An organic compound contains only carbon, hydrogen, nitrogen and oxygen…
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An organic compound contains only carbon, hydrogen, nitrogen and oxygen. A sample is burnt completely in excess oxygen. The products are passed through absorbers and give of and of . In a second experiment, the nitrogen in an identical sample is converted entirely to , which measures at STP (273 K, 1 atm, molar volume ). The molar mass found by mass spectrometry is . Atomic masses in : H 1.0, C 12.0, N 14.0, O 16.0.
- What is the percentage of carbon by mass?
- What is the percentage of nitrogen by mass?
- What is the percentage of oxygen, found by difference?
- What is the molar mass of the empirical formula unit?
- How many hydrogen atoms are there in one molecule?
The worked solution
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Plan. Masses of C, H and N from the data, O by difference, then mole ratios.
Masses. C: . H: . N: of , which is . O: .
(a), (b), (c) Divide by : C , N , O (and H is ).
Mole ratio. C , H , N , O mol. Dividing by the smallest () gives , so the empirical formula is .
(d) .
(e) , so the molecular formula is and there are hydrogen atoms.
Check. Burning of forward gives of , of and of , exactly the data.
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