Q43Long answer
A sample of ethane (C2H6) gas has the same mass as 1.5 ×1020 molecules of methane (CH4). How many C2H6 molecules does the sample of gas contain?
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Mass of 1 molecule of CH4 = 1.5 × 1020 × 16 Mass of 1.5 ×1020 molecules of methane = g Mass of 1 molecule of C2H6 = g 1.5 × 1020 × 16 Mass of molecules of C2H 6 is = g 1.5 × 1020 × 16 NA ∴ Number of molecules of ethane = × = 0.8 × 10 20 NA 30