An organic compound is found to contain 6 parts of carbon, 1 part of hydrogen and 8 parts of
oxygen by mass.
6 g of a gaseous sample of the compound would have a volume of 2.4 dm3 at room temperature
and pressure.
Which formula (A–E) is the molecular formula for this compound?
(Ar: H = 1; C = 12; O = 16)
(1 mole of any gas occupies 24 dm 3 at room temperature and pressure)

Answer:

The key steps in this solution are finding the molecular mass, from the mass, volume and mole
information, and then dividing this mass over the 3 elements to figure out how many atoms of each
are present in the molecule.
Using the molar gas volume given (1 mole = 24 dm 3
), we can deduce that 2.4 dm 3 of the compound
must contain 0.1 moles.
So 0.1 moles of the compound weighs 6 g.
So 1 mole of the compound would weigh 60 g. Hence the relative molecular mass must be 60.
The ratio of C : H : O by mass is 6 : 1 : 8; dividing the mass into this ratio gives us the mass of each
element in the molecule.
C: 6
15 × 60 H: 1
15 × 60 O: 8
15 × 60
The relative masses are: C: 24 H: 4 O: 32
Using the Ar values, we can see this relates to 2 carbons, 4 hydrogens, and 2 oxygens.
The molecular formula is C2H4O2
The answer is B