We have to determine the molecular formula of a compound. We know that its empirical formula is CH. To find the molecular formula we have to compare the molar mass of the empirical formula with the molar mass of the compound (65.09 g/mol).
Let's start determining the molar mass of the empirical formula. The atomic masses of C and H are:
C: 12.01 amu H: 1.01 amu
With those values we can calculate the molar mass of CH:
molar mass of CH = 1 * 12.01 + 1 * 1.01
molar mass of CH = 13.02 g/mol
If we divide the molar mass by the molar mass of the empirical formula we will get the relationship between them.
x = molar mass/molar mass of the empirical formula
x = 65.09 g/mol / (13.02 g/mol)
x = 5
Finally to get the molecular formula we have to multiply each element by that ratio.
Answer: C₅H₅ is the molecular formula.