2.4.3 Concentration of Solutions Explained
Key Concepts
1. Definition of Concentration
Concentration refers to the amount of solute present in a given amount of solvent or solution. It is a measure of how much of a substance is dissolved in another substance.
2. Units of Concentration
Concentration can be expressed in various units, including:
- Molarity (M): Moles of solute per liter of solution.
- Molality (m): Moles of solute per kilogram of solvent.
- Mass Percent (%): Mass of solute divided by the mass of the solution, multiplied by 100.
- Volume Percent (% v/v): Volume of solute divided by the volume of the solution, multiplied by 100.
- Parts per Million (ppm): Number of parts of solute per million parts of the solution.
3. Dilution
Dilution is the process of reducing the concentration of a solution by adding more solvent. The amount of solute remains constant, but the volume of the solution increases.
Detailed Explanation
Molarity (M)
Molarity is the most common unit of concentration in chemistry. It is calculated using the formula:
M = moles of solute / liters of solution
For example, a 1 M solution of sodium chloride (NaCl) contains 1 mole of NaCl dissolved in 1 liter of solution.
Molality (m)
Molality is used to express concentration in terms of the mass of the solvent rather than the volume of the solution. It is calculated using the formula:
m = moles of solute / kilograms of solvent
For example, a 1 m solution of glucose (C6H12O6) contains 1 mole of glucose dissolved in 1 kilogram of water.
Mass Percent (%)
Mass percent is a simple way to express concentration based on the mass of the solute and the solution. It is calculated using the formula:
Mass Percent = (mass of solute / mass of solution) × 100
For example, a 10% mass percent solution of sugar in water contains 10 grams of sugar dissolved in 90 grams of water.
Volume Percent (% v/v)
Volume percent is used for solutions where both the solute and solvent are liquids. It is calculated using the formula:
Volume Percent = (volume of solute / volume of solution) × 100
For example, a 50% volume percent solution of ethanol in water contains 50 milliliters of ethanol in 100 milliliters of solution.
Parts per Million (ppm)
Parts per million is used to express very low concentrations. It is calculated using the formula:
ppm = (mass of solute / mass of solution) × 1,000,000
For example, a solution with 1 ppm of lead contains 1 milligram of lead in 1 kilogram of solution.
Dilution
Dilution is a common laboratory procedure where a concentrated solution is made less concentrated by adding more solvent. The formula for dilution is:
M1V1 = M2V2
Where M1 and V1 are the initial molarity and volume, and M2 and V2 are the final molarity and volume.
Examples and Analogies
Example: Molarity Calculation
If you dissolve 2.5 grams of sodium chloride (NaCl) in 500 milliliters of water, you can calculate the molarity as follows:
Moles of NaCl = 2.5 g / 58.44 g/mol = 0.0428 mol
Molarity = 0.0428 mol / 0.5 L = 0.0856 M
Analogy: Concentration as a Recipe
Think of concentration as a recipe. Just as a recipe specifies the amount of each ingredient to use, concentration specifies the amount of solute to use in a given amount of solvent. For example, a recipe for lemonade might call for 1 cup of sugar per liter of water, which is similar to a 1 M solution of sugar in water.
Example: Dilution Process
If you have a 2 M solution of hydrochloric acid (HCl) and you want to dilute it to 0.5 M, you can use the dilution formula:
M1V1 = M2V2
2 M × V1 = 0.5 M × 1 L
V1 = 0.25 L
So, you would take 0.25 liters of the 2 M HCl solution and add enough water to make 1 liter of 0.5 M solution.
Conclusion
Understanding the concentration of solutions is crucial for various scientific and practical applications. By mastering the different units of concentration and the process of dilution, you can accurately prepare and manipulate solutions for experiments and everyday use.