Possible answer:

Identifying the Molecular Formula of a Hydrocarbon

The molecular formula of a hydrocarbon can be determined from its percentage composition and vapour density using the following steps:

Step 1: Calculate the empirical formula of the hydrocarbon based on the percentage of carbon (C) and hydrogen (H) atoms in its composition. Assume a 100 g sample to simplify the calculation.

Given that the hydrocarbon contains 80% C, the mass of C in 100 g of the sample is:

80 g C = (80/12) mol C

Similarly, the mass of H in 100 g of the sample is:

(100 - 80) g H = 20 g H = (20/1) mol H

Thus, the empirical formula of the hydrocarbon is:

C5H4 = (80/12)/(20/1) = 6.67/20 ≈ 1:1

Step 2: Calculate the molecular formula of the hydrocarbon by multiplying the empirical formula by a whole number (n) that corresponds to the ratio of the molecular mass (MM) to the empirical formula mass (EFM). Use the vapour density (VD) to determine the MM/EFM ratio.

The VD of a gas is the ratio of its density to the density of hydrogen gas (H2), which is assigned a value of 1. Therefore, the VD of the hydrocarbon is:

VD = (MM/EFM)/1 = MM/EFM

If we assume that the hydrocarbon exists as a monomer in the gas phase, then its VD can be related to its molecular mass (MM) as follows:

VD = MM/2.016

Solving for MM/EFM, we get:

MM/EFM = VD/2.016

Substituting the given value of VD = 30, we get:

MM/EFM = 30/2.016 ≈ 14.88

The closest whole number to 14.88 is 15, so we can multiply the empirical formula by 15 to obtain the molecular formula, which is:

C5H4 × 15 = C75H60 = (CH3)5C15H45 = (C2H6)5C5H15 = C4H12

Therefore, the possible molecular formula of the hydrocarbon is C4H12, which corresponds to n-pentane or its isomers, such as neopentane or isopentane.

Conclusion:

The hydrocarbon with 80% C and a vapour density of 30 has a molecular formula of C4H12, which can be derived from its empirical formula (C5H4) and the MM/EFM ratio of 15.