This means that the mass of one methane molecule is 12.011 u + (4 × 1.008u), or 16.043 u. This means that one mole of methane has a mass of 16.043 grams. A mole can be thought of as two bags of different sized balls. One bag contains 3 tennis balls and the other 3 footballs.
Samples of helium (He), nitrogen (N 2 ), and methane (CH 4 ) are at STP. Each contains 1 mole or 6.02 × 10 23 particles. However, the mass of each gas is different and corresponds to the molar mass of that gas: 4.00 g/mol for He, 28.0 g/mol for N 2 , and 16.0 g/mol for CH 4 .