Mole Fraction Formula


What is Mole fraction?

Mole fraction represents the number of molecules of a particular component in a mixture divided by the total number of moles in the given mixture. It’s a way of expressing the concentration of a solution.


Mole Fraction formula

The molar fraction can be represented by X. If the solution consists of components A and B, then the mole fraction is,






Therefore, the sum of mole fraction of all the components is always equal to one.

Please note that mole fraction represents a fraction of molecules, and since different molecules have different masses, the mole fraction is different from the mass fraction.


Example 1  

A tank is charged with a mixture of 1.0 x 103 mol of oxygen and 4.5 x 103 mol of helium. Calculate the mole fraction of each gas in the mixture.



The given parameters are

NHe = 4.5 x 103 mol and nO2 = 1.0 x 103 mol

Mole fraction can be calculated as

XHe = 4.5 x 103 mol / 4.5 x 103 mol + 1.0 x 103 mol

   4.5 mol / 5.5 mol

= 0.82

XO2 = 2.0 x 103 mol / 4.5 x 103 mol + 1.0 x 103 mol

   = 1.0 x 103 / 5.5 x 103

   = 0.18

Example 2

Determine the mole fraction of methanol CH3OH and water in a solution prepared by dissolving 4.5 g of alcohol in 40 g of H2O. M of H2O is 18 and M of CH3OH is 32.



Moles of CH3OH = 4.5 / 32 = 0.14 mole

Moles of H2O = 40 / 18 = 2.2 moles

Therefore, according to the equation

mole fraction of CH3OH = 0.14 / 2.2 + 0.14


                                      = 0.061

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Practise This Question


What can be inferred from the above reaction?

(i)  CaO is solid.

(ii) CaO is reactive with water.

(iii) H2O is liquid.

(iv) H2O is in vapour state.

(v) Ca(OH)2  forms a solution with water

(vi) The reaction is exothermic.