Solution Concentration and Dilution

In a given quantity of solvent, different amounts of solutes can be added to create solutions of varying concentrations.

In a dilute solution, the proportion of solute relative to the solvent is small, whereas for a concentrated solution, the proportion is large.

The concentration of a solution is expressed as molarity, which is the number of moles of solute per liter of solution.

The denominator is the volume of the solution and not the volume of the solvent. Therefore, to make 1 liter of a 1-molar solution, 1 mole of solute is added to a volumetric flask, which is then filled to the 1-liter demarcation with the solvent. Similarly, if 1 mole of solute is added to another flask and then filled to the 0.5 liter mark with solvent, this would give a 2-molar solution.

The molarity of the solution can be used as a conversion factor between the moles of the solute and the volume of the solution.

Suppose 237 grams of potassium permanganate is dissolved in water to make a 3-molar solution. To calculate the total volume of the solution, first, the mass of the solute is converted to moles. Then, the number of moles is divided by the molarity to get a volume of 0.5 liters.

If more water is added, the total volume of the solution increases, but the number of moles of the solute stays the same. This means that the concentration of the solution has decreased, or the solution has been diluted.

Dilution of a concentrated, or stock, solution is performed using the dilution equation, which relates the volumes and molarities of a solution before and after the dilution process. M₁ and V₁ are the molarity and volume of the initial concentrated solution, and M₂ and V₂ are the molarity and volume of the final diluted solution.

For example, the dilution equation can be used to prepare a solution of potassium permanganate, which is used medicinally as a general disinfectant. What volume of a 2-molar stock solution is needed to prepare 1 liter of a 0.2-molar solution of potassium permanganate?

Knowing M₁, M₂, and V₂ and solving the dilution equation for V₁ yields 0.1 liters. Therefore, 0.1 liters of the stock solution is added to a new volumetric flask, and solvent is added to make 1 liter of the 0.2-molar solution.

After the solution is diluted, although the concentration has changed, the number of moles of potassium permanganate stays the same.