The part of the total molecular mass of a compound given by a specific element is found by multiplying the atomic weight of that element by the number of its atoms in the compound and dividing by the total weight of the molecule. If a single molecule weighs 100 amu and a particular element accounts for 5 amu of that mass, that element composes 5% of the mass of the compound.
Example:
Methyl isocyanate ( was the toxic gas responsible for the disaster in Bhopal, India. What percent of the total mass is given by each of the elements?
The total molecular mass is 3(1.008) + 2(12.01) + (14.01) + (15.80) = 57.05 amu.
(1) %H = 3(1.008)/57.05 = .05300 = 5.300%
(2) %C = 2(12.01)/57.05 = .4210 = 42.10%
(3) %N = 14.007/57.05 = .2455 = 24.55%
(4) %O = 15.80/57.05 = .2770 = 27.70%
The total of the percentages differs from 100 because of rounding during calculations.
