Mole Fraction

The mole fraction of a solution component X_i  is the fraction of moles of component i of the total number of moles of all components in solution.  

		moles of component i
Xi	=	------------------------
		total moles of solution Example problem What is the mole fraction of each component in a solution in which 3.57 g of sodium chloride, NaCl, is dissolved in 25.0 g of water? Solution First, convert from mass of NaCl to moles of NaCl. 1 mole NaCl 3.57 g NaCl x --------------------- = 0.0611 mole NaCl 58.44 g NaCl   Next, convert from mass of water to moles of water. 1 mole H2O 25.0 g H2O x --------------------- = 1.39 mole H2O 18.02 g H2O Substitute these two quantities into the defining equation for mole fraction. 0.0611 mol NaCl XNaCl = -------------------- = 0.0421 (0.0611 + 1.39) mol solution 1.39 mole H2O Xwater = -------------------- = 0.958 (0.0611 + 1.39) mol solution  Note that within the limits of the significant figures that XNaCl + Xwater = 1.  The sum of the mole fractions for a solution will equal 1.  In our example above, after we have calculated one mole fraction we could have subtracted it from 1 to obtain the other.