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How to Determine the Osmolarity

Osmolarity is a measure of the concentration of a solute in a solution. Osmolarity is measured in osmoles of solute per liter, or Osm/L. The difference between osmolarity and molarity is that osmolarity is the measure of the moles of solute atoms as opposed to moles of solute as a whole. The idea of osmolarity arises because sometimes a solute dissociates in a solution. That is, the molecules can break apart into their component atoms.

Instructions

  1. Osmolarity of a Mixture

    • 1

      Measure the amounts of each component substance within the mixture.

    • 2

      Determine the number of osmoles per unit of measure, which are particles that will not disassociate when disolved.

    • 3

      Multiply the osmoles per unit/measure for each substance by the volume of that substance. For example, if a solution is made up of three substances, one with a volume of 50 liters and an osmoles per unit/measure of 2 osmoles per liter, one with 10 L and 4 Osm/L and one with 5 L and 2 Osm/L, multiply the volume of each substance by its Osm/L. So, (50 L * 2 Osm/L) = 100 Osmoles, (10 L * 4 Osm/L) = 40 Osmoles and (5 L * 2 Osm/L) = 10 Osmoles.

    • 4

      Add the values of osmoles together to determine the osmolarity of the whole mixture. For example, if three component substances of a mixture contain 100 Osmoles, 40 Osmoles and 10 Osmoles, the entire mixture contains (100 Osm + 40 Osm + 10 Osm) = 150 Osm.

    Osmolarity from Molarity

    • 5

      Add up the total number of atoms per molecule of substance. For example, MgCl2 has three atoms, 1 Mg atom and 2 Cl atoms.

    • 6

      Measure the molarity of the solution. Molarity equals the number of moles of a solute (the substance being dissolved) divided by the number of liters of the entire solution.

    • 7

      Multiply the molarity of the solution by the number of atoms per molecule. For example, a solution of MgCl2, NaCl and glucose with a molarity of 3 M and (1 Mg + 3 Cl + 1 Na + 1 glucose) = 6 atoms, becomes: (3 M * 6 atoms) = 18 Osm.


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