Look at the Periodic Table, any Periodic Table, and the elemental masses are quoted in #g*mol^-1#. You should be provided with a Periodic Table in every exam in Chemistry and Physics you ever sit. You do, however, have to be able to use the Table.
Look at such table, the molar masses of each element are quoted: #H, 1.00794*g#; #He, 4.003*g#; #Li, 6.941*g#;.......... #Fe, 55.85*g#; etc. These would be whole numbers but for the existence of isotopes, i.e. elements with different atomic masses, and the weighted average is reported.
The weights reported refer to the mass of #"Avogadro's number"# of the element's atoms, where #"Avogadro's number"# #=# #N_A# #=# #6.022xx10^23*mol^-1#. And thus #N_A# is the link between the micro world of atoms and molecules, which we can't see but can observe and whose behaviour we can predict, to the macro world of grams, and litres, which we can measure and quantify.
And thus, given a mass of element, or a molecular mass, we can tell precisely how many molecules/atoms/particles that mass represents, and what mass of co-reactant is an equivalent quantity.
