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

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#### 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 »

#### 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)

#### 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%

#### 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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#### 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

#### 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.

#### 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

#### 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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#### 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.

#### 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.