Calculate the mass of atoms of carbon and 3 molecules of hydrogen

The mass of atoms of carbon and 3 molecules of hydrogen : 18 g/mol

An atomic mass unit ( amu or "u") is a relative atomic mass of 1/12 the mass of an atom of carbon-12.  

The molar mass(molecular mass-formula mass-molecular weight(MW)) of a compound is the sum of the relative atomic mass (Ar) of the constituent elements of the compound

Can be formulated :  

M AxBy = (x.Ar A + y. Ar B)  

The mass of atom of Carbon(C)⇒Ar = 12 g/mol

The mass of 1 molecule of Hydrogen - H₂(MW) : 2 g/mol

The mass of 3 molecules of Hydrogen : 3 x 2 = 6 g/mol

So the mass of atoms of carbon and 3 molecules of hydrogen :

the product of the pressure and volume is also constant