Percent Composition Formula - Practice Questions & MCQ

Percentage Combination:

percentage combination of the compound is the relative mass of the each of the constituent element in 100 parts of it.

Let us take an example of water (H₂O), it contains hydrogen and oxygen, the percentage composition of both these elements can be calculated as follow:

The molar mass of water = 18.02 g

One can check the purity of a given sample by analyzing percentage composition.

Equivalent Weight: 

• Equivalent weight is the weight of an element or a compound which combines with or displaces 1.008 part by weight of H₂ or 8 part by weight of O₂, or 35.5 part by weight of Cl₂.

• Equivalent weight is a number and when it is 
  denoted in grams, it is called gram equivalent.

• It depends upon the nature of chemical reaction 
  in which substance takes part 

Methods To Find Equivalent Weight: 

• For Acids:
  E = (Molecular weight) / (Protocity or Basicity of Acid)
  For Example, for H3PO4, E = M/3
  For H₂SO₄  , E =M/2 
• For Bases:
  E = (Molecular weight) / (Acidity or number of OH^- ions)
  For Example, for Ca(OH)₂, E = M/2
  For Al(OH)₃, E =M/3 
• For Ions:
  E = (Molecular weight) / (Charge on ion)
  For Example, for SO₄^2-, E = M/2
  For PO₄^3-, E = M/3 
• For Compounds:
  E = (Molecular weight) / (Valency of cation or anion)
  For Example, for CaCO₃, E = M/2
  For AlCl₃, E =M/3 
• For Redox Reactions:
  E = (Molecular weight) / (Change in oxidation number)
  For Example, for KMnO₄
  (a) In acidic medium: E = M/5 
  
  
  5 unit change in oxidation number. 
  
  (b) In basic medium: E = M/1
  
  
  one unit change in oxidation number 
  
  (c) In neutral medium: E = M/3 
  
  
  3 unit change in oxidation number 
• For Acidic Salt: 
  E = (Molecular weight) / (Number of replaceable H-atoms) 
  For example, for H₃PO₄ 

• Metal displacement method 
  E₁ / E₂ = W₁ / W₂