High School Chemistry: Solutions - Concentration - Dilute vs. Concentrated

Concentration (Dilute vs Concentrated) Study Guide

1. What This Is (In Plain English)

Concentration is how much of a substance is mixed with a certain amount of something else, like water or air. Think of it like making a favorite drink: if you add a lot of sugar to a small amount of water, it's super sweet, but if you add just a little sugar to a big glass of water, it's not as sweet.

In real life, concentration matters because it affects how things work. For example, if a medicine is too concentrated, it might be too strong for our bodies, but if it's too diluted, it might not work at all. Without understanding concentration, we wouldn't have safe and effective medicines, or even tasty drinks like our favorite flavor of soda!

2. Key Ideas & Definitions

• Concentration: The amount of a substance mixed with a certain amount of something else.
  • Definition: Think of it like the ratio of sugar to water in your favorite drink.
  • Example: If you have 10 grams of sugar and 100 grams of water, the concentration is 10:100 or 1:10.
• Dilute: A solution with a low concentration of a substance.
  • Definition: It's like adding a lot of water to a small amount of juice.
  • Example: If you mix 10 grams of sugar with 1000 grams of water, it's a dilute solution.
• Concentrated: A solution with a high concentration of a substance.
  • Definition: It's like adding a small amount of juice to a lot of water.
  • Example: If you mix 100 grams of sugar with 100 grams of water, it's a concentrated solution.
• Molarity (M): A unit of concentration that measures the number of moles of a substance per liter of solution.
  • Definition: It's like counting the number of sugar molecules in a certain amount of water.
  • Example: If you have 1 mole of sugar in 1 liter of water, the molarity is 1 M.
• Normality (N): A unit of concentration that measures the number of equivalents of a substance per liter of solution.
  • Definition: It's like counting the number of sugar molecules that can react with something else.
  • Example: If you have 1 equivalent of sugar in 1 liter of water, the normality is 1 N.
• Percent Solution (%): A unit of concentration that measures the percentage of a substance in a solution.
  • Definition: It's like measuring the percentage of sugar in your favorite drink.
  • Example: If you have 10 grams of sugar in 100 grams of water, the concentration is 10%.
• Solute: The substance that is dissolved in a solution.
  • Definition: It's like the sugar in your favorite drink.
  • Example: Sugar is the solute in a sugar-water solution.
• Solvent: The substance that dissolves the solute.
  • Definition: It's like the water in your favorite drink.
  • Example: Water is the solvent in a sugar-water solution.
• Solution: A mixture of a solute and a solvent.
  • Definition: It's like the sugar-water mixture in your favorite drink.
  • Example: A sugar-water solution is a mixture of sugar and water.

3. How To Do It (Step-by-Step)

Calculating Concentration

1. Determine the amount of solute and solvent in the solution.
2. Calculate the ratio of solute to solvent.
3. Express the concentration as a percentage, molarity, or normality.
4. Use the formula: Concentration = (Amount of solute / Amount of solvent) x 100 (for percentage) or Concentration = (Number of moles of solute / Volume of solution in liters) (for molarity or normality).
5. Sample numbers: If you have 20 grams of sugar and 200 grams of water, the concentration is (20 / 200) x 100 = 10%.

Making a Concentrated Solution

1. Determine the desired concentration of the solution.
2. Calculate the amount of solute needed.
3. Measure out the correct amount of solute.
4. Add the solute to the solvent and mix well.
5. Sample numbers: If you want to make a 20% sugar solution, you need 20 grams of sugar for every 100 grams of water.

4. Watch Out! (Common Mistakes)

• Mistake: Forgetting to convert units when calculating concentration.
  • Fix: Make sure to convert units correctly, like grams to moles or liters to milliliters.
• Mistake: Not accounting for the density of the solute when making a concentrated solution.
  • Fix: Use a density column or a balance to accurately measure the amount of solute needed.
• Mistake: Using the wrong unit of concentration (e.g., using molarity when normality is required).
  • Fix: Double-check the unit of concentration required for the problem or experiment.
• Mistake: Not considering the effects of temperature on concentration.
  • Fix: Take into account the effects of temperature on the solute and solvent, and adjust the concentration accordingly.

5. Practice Problems

Problem 1

A solution contains 30 grams of salt and 150 grams of water. What is the concentration of the solution in percentage?

Solution:

1. Determine the amount of solute and solvent: 30 grams of salt and 150 grams of water.
2. Calculate the ratio of solute to solvent: 30 / 150 = 1 / 5.
3. Express the concentration as a percentage: (1 / 5) x 100 = 20%.
4. Answer: The concentration of the solution is 20%.

Problem 2

A concentrated solution of sugar contains 50 grams of sugar per 100 grams of water. What is the concentration of the solution in molarity?

Solution:

1. Determine the amount of solute and solvent: 50 grams of sugar and 100 grams of water.
2. Calculate the number of moles of sugar: 50 grams / 18.02 g/mol = 2.78 mol.
3. Calculate the volume of the solution in liters: 100 grams / 1000 g/L = 0.1 L.
4. Express the concentration as molarity: 2.78 mol / 0.1 L = 27.8 M.
5. Answer: The concentration of the solution is 27.8 M.

6. Cram Sheet

• Concentration is the amount of a substance mixed with a certain amount of something else.
• Dilute solutions have a low concentration of a substance.
• Concentrated solutions have a high concentration of a substance.
• Molarity measures the number of moles of a substance per liter of solution.
• Normality measures the number of equivalents of a substance per liter of solution.
• Percent solution measures the percentage of a substance in a solution.
• Solute is the substance that is dissolved in a solution.
• Solvent is the substance that dissolves the solute.
• Solution is a mixture of a solute and a solvent.
• Mass stays the same during a phase change; energy is what changes.
• Concentration is affected by temperature.
• Use the correct unit of concentration for the problem or experiment.

7. Where to Learn More

• Amoeba Sisters: A fun and educational YouTube channel that covers various chemistry topics, including concentration.
• PhET Simulations: Interactive simulations that allow you to explore and learn about concentration and other chemistry concepts.
• Chemistry LibreTexts: A free online textbook that covers concentration and other chemistry topics in detail.

Remember, concentration is all about the amount of a substance mixed with something else. Practice calculating concentration, making concentrated solutions, and avoiding common mistakes to become a pro at chemistry!