Question

calculate the total number of molecules in 1.6g of methane

Answer 1

There are Avogadro number of molecules in one mole of any substance.
So,
There are Avogadro number of molecules in 16 grams(1 mole) of methane(CH4).
that is there are 6.023 x 10^23 molecules of methane in 16 grams of methane.
(where 6.023 x 10^23 is known as Avogadro number)

so, in 1.6 grams of methane, there are 6.023 x 10^22 molecules