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 co…
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.
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Q1: What is molarity and how is it calculated?
Molarity (M) is the number of moles of solute per liter of solution, expressed in mol/L. To calculate molarity, divide the moles of solute by the total volume of solution in liters. For example, dissolving 1 mole of solute in a volumetric flask and filling to the 1-liter mark creates a 1-molar solution. The key is using the total solution volume, not just the solvent volume, since solutes can affect the overall volume.
Q2: Why is the total solution volume used in the molarity equation instead of solvent volume?
The total solution volume is used because solutes can change the volume of a solution depending on their interactions with the solvent. Using solvent volume alone would give an inaccurate concentration. The molarity equation accounts for this by including the combined volume of both solute and solvent, ensuring precise concentration measurements for chemical reactions in aqueous solutions.
Q3: How do you prepare a solution of specific molarity using a volumetric flask?
To prepare a solution of specific molarity, add the calculated moles of solute to a volumetric flask, then add solvent until the liquid reaches the marked line for your desired volume. For instance, to make 1 liter of a 1-molar solution, add 1 mole of solute and fill to the 1-liter mark. This ensures the total solution volume equals your target volume and produces the correct concentration.
Q4: What happens to the number of moles of solute when a solution is diluted?
The number of moles of solute remains constant during dilution. When solvent is added to a concentrated solution, the total volume increases but the amount of solute stays the same, causing the concentration to decrease. This principle is expressed mathematically as M1V1 = M2V2, where the product of molarity and volume equals moles before and after dilution.
Q5: How do you use the dilution equation to prepare a dilute solution from a stock solution?
The dilution equation M1V1 = M2V2 relates the molarity and volume of a stock solution to the desired dilute solution. Identify the known values: M1 and V1 from the stock solution, M2 and V2 from your target solution. Solve for the unknown variable, typically V1, to determine how much stock solution to transfer to a new flask, then add solvent to reach the final volume.
Q6: What is the difference between a dilute and concentrated solution?
A dilute solution has a small proportion of solute relative to solvent, resulting in low concentration. A concentrated solution has a large proportion of solute relative to solvent, resulting in high concentration. Concentrated solutions, called stock solutions, are often stored in laboratories and diluted as needed to create solutions of lower concentrations for specific applications.
Q7: Can you convert between mass of solute and solution volume using molarity?
Yes, molarity serves as a conversion factor between moles of solute and solution volume. First convert mass to moles using molar mass, then use molarity to find volume by dividing moles by molarity. For example, 237 grams of potassium permanganate converted to moles, then divided by 3 M, yields 0.5 liters of solution. This allows precise preparation of solutions with known concentrations.