: Percentage yield above 100%?
« Reply #9 on: August 26, 2020, 04:44:48 AM »
I recommend to write the units each and every time you make a computation, and double-check their consistency.

Secret: professional scientists don't memorize formulas. They compute according to the units.

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