Question

An unknown compound contains only carbon, hydrogen, nitrogen, and oxygen. Complete combustion of a 2.77 g sample of the compound gives 6.64 g CO2 and 1.67 g H2O. Another sample of mass 1.69 g was found to contain 0.143 g N. The molar mass of the compound was found to be 165 g mol−1. Find the molecular formula of the unknown compound

Answer 1

The molecular formula of the unknown compound is determined as  C₉H₁₁NO₂.

Molecular formula of the compound

The molecular formula is calculated as follows;

CHNO  +  O₂ ------------> CO₂  + H₂O

Mass of carbon, C:  = (6.64 x 12)/44 = 1.81 g in 2.77 g sample

Mass of hydrogen, H: = (1.67 x 2)/18 = 0.186 g in 2.77 g sample

Mass of Nitrogen, N: = (2.77 x 0.143)/1.69 = 0.234 g

Mass of oxygen, O:  = 2.77 g - 1.81 g - 0.186 g - 0.234 g = 0.54 g

molar ratio of the elements:

C = 1.81 g = 0.15 mol

H = 0.186 g = 0.186 mol

N = 0.234 g = 0.017 mol

O = 0.54 g = 0.0337 mol

divide through with the smallest number of moles (0.017 mol);

C = 9

H = 11

N = 1

O = 2

Molecular formula = C₉H₁₁NO₂

Check the molar mass of the compound = (9 x 12) + (11 x 1) + (14) + (2 x 16) = 165 g/mol

Thus, the molecular formula of the unknown compound is determined as  C₉H₁₁NO₂.

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