How to Convert Percent to Molarity

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A lab may use either percent weight per volume or molarity to communicate the concentration of a chemical solution. Both of these describe the strength of a solution, although in different ways. Percent weight per volume gives the grams of chemical per 100 ml of solution, whereas molarity gives the number of moles of chemical per liter of solution. A mole is a unit that stands for a specific number of atoms or molecules; 6.02 x 10^23. You can convert from percent to molarity once you know the gram molecular weight of your chemical.

Things You'll Need

  • Periodic table
  • Calculator
  • Write down the chemical formula for the compound in your solution. This formula will specify the elements in the compound and how many atoms of each element are in a molecule of that compound. If you had a salt solution, for example, you would write out the chemical formula for sodium chloride; NaCl.

  • Look up the atomic weight for each element in the chemical formula. You can find these weights on a standard periodic table of the elements. In the case of the example, the atomic weight of sodium (Na) is 22.99 and that of chlorine (Cl) is 35.45.

  • Multiply each element's atomic weight by the number of atoms of that element per molecule of compound. Add together all the products of those multiplications. The value you obtain will be the gram molecular weight of that chemical, in units of grams per mole. For NaCl, there is only one sodium atom and one chlorine atom per molecule, so you would calculate:

    (1)(22.99) + (1)(35.45) = 58.44 grams per mole.

  • Multiply the value of percent concentration by 10. Since percent weight per volume is grams chemical per 100 milliliters solution, this calculation gives the grams of chemical in each liter of solution -- because there are 1,000 ml in a liter. If a solution of NaCl is 12 percent, you would multiply that value by 10 to determine there are 120 g of NaCl per liter.

  • Divide the value of grams you just calculated by the gram molecular weight of the chemical. This calculation provides the number of moles of chemical per liter of solution, which is the definition of molarity; at this point you have converted the original percent concentration into molarity. In the case of the NaCl solution, 120 g divided by 58.44 g per mole gives a molarity of 2.05.

Tips & Warnings

  • Make sure your percent concentration is a percent weight per volume, since there are other types of percent concentrations.

References

  • Photo Credit Creatas/Creatas/Getty Images
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