A Level Chemistry Revision "Calculations Involving the Avogadro Constant Part 1"

Introduction

This tutorial will guide you through the calculations involving the Avogadro constant, a fundamental concept in A Level Chemistry. Understanding how to calculate the number of atoms in a sample is crucial for stoichiometry and various applications in chemistry. This guide will cover how to determine the number of atoms in both pure and impure samples, providing practical examples to enhance your comprehension.

Step 1: Understanding the Avogadro Constant

• The Avogadro constant (approximately (6.022 \times 10^{23})) represents the number of atoms or molecules in one mole of a substance.
• To use the Avogadro constant in calculations, remember the following:
  • 1 mole of any substance contains (6.022 \times 10^{23}) representative particles (atoms, molecules, or formula units).
  • The molar mass of an element (found on the periodic table) is the mass of one mole of that element in grams.

Practical Tip

• Always ensure your mass is in grams and your molar mass is in grams per mole (g/mol) for accurate calculations.

Step 2: Calculating the Number of Atoms in a Pure Sample

1. Find the mass of the element in grams.
2. Determine the molar mass of the element from the periodic table.
3. Calculate the number of moles using the formula: [ \text{Number of moles} = \frac{\text{Mass of element (g)}}{\text{Molar mass (g/mol)}} ]
4. Calculate the number of atoms by multiplying the number of moles by the Avogadro constant: [ \text{Number of atoms} = \text{Number of moles} \times 6.022 \times 10^{23} ]

Example

• If you have 12 g of carbon (C):
  • Molar mass of C = 12 g/mol
  • Number of moles = ( \frac{12 \text{ g}}{12 \text{ g/mol}} = 1 \text{ mole} )
  • Number of atoms = ( 1 \text{ mole} \times 6.022 \times 10^{23} = 6.022 \times 10^{23} \text{ atoms} )

Step 3: Calculating the Number of Atoms in an Impure Sample

1. Estimate the mass of the element in the impure sample.

   • For example, consider a coin that contains a certain percentage of a metal (e.g., copper).

2. Determine the percentage composition of the desired element in the sample.

3. Calculate the mass of the pure element in the sample: [ \text{Mass of pure element} = \text{Total mass of sample} \times \left(\frac{\text{Percentage of element}}{100}\right) ]

4. Repeat the calculations from Step 2 to find the number of moles and then the number of atoms.

Example

• If a coin weighs 10 g and contains 75% copper:
  • Mass of pure copper = ( 10 \text{ g} \times 0.75 = 7.5 \text{ g} )
  • Molar mass of Cu = 63.5 g/mol
  • Number of moles of Cu = ( \frac{7.5 \text{ g}}{63.5 \text{ g/mol}} \approx 0.118 \text{ moles} )
  • Number of atoms of Cu = ( 0.118 \text{ moles} \times 6.022 \times 10^{23} \approx 7.09 \times 10^{22} \text{ atoms} )

Conclusion

In this tutorial, you learned how to use the Avogadro constant to calculate the number of atoms in both pure and impure samples. Remember the steps:

• Understand the Avogadro constant.
• Calculate for pure samples using mass and molar mass.
• Apply percentage composition for impure samples.

These skills are essential for A Level Chemistry and will be useful in various real-world applications, such as analyzing materials in chemistry labs. For further practice, continue to explore more advanced calculations in the next video or additional resources.