Acompound has a molar mass of 42.1 g/mol. it is 85.6% carbon and 14.4% hydrogen. what is the molecular formula?

To determine the molecular formula of the compound given, we first need to determine the empirical formula. To do that we assume to have 100 grams sample of the compound with the given composition. Then, we calculate for the number of moles of each element. We do as follows:
         mass       moles        ratio
C      85.6        7.13              1
H      14.4        14.26            2

Dividing the number of moles of each element with the smallest value, we will have the empirical formula:

CH2

To determine the molecular formula, we multiply a value to the empirical formula. Then, calculate the molar mass and see whether it is equal to the one given (42.1 g/mol).

              molar mass 
CH2          14.03
C2H4        28.06

C3H6        42.09

Most likely, the molecular formula of the compound would be C3H6.

CH2

Explanation:

it was hard to find above, so I restated it.

the ideal gas law equation is as follows

pv = nrt

where p - pressure

v - volume

n - number of moles

r - universal gas constant

t - temperature

so if the pressure, volume and temperature are already known

we are left with n and r

since r is the universal gas constant that has a known fixed value then r too is known

so we are left with 'n'

once we know temperature volume and pressure

we can find the number of moles of gas present using the ideal gas law equation

this is not my own answer, this is an answer for the same question by the user blahblahmali. i do not claim ownership of this account.