What is the formula mass of (NH4)2O?
The formula mass of an ionic compound is the mass, in grams, of one mole of the formula unit. To find this value, find the sum of the mass of each atom in the formula. When a polyatomic ion (such as the NH4+ in this formula) has parentheses with a subscript outside, multiply everything inside the parentheses by the subscript:
2(N) + 8(H) + O
2(14.0 g/mol) + 8(1.00 g/mol) + 16.0 g/mol = 52.0 g/mol
Molar mass is expressed in grams per mole for practical purposes such as calculations. The mass numbers from the periodic table that were used to arrive at the formula mass represent two things:
1) The mass of 6.02 x 0^23 (1 mole) atoms of the element, in grams
2) The mass of an individual atom of the element in atomic mass units (amu).
Avogadro's number, 6.02 x 10^23, is the quantity that relates the grams to atomic mass units.
The mass numbers were originally assigned as relative masses. Hydrogen, the lightest element, was given a mass number of 1 and the mass numbers assigned to other elements were their masses as compared to hydrogen's. The masses are now based on the carbon-12 atom having a mass number of 12.000.