The atomic mass of an element is actually proportional average of all the different naturally occurring isotopes of an element. These isotopes all have different atomic masses, so when an atomic mass is given for an element, it is a proportional average of the different isotopes.
As for determining the mass of a number of moles of a compound, there are a couple of easy steps to follow. Your example methane has an atomic mass of 16.06 g/mol. This tells you that for every mole of methane there are 16.06 grams of methane. You can simply multiply this number by the number of moles of methane you need to determine how many grams of methane you need. In this case, 0.63 moles of methane is equal to 10.12grams of methane when rounded to four significant figures.