Percent Composition

Per cent composition of the element in the molecule is defined the ratio of the mass of the different elements present in the molecule expressed in term of percentage.    The easiest method for  determining the percentage mass of a particular element in the compound is to find out the molecular mass of the element.  The molecular mass of the element is calculated by adding all the atomic mass of the element in a compound  The find out the mass of the particular element that is represented in the molecule

The next step is to divide the mass of element upon the molar mass of the molecule and converting them in percentage.   For example for determining the % composition of the A in A3B4

% of A       =    (3 x atomic mass of A/(3 x atomic mass of A + 4 x atomic mass of B)) x 100

Example 1 for the determination of percent composition

Q. Calculate the % composition of  each element in NaOH?

Ans:  There are three element in NaOH.  Sodium, oxygen and hydrogen.

The molecular mass of NaOH = 23 + 16 +1 = 40g/mole

% composition of Na = (23/40) x 100 =57.5%

% composition of Oxygen = (16/40) x 100 = 40%

% composition of Hydrogen = (1/40)  x 100 =  2.5%

Example of percentage composition 2

Q.  Calculate the percentage composition of H2SO4?

Answer:  There are three element in H2SO4 they are hydrogen, sulfur and oxygen

The molecular mass of H2SO4 = 2x 1+32 + 4 x16 =98g/mole

% composition of Hydrogen = (2/98) x 100 = 2.04%

% composition of sulfur  = (32/98) x 100 = 32.65%

% composition of oxygen   = (64/98) x 100 = 65.31%

Excercise

Find the % composition of Carbon in carbon dioxide?
Find the % composition of chromium in potassium dichromate
Find the pecentage composition of water in copper(II) sulfate pentahydrate
Find the percentage composition of Mn in KMnO4