# 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?

Number of molecules of methane (CH4) = 1.5 ×1020

Mass of 1 mole of CH4 = 16 g

Mass of 1 mole of ethane (C2H6) = 30g

1 mole of substances contains 6.022 X 1023 molecules.

Thus, the mass of 6.022 X 1023 molecules of CH4 = 16 g

Thus, the mass of 1.5 ×1020 molecules of methane = 1.5 ×1020 × 16/ 6.022 X 1023

= 3.98 x 10-3g

Mass of molecules of C2H6 is = Mass of CH4(given)

Mass of 6.022 X 1023 molecules of C2H6 = 30g

Therefore, 3.98 x 10-3g of ethane will contain = 6.022 x 1023 x 3.98 x 10-3 / 30

= 0.8 × 1020 molecules

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