# Stuck on this question!

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#1
Hi guys, just wondering if someone could give me a hand with this...

Elemental analysis is a hugely powerful technique for determining a compound's identity and purity. Routine analysis for the determination of C, H and N by mass involves burning the substance to produce CO2, H2O and NOx. The gas stream is reduced to convert NOx to N2. 0.1156 g of an unknown compound is combusted to give 0.1638 g of CO2, 0.1676 g of H2O and 0.05215 g of N2.

A) Calculate the percentage by mass of C, H and N in the unknown compound.
B) Determine the empirical formula of the unknown compound.
C) Suggest a plausible identity for the unknown compound.

I'm struggling with part A, I'm not sure how to get started. I could calculate the number of moles of CO2, H2O and N2, but I'm not sure how that would help.

If anybody could point me in the right direction, it would be much appreciated!

Thanks!

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5 years ago
#2
(Original post by DanTaylor)
Hi guys, just wondering if someone could give me a hand with this...

Elemental analysis is a hugely powerful technique for determining a compound's identity and purity. Routine analysis for the determination of C, H and N by mass involves burning the substance to produce CO2, H2O and NOx. The gas stream is reduced to convert NOx to N2. 0.1156 g of an unknown compound is combusted to give 0.1638 g of CO2, 0.1676 g of H2O and 0.05215 g of N2.

A) Calculate the percentage by mass of C, H and N in the unknown compound.
B) Determine the empirical formula of the unknown compound.
C) Suggest a plausible identity for the unknown compound.

I'm struggling with part A, I'm not sure how to get started. I could calculate the number of moles of CO2, H2O and N2, but I'm not sure how that would help.

If anybody could point me in the right direction, it would be much appreciated!

Thanks!

Posted from TSR Mobile
Calculate the moles of each like you said.

Then you know that:

moles of carbon dioxide = moles of carbon (1 carbon atom per molecule)
moles of water x 2 = moles of hydrogen atoms (2 hydrogen atoms per molecule)
moles of nitrogen x 2 = moles of nitrogen atoms (2 nitrogen atoms per molecule)
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#3
(Original post by charco)
Calculate the moles of each like you said.

Then you know that:

moles of carbon dioxide = moles of carbon (1 carbon atom per molecule)
moles of water x 2 = moles of hydrogen atoms (2 hydrogen atoms per molecule)
moles of nitrogen x 2 = moles of nitrogen atoms (2 nitrogen atoms per molecule)
Thanks so much! So once I've calculated moles, how would I go about calculating percentage by mass with them values?

Posted from TSR Mobile
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5 years ago
#4
(Original post by DanTaylor)
Thanks so much! So once I've calculated moles, how would I go about calculating percentage by mass with them values?

Posted from TSR Mobile
You don't need to! You already have the mole ratio. Just adjust to obtain integral values (divide through by the smallest value).
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