The mole fraction of a solution component Xi is the fraction of moles of component i of the total number of moles of all components in solution.
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moles of component i
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Xi
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=
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total moles of solution
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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?
First, convert from mass of NaCl to moles of NaCl.
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1 mole NaCl
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3.57 g NaCl
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x
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=
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0.0611 mole NaCl
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58.44 g NaCl
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Next, convert from mass of water to moles of water.
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1 mole H2O
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25.0 g H2O
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x
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1.39 mole H2O
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18.02 g H2O
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Substitute these two quantities into the defining equation for mole fraction.
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0.0611 mol NaCl
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XNaCl
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0.0421
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(0.0611 + 1.39) mol solution
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1.39 mole H2O
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Xwater
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0.958
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(0.0611 + 1.39) mol solution
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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.