Final answer:
The molecular formula of a gaseous compound with 81.81% C and 18.19% H is determined by first calculating the empirical formula from these mass percentages.
Step-by-step explanation:
To determine the molecular formula of a gaseous compound containing hydrogen and carbon, which is found to be 81.81% C and 18.19% H by mass, and has a mass of 0.646 g at 329 mL and STP, we first calculate the empirical formula based on the given percentages.
Assuming a 100 g sample, we have 81.81 g of carbon and 18.19 g of hydrogen. The number of moles for carbon is calculated by dividing its mass by its molar mass (12.01 g/mol for C) and for hydrogen by dividing its mass by its molar mass (1.01 g/mol for H).
The empirical formula is determined by dividing the number of moles of each element by the smallest number of moles obtained. The empirical mass (EM) is then calculated by adding the atomic masses of the elements in the empirical formula.