The number of molecules within a mole of a substance is given by Avogadro's number, 6.0221413x10^23. All you have to do is multiply the number of moles you have by Avogadro's number.

So molecules = moles * Avogadro's number, then molecules of CO2 would be 1.90x10^-2 * 6.0221413x10^23 = 1.144206847x10^22 molecules, rounded to 1.14x10^22 to account for significant figures.

To find moles of ethane (C2H6), you simply need to do the reverse process, where moles = molecules / Avogadro's number. Therefore 3.771x10^23 / 6.0221413x10^23 = .6261892261 moles, rounded to .6262 moles to account for significant figures.

1 mol is equivalent to 6.022E23 molecules. To find out how many moles are in the given amount of molecules, you will divide 3.771E23 / 6.022E23