Final answer:
The molecular formula for ethene (not ethane as mistakenly mentioned) is C2H4. This is determined by dividing the percentage composition by the atomic masses of carbon and hydrogen and multiplying the empirical formula CH2 by 2 because the molar mass of ethene is 28 g/mol.
Step-by-step explanation:
To find the molecular formula for ethane, we must first correct the initial typo and recognize that the gas in question is actually ethene (also called ethylene), which is a different compound than ethane. Now, ethene has a molecular mass of 28 g/mol and is composed of 85.7% carbon and 14.3% hydrogen by mass. Given the atomic masses of carbon (approximately 12 g/mol) and hydrogen (approximately 1 g/mol), we can deduce the empirical formula by dividing the percentage composition by the respective atomic masses.
For carbon: 85.7 g of carbon / 12 g/mol = approx. 7.14 moles of carbon
For hydrogen: 14.3 g of hydrogen / 1 g/mol = approx. 14.3 moles of hydrogen
The ratio of moles of carbon to hydrogen is roughly 1:2, hence the empirical formula for ethene would be CH2. Since the empirical formula mass (CH2) is 14 g/mol and the given molar mass is 28 g/mol, the molecular formula is obtained by multiplying the empirical formula by a factor of 2. Therefore, the molecular formula for ethene is C2H4.