This method is used when the solution is constituted by mixing two or more components. It is defined as the ratio of number of moles of one component to the tota number of moles of the solution (i.e., all the components). Taking three components A, B and C.

 

Components           A              B              C

 

Mass (in grams)      W₁            w₂             w₃

 

Molecular mass       m₁            m₂            m₃

 

No. of g moles        w₁             w₂             w₃

                              ___  _________________

 

 

Total number of g moles = w₁/m₁ + w₂/m₂ + w₃/m₃

 

 Thus, Mole fraction of A – w₁/m₁/w₁/m₁ + w₂/m₂ + w₃/m₃ = f_A

 

Mole fraction of B = w₂/m₂/w₁/m₁ + w₂/m₂ + w₃/m₃ = f_B

 

Mole fraction of C = w₃/m₃/w₁/m₁ + w₂/m₂ + w₃/m₃ = f_C

 

The sum of mole fractions of a solution is equal to 1,

 

i.e.,               f_A + f_B + f_C = 1.

 

In a binary solution,

 

Mole fraction of solute + Mole fraction of solvent = 1

 

Let n moles of solute (A) and N moles of solvent (B) be present in a solution.

 

Mole fraction of solute = n/N+n = X_A

 

Mole fraction of solvent = N/N+n = X_B

 

Thus,             X_A + X_B = 1