Year 10 Chemistry: Using Molar Concentrations of Solutions

Introduction to Molar Concentration

Hello, Year 10! Today, we’re going to dive into the concept of molar concentration. This is a key idea in chemistry, especially when we work with solutions.

What is Molar Concentration?

Molar concentration, also known as molarity, tells us how much of a substance is dissolved in a certain volume of solution. It is measured in moles per litre, which we write as mol/L or just M.

Molarity Formula

The formula for calculating molarity is:

$$

\text{Molarity (M)} = \frac{\text{Number of moles of solute}}{\text{Volume of solution in litres}}

$$

Example

Let’s say you have 2 moles of salt (sodium chloride) dissolved in 1 litre of water. The molarity would be:

$$

M = \frac{2 \text{ moles}}{1 \text{ L}} = 2 \text{ M}

$$

This means you have a 2 molar solution of salt in water.

Key Rules for Molar Concentration

1. Concentration Increases with More Solute: If you add more solute (the substance being dissolved), the molarity increases.
2. Concentration Decreases with More Solution: If you add more solvent (the liquid doing the dissolving), the molarity decreases.
3. Units Matter: Always remember to convert your volume to litres because molarity is expressed in moles per litre.

Tips and Tricks

• Convert Volume: If you’re given volume in millilitres (mL), remember to convert it to litres (1 L = 1000 mL).
• Use a Calculator: When calculating moles, use a calculator to ensure accuracy.
• Practice: The more you practice calculating molarity, the easier it will become!

Questions

Easy Level Questions

1. What is molarity?
2. What is the unit of molarity?
3. If you have 1 mole of sugar in 1 L of water, what is the molarity?
4. How would you write 0.5 moles per litre using the proper notation?
5. If you dissolve 3 moles of salt in 2 L of water, what is the molarity?
6. What happens to molarity if you add more solute?
7. What happens to molarity if you add more water?
8. How many moles are in a 1 M solution of NaCl in 1 L?
9. Is it possible to have a 0 M solution? Why or why not?
10. What is the molarity of a solution with 4 moles of KCl in 4 litres of water?

Medium Level Questions

11. Calculate the molarity of a solution with 5 moles of solute in 2 litres of solution.
12. If you have a 2 M solution and dilute it to 5 L, how many moles do you have?
13. A solution has a molarity of 3 M. If you have 0.5 L of this solution, how many moles of solute are present?
14. If you dilute a 6 M solution to 3 L, what will be the new molarity?
15. How many litres of a 1.5 M solution would contain 3 moles of solute?
16. What is the molarity of a solution that has 0.2 moles in 1.5 L of solution?
17. If you double the amount of solute in a solution, what happens to the molarity?
18. How do you convert 250 mL of solution to litres?
19. If you have a 1.5 M NaOH solution, how many moles are in 2 L?
20. What is the concentration in mol/L if you dissolve 10 grams of NaCl in 2 L of water? (Molar mass of NaCl = 58.5 g/mol)

Hard Level Questions

21. You have a solution with a molarity of 0.75 M. If you want to prepare 2 L of this solution, how many moles of solute do you need?
22. A solution contains 4 moles of solute in 0.5 L. What is its molarity?
23. If you have 3 L of a 1.2 M solution, how many grams of solute are present? (Molar mass = 58.5 g/mol)
24. How would you prepare 500 mL of a 0.25 M solution from a 1 M stock solution?
25. A chemical reaction requires 0.3 moles of a reactant in a 2 M solution. What volume of solution do you need?
26. If you dilute 10 mL of a 2 M solution to 100 mL, what is the new molarity?
27. How many moles are in 0.75 L of a 3 M solution?
28. What is the molarity if you have 8 grams of solute in 0.5 L? (Molar mass = 32 g/mol)
29. You have a 2.5 M solution and you want to make it 1 M. What volume of solvent do you need to add if you start with 200 mL?
30. If you have 1.5 moles of a substance in 3 L of solution, what is the molarity?

Answers and Explanations

Easy Level Answers

1. Molarity is the concentration of a solution in moles per litre.
2. The unit of molarity is mol/L or M.
3. The molarity is 1 M.
4. 0.5 M.
5. The molarity is 1.5 M.
6. Molarity increases.
7. Molarity decreases.
8. There is 1 mole.
9. Yes, because it means no solute is present.
10. The molarity is 1 M.

Medium Level Answers

11. Molarity = $$\frac{5 \text{ moles}}{2 \text{ L}} = 2.5 \text{ M}$$
12. You have 10 moles.
13. There are 1.5 moles of solute.
14. The new molarity is 2 M.
15. You would need 2 L.
16. The molarity is $$\frac{0.2 \text{ moles}}{1.5 \text{ L}} = 0.133 \text{ M}$$
17. Molarity doubles.
18. 0.25 L.
19. There are 3 moles of NaOH.
20. The concentration is 0.34 M.

Hard Level Answers

21. You need 1.5 moles.
22. The molarity is 8 M.
23. There are 351 grams of solute.
24. You need to add 500 mL of water.
25. You need 150 mL of solution.
26. The new molarity is 0.2 M.
27. There are 2.25 moles.
28. The molarity is 0.25 M.
29. You need to add 800 mL of solvent.
30. The molarity is 0.5 M.

I hope this guide helps you understand how to use molar concentrations in your chemistry studies! Remember to practice, and you’ll become more confident with these concepts. Happy studying!