#"Formula mass"# is the sum of the #"atomic masses"#.........

Explanation:

Take #CO_2#, the atomic mass of carbon is #12.011*g*mol^-1#, that of oxygen is #15.999*g*mol^-1#. Sum the atomic masses, i.e. #(12.011+2xx15.999)*g*mol^-1#, and we get #44.0*g*mol^-1#, which is the formula mass of #"carbon dioxide"#.