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  •  » Calculating percent composition of a mixture

#1 2009-10-04 08:29:25


Calculating percent composition of a mixture wrote:

What is the maximum amount of benzaldehyde impurity that could be in a sample of anisole
(methoxybenzene) before it "failed" elemental analysis according to JOC standards.

I started this by finding the theoretical amounts, but not sure where to go from there. Help pls!

This is an interesting question, one that I had not thought about giving to students. Okay, lets just think about it for a minute. If we had pure anisole and benzaldehyde, the values would be:

MF: C7H8O (anisole)
Molecular Weight: 108.14
Elemental Analysis: C, 77.75; H, 7.46; O, 14.80

Chemical Formula: C7H6O (benzaldehyde)
Molecular Weight: 106.12
Elemental Analysis: C, 79.22; H, 5.70; O, 15.08

The differences in the percent carbon and hydrogen are 1.47 and 1.76% (7.47-5.70). Those values will change linearly with the amount of benzaldehyde present in the sample. Thus, if the sample contained 10% of benzaldehyde, then the percent composition would be 0.17% low for hydrogen. If the sample contained 100% benzaldehyde, the percent composition would be 5.70% or 1.76% low for hydrogen.

In order for the percent composition to be 0.4% low for hydrogen, then 0.22 of anisole must be benzaldehyde.


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