How do u calculate gram formula m?

simply add the atomic mes of each. For example, Nitrogen dioxide, you would do 14 + 16 + 16, because nitrogen dioxide is "NO2" and you have 1 N and 2 O. N = 14g/mol; O=16g/mol. So NO2 would be 46g/mol. remember, gram formula m is the m per mole of the substance.