4.5:
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.
4.5:
Solution Concentration and Dilution
The relative amount of a given solution component is known as its concentration. Often, though not always, a solution contains one component with a concentration that is significantly greater than that of all other components. This component is called the solvent and may be viewed as the medium in which the other components are dispersed or dissolved. Solutions in which water is the solvent are, of course, very common on our planet. A solution in which water is the solvent is called an aqueous solution.
A solute is a component of a solution that is typically present at a much lower concentration than the solvent. Solute concentrations are often described with qualitative terms such as dilute (of relatively low concentration) and concentrated (of relatively high concentration).
Concentrations may be quantitatively assessed using a wide variety of measurement units, each convenient for particular applications. Molarity (M) is a useful concentration unit for many applications in chemistry. Molarity is defined as the number of moles of solute in exactly 1 liter (1 L) of the solution and has the units of ‘mol/L’.
Note that in the molarity equation, the volume of solution, and not the volume of solvent, is used. This is because, depending on the nature of interactions between the solute and solvent, the solute can change the volume of the solution. Hence, in the molarity equation, we use the total solution volume (i.e., solvent volume + solute volume). Because solution volumes vary with temperature, molar concentrations will likewise vary. When expressed as molarity, the concentration of a solution with identical numbers of solute and solvent species will be different at different temperatures, due to the contraction/expansion of the solution.
Dilution of Solutions
Dilution is the process whereby a solution is made less concentrated (or more dilute) by the addition of solvent. For example, a glass of iced coffee becomes increasingly dilute, and less sweet, as the ice melts. In laboratories, solutions are often stored in their concentrated forms, called stock solutions. Solutions of lower concentrations are prepared from stock through dilution.
where M and V are concentration and volume, respectively, and the subscripts “1” and “2” refer to the solution before and after the dilution, respectively.
Now, since the product of molarity and volume equals moles, the number of moles before and after dilution stays the same.
Thus, dilution does not change the amount of solute in the solution.
This text is adapted from OpenStax Chemistry 2e, Section 3.3: Molarity.