Expressing Solution Concentration

If a small amount of solute is added to a large quantity of solvent, a dilute solution is formed. The concentrations of very dilute solutions, such as trace metal impurities in water, are often expressed in parts per billion or ppb, or in parts per million or ppm.

Ppm of a solute can be calculated by dividing the mass of solute by the mass of solution and multiplying times one million.

If a large amount of solute is added to a small quantity of solvent, a concentrated solution is formed.

Commonly used units include relative units expressed as percentages, such as: mass percent, which is the mass of solute divided by the mass of solution times one hundred; volume percent, which is the volume of solute divided by the volume of solution times one hundred; and mass per volume percent, which is the mass of solute divided by the volume of solution times one hundred.

In the laboratory, these units are used along with mole fraction, molarity, and molality.

A mole fraction of component i, which can be either solute or solvent, is the number of moles of the component divided by the total number of moles of all components in the solution. This is a ratio with no units, denoted by the letter χ_i.

Molarity is the number of moles of solute in 1 liter of solution. Its symbol is uppercase M and the units are moles per liter.

Molality is the number of moles of solute in 1 kilogram of solvent. Its symbol is a lowercase italicized m and the units are moles per kilogram.

In molality, the mass of the solvent is considered, whereas, in molarity, the volume of the entire solution is taken into account.

To prepare a 1 molar or 1 molal solution, 1 mole of a solute, such as iodine, is dissolved in a solvent, such as carbon tetrachloride.

The final volume of the solution is made up to 1 liter to form a 1 molar solution.

For a molal solution, 1 kilogram of carbon tetrachloride is weighed out. Given that the density of carbon tetrachloride is 1.59 grams per milliliter, the volume of 1 kilogram of solvent would be 629 milliliters.

So the final concentration of 1 molal solution in carbon tetrachloride would be equal to 1.59 molar.