Mass of 1 molecule of CH_{4} = 16g/ N_{A}

Mass of 1.5 ×1020 molecules of methane = ( 1.5 ×10^{20 } × 16 / N_{A}) g

Mass of 1 molecule of C_{2}H_{6} = 30 / N_{A} g

Mass of molecules of C_{2}H_{6} is = 1.5 ×10 ^{20}×16/ N_{A} g

∴ Number of molecules of ethane = (1.5 X 10^{20} X 16 / N_{A}) X N_{A} / 30 = 0.8 X 10^{20 }