How to find the empirical formula being given only masses?

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elizabethrojas1G
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Joined: Fri Sep 25, 2015 3:00 am

How to find the empirical formula being given only masses?

Postby elizabethrojas1G » Mon Sep 28, 2015 6:53 pm

A compound found in the nucleus of a human cell was found to be composed of carbon, hydrogen, oxygen, and nitrogen. A combustion analysis of 1.35 g of the compound produced 2.20 g of CO2 and 0.901 g of H2O. When a separate 0.500g sample of the compound was analyzed for nitrogen, 0.130g of N2 was produced. What is the empirical formula of the compound?

I vaguely remember solving this problem in high school. I've started the problem but I don't think I did it well. Can anyone explain it please?

Chem_Mod
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Re: How to find the empirical formula being given only masse

Postby Chem_Mod » Mon Sep 28, 2015 10:30 pm

First calculate the mass percentages of C, H, and N:

for example for C:
2.20 g CO2 * (mol CO2 / 44g CO2) * (1 mol C / 1 mol CO2) * ( 12 g C / 1 mol C) = x grams C

x grams C / 1.35 grams compound = mass % of C

Once you did this for C, H, and N, subtract them from 100% to get the mass percentage of O. Then getting from mass percentages to empirical formula should be straightforward (assume 100 grams, etc!)

Crystal Eshraghi 2L
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Re: How to find the empirical formula being given only masse

Postby Crystal Eshraghi 2L » Mon Sep 28, 2015 10:33 pm

Hi Elizabeth!

As far as I know, the way you solve this problem is by initially separating your calculations into two steps. You first find the grams of carbon (C) and hydrogen (H) present in the compound in the first chemical reaction of: compound + O2 --> CO2 + H2O by using the apparent molar ratios, and since nitrogen gas (N2) is not a product of this first particular reaction, you would save that calculation for later. So after doing this you would end up with 0.6 g of C and 0.1 g of H. You then calculate the grams of nitrogen present in the compound in the second chemical reaction of: compound + 02 --> N2 by using the appropriate molar ratios, ending up with 0.13 g of N.

In order to find the percentage composition for C, H, and N, you would then have to divide the grams of the particular element in question by the total grams present of its source compound. So for C and H, you would divide by 1.35 g of compound, and for N, you would divide by 0.5 g of compound. After finding the percentage compositions of each of these elements, all you have to do it subtract their sum from 100%, with this difference representing the remaining percentage of oxygen (O) present in the compound. Then, all that's left to do is calculate the empirical formula of the compound by the usual methods (i.e. using a 100 gram sample of compound, converting grams present in sample of compound to moles, dividing by smallest value, etc.).

I hope this helped!

elizabethrojas1G
Posts: 44
Joined: Fri Sep 25, 2015 3:00 am

Re: How to find the empirical formula being given only masse

Postby elizabethrojas1G » Mon Oct 12, 2015 7:28 pm

thank you


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