What is the molecular formula of this compound ?

Question

Elemental ananalysis of a compound results in the following data
65.45 % C
5.49 % H
29.06 % O

5.51 g of the compound fills a 192.0 ml bottle with a pressure of 4782.0 mm Hg at a temperature of 21.3oC.

Answer 1

Atomic mass of C: 12.011g/mol
Atomic mass of H: 1.0079g/mol
Atomic mass of O: 15.999g/mol
So now we need to convert the weight ratio of C:H:O:
65.45 : 5.49 : 29.06
to the molar ratio of C : H : O:
5.449 : 5.447 : 1.816 = 3 : 3 : 1
Thus the emperical formula is C3H3O and the emperical formula weight is 55.056 g/mol. The molecular weight must be an integral multiply of that.
Now, 192.0 ml gas with a pressure of 4782.0 mm Hg at 21.3oC can be figured out with the ideal gas law:
n = PV/RT = 0.1920*(4782/760.0)/(0.08206*294.5) = 0.04999 (mol)
thus the molecular mass for this compound is:
5.51g/0.04999mol = 110.2 g/mol
Therefore, the molecular formula of this compound must be: C6H6O2.