November 28 2010 - Percent Composition

The percent composition of a compound is a relative measure of the mass of each element present in the compound.  


To calculate the percent composition (percentage composition) of a compound
1.Calculate the molecular mass (molecular weight, formula mass, formula weight), MM, of the compound
2.Calculate the total mass of each element present in the formula of the compound
3.Calculate the percent compositon (percentage composition): % by weight (mass) of element
    = (total mass of element present ÷ molecular mass) x 100


Eg.
Calculate the molecular mass (MM):MM = (2 x 22.99) + 32.06 + (4 x 16.00) = 142.04


Calculate the total mass of Na present:2 Na are present in the formula, mass = 2 x 22.99 = 45.98


Calculate the percent by weight of Na in Na₂SO₄:%Na = (mass Na ÷ MM) x 100 = (45.98 ÷ 142.04) 
x 100 = 32.37%
Calculate the total mass of S present in Na₂SO₄:1 S is present in the formula, mass = 32.06


Calculate the percent by weight of S present:%S = (mass S ÷ MM) x 100 = (32.06 ÷ 142.04) x 100 
= 22.57%


Calculate the total mass of O present in Na₂SO₄:4 O are present in the formula, mass = 4 x 16.00 = 64.00


Calculate the percent by weight of O in Na₂SO₄:%O = (mass O ÷ MM) x 100 = (64.00 ÷ 142.04) x 100 = 45.06%


The answers above are probably correct if %Na + %S + %O = 100, that is,     32.37 + 22.57 + 45.06 = 100