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Question
Solve the following problem:
An organic compound containing oxygen, carbon, hydrogen, and nitrogen contains 20% carbon, 6.7% hydrogen, and 46.67% nitrogen. Its molecular mass was found to be 60. Find the molecular formula of the compound.
Solution
Given: Percentage of carbon, hydrogen, nitrogen = 20%, 6.7%, 46.67% respectively.
Molar mass of the compound = 60 g mol–1
To find: The molecular formula of the compound
Calculation: % carbon + % hydrogen + % nitrogen = 20 + 6.7 + 46.67 = 73.37%
This is less than 100%. Hence, the compound contains adequate oxygen so that the total percentage of elements is 100%.
Hence, % of oxygen = 100 – 73.37 = 26.63%
Moles of C = `"% of C"/"Atomic mass of C"=20/12` = 1.667 mol
Moles of H = `"% of H"/"Atomic mass of H"=6.7/1.0` = 6.700 mol
Moles of N = `"% of N"/"Atomic mass of N"=46.67/14` = 3.334 mol
Moles of O = `"% of O"/"Atomic mass of O"=26.63/16` = 1.664 mol
Hence, the ratio of number of moles of C:H:N:O is
`1.667/1.664` ≈ 1, `6.700/1.664` = 4, `3.334/1.664` = 2, and `1.664/1.664` = 1
Hence, empirical formula is CH4N2O.
Empirical formula mass = 12 + 4 + 28 + 16 = 60 g mol–1
Hence, Molar mass = Empirical formula mass
∴ Molecular formula = Empirical formula = CH4N2O
Molecular formula of the compound = CH4N2O
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