A Level Chemistry Revision "Working with Solutions 2"

Introduction

This tutorial focuses on calculations involving the concentration of solutions, specifically when given the mass of a chemical rather than the number of moles. It is designed for A Level Chemistry students but can also benefit those studying International A Level Chemistry. By following the steps outlined below, you will learn how to convert mass into moles and use this information to calculate concentrations.

Step 1: Understand the Relationship Between Mass and Moles

To begin, familiarize yourself with the key relationship in chemistry:

• Moles are a measure of the amount of a substance.

• The formula to convert mass into moles is:

  [ \text{Moles} = \frac{\text{Mass (g)}}{\text{Molar Mass (g/mol)}} ]

Practical Tips

• Always ensure you have the correct molar mass for the substance you are working with.
• Use a periodic table to find the atomic masses of elements to calculate the molar mass accurately.

Step 2: Calculate Molar Mass

Before performing calculations, you need to determine the molar mass of the chemical involved.

1. Identify the chemical formula of the substance.
2. Break down the formula into its constituent elements.
3. Sum the atomic masses of all elements in the formula according to their quantities.

Example

For sodium chloride (NaCl):

• Sodium (Na) has an atomic mass of approximately 23 g/mol.
• Chlorine (Cl) has an atomic mass of approximately 35.5 g/mol.
• Molar mass of NaCl = 23 + 35.5 = 58.5 g/mol.

Step 3: Convert Mass to Moles

Now that you have the molar mass, you can convert the given mass of the substance into moles.

1. Use the formula from Step 1.
2. Plug in the values for mass and molar mass.

Example Calculation

If you have 10 grams of NaCl: [ \text{Moles of NaCl} = \frac{10 \text{ g}}{58.5 \text{ g/mol}} \approx 0.171 \text{ moles} ]

Step 4: Calculate Concentration

Once you have the number of moles, you can calculate the concentration of the solution.

1. Use the formula for concentration:

[ \text{Concentration (mol/L)} = \frac{\text{Moles}}{\text{Volume (L)}} ]

2. Ensure that the volume of the solution is in liters.

Example Calculation

If the solution volume is 0.5 L: [ \text{Concentration} = \frac{0.171 \text{ moles}}{0.5 \text{ L}} \approx 0.342 \text{ mol/L} ]

Step 5: Practice with Examples

To solidify your understanding, practice with various scenarios. Here are three examples to try:

1. Calculate the concentration of a solution made by dissolving 20 grams of potassium nitrate (KNO3) in 1 liter of water (molar mass of KNO3 = 101 g/mol).
2. Determine the moles of magnesium sulfate (MgSO4) in 15 grams and the concentration if dissolved in 250 mL of solution (molar mass = 120.4 g/mol).
3. Find the concentration of a solution containing 30 grams of calcium chloride (CaCl2) in 750 mL of water (molar mass = 110.98 g/mol).

Conclusion

In this tutorial, you learned how to convert mass into moles and calculate the concentration of solutions. Key steps include understanding the mass-mole relationship, calculating molar mass, converting mass to moles, and determining concentration. To further enhance your skills, practice with different substances and scenarios. For more in-depth learning, consider reviewing additional A Level Chemistry resources.