Calculate the mass of 9.85*10 ^19 methane molecules

Methane has molecular weight = 16 ;

16 g of methane contains 6.023 X 10^23 molecules.

Mass of 9.85 X 10^19 molecules = 16 X {9.85 X 10^19/6.023 X 10^23} g

= 26.17 X 10^-4 g.

(9.85 x 10¹⁹) / (6.022 x 10²³) = ~0.0001635 moles
0.0001635 x 16.042 = 0.002622867g

16.04 methane's atomic mass (C=12.01+H=1.008 x 4)
(6.022 x 10²³) is number of atoms/molecules in a mole

1mole ch4 is 16 grams 1 mole is 6.022 * 10^23 molecules then you now how much each molecule weighs then multiply by amount.