1) Benzene, C6H6
Molar mass = 78.11 g/mol
Carbon: ((6 x 12.011) / 78.11) x 100 = 92.26%
Hydrogen: ((6 x 1.008) / 78.11) x 100 = 7.74%
2) Acetylene, C2H2
Molar mass = 26.04 g/mol
Carbon: ((2 x 12.011) / 26.04) x 100 = 92.25%
Hydrogen: ((2 x 1.008) / 26.04) x 100 = 7.75%
Please note that, within reasonable limits, the percentage compositions are the same for benzene and acetylene. This is because they both have the same empirical formula, which is the lowest whole-number ratio of atoms in a molecule. Both substances have a 1:1 ratio of carbon to hydrogen.
3) C7H16
We know that all nine compounds with this empirical formula will have the same percentage composition, which is: C = 89.30%, H = 10.7%.
4 and 5) If you find definitions quite different from mine, I'd like to see them.