Calculating Mole Fraction

Known                                                                 Unknown

Mass of solute                                            Concentration of solution

Mass of solvent

Moles of solute

Moles of solvent


Mole fraction  is the ratio of the number of moles of solute in the solution to the total faction for the solute.

XA = nA / nA + nB

XB = nB / nA + nB

The symbol X is commonly used for mole fraction, with a subscript to indicate the solvent or solute. The mole fraction for the solvent is XA and the mole fraction for the solute is XB while nA represents the number of moles of solvent and nB stands for the number of moles of solute.

For example, we will do the mole fraction of hydrochloric acid (HCl) in the aqueous solution with 33.6 g HCl and 66.4 g of H2O.

nHCl = 33.6 g HCl X 1 mol HCl / 36.5 g HCl

nHCl = 0.92 mol HCl

nH2O = 66.4 g H2O X 1 mol H2O/  18 g H2O

nH2O = 3.68 mol H2O

With what is shown above, the mole fractions of hydrochloric acid and water can be expressed as:

XHCL = nHCl / nHCl + nH2O

 0.92 mol HCl / 0.92 mol HCl +3.68 mol H2O

XHCl = 0.2

XH2O = nH2O/ nHCl + nH2O

XH2O = 3.68 mol H2O / 0.92 mol HCl +3.68 mol H2O

XH2O = 0.8

ALL mole fraction answers should end up being a whole (i.e. 1, 100, ect)



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