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How to Calculate Mole Fraction Given Only Percentage

Chemical calculations require frequent conversion back and forth between unit systems. Expressions of concentration for a solute-solvent system include molarity, molality, weight/weight percent and ppm. Convert weight/weight percent concentration to mole fraction by converting the percentages to moles of solute and solvent. Once you convert the moles to weights, you can calculate the mole fraction of each component.

Things You'll Need

Periodic table
Calculator

Instructions

- 1
  Identify all the components of the system of interest. For a solution, the components are the solute and the solvent. For example, assume the system of interest is a sodium chloride solution. Sodium chloride is the solute and water is the solvent. Further assume that the solution contains 7.5% NaCl.
- 2
  Calculate the mole percent of each component. Mole % is equal to moles fraction of component of solution divided by the total number of moles of both solute and solvent. Multiply the resulting values by 100 to achieve the mole percent. For example, using the solution above of salt mixed with water, the salt is 7.5% and the water is 92.5%. Assume we are discussing 100 g of solution, and convert this value of weight percent to a mole percent. You have 7.5 grams of sodium chloride and 92.5 grams of water. Following this calculation through, the moles of NaCl is 7.5 g / 58.44 g (molecular weight of sodium chloride), so grams of sodium chloride / molecular weight of sodium chloride = 7.5 / 58.44 = 0.128 moles. The same calculation for the water yields 92.5 / 18.02 = 5.133 moles.
- 3
  Calculate the mole fraction of solute in solution. The equation for the mole fraction of solute is mole fraction (solute) = moles (solute) / [ moles (solute) + moles (solvent)]. Continuing the example, mole fraction (NaCl) = moles (NaCl) / [moles (NaCl) + moles (water)] = 0.128 / (0.128 + 5.133) = 0.128 / 5.261 = 0.0243.
- 4
  Calculate the mole fraction of the solvent in solution by the following formula moles fraction (solvent) = moles (solvent) / [moles (solute) + moles (solvent)] = 5.133 / [ 0.128 + 5.133] = 5.133 / 5.261 = 0.9757.

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