formula weight, in chemistry, a quantity computed by multiplying the

atomic weight (in atomic mass units) of each

element in a

formula by the number of atoms of that element present in the formula, and then adding all of these products together. For example, the
formula weight of water (H2O) is two times the atomic weight of hydrogen plus one times the atomic weight of oxygen. Numerically, this is (2×1.00797)+(1×15.9994)=2.01594+15.9994=18.01534. If the
formula used in computing the formula weight is the molecular formula, the formula weight computed is the

molecular weight. The percentage by weight of any atom or group
of atoms in a compound can be computed by dividing the total weight of the atom (or group of atoms) in the formula by the formula weight and multiplying by 100. For example, the weight percentage of
hydrogen in water is de...

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