Mass of 1 molecule of CH4 = 16 g / NA
Mass of 1.5 x 1020 molecules of methane = (1.5 × 1020× 16 / NA) g
Mass of 1 molecule of C2H6 = (30 / NA) g
Mass of molecules of C2H6 = (1.5 × 1020× 16 / NA) g
∴ Number of molecules of ethane = (1.5 × 1020× 16 / NA) × (𝑁𝐴 / 30)
= 0.8 × 1020