The molecular weight of C3H8 is 44 g/mol.

Given that the relative molecular mass (Mr) of C3H8 is 100, we can calculate the mass of carbon in one mole of C3H8 using the following equation:

Mr = (n_C x Mr_C) + (n_H x Mr_H)

Where:
n_C = Number of carbon atoms
n_H = Number of hydrogen atoms
Mr_C = Atomic mass of carbon = 12 g/mol
Mr_H = Atomic mass of hydrogen = 1 g/mol

Substitute the values:

100 = (3 x 12) + (8 x 1)
100 = 36 + 8
100 = 44

So, the mass of carbon in 1 mole of C3H8 is 36 g/mol.

Therefore, the mass percent of carbon in C3H8 is:
w(C) = (mass of carbon / molecular weight of C3H8) x 100
w(C) = (36 / 44) x 100
w(C) ≈ 81.82%

Therefore, the mass percent of carbon in C3H8 is approximately 81.82%.