A Level Chemistry Revision "Relative Molecular Mass and Relative Formula Mass"

Introduction

This tutorial will guide you through the concepts of relative molecular mass and relative formula mass, essential topics in A Level Chemistry. Understanding how to calculate these masses is crucial for solving chemical problems, especially when dealing with molecular and ionic compounds.

Step 1: Understanding Relative Molecular Mass

Relative molecular mass (Mr) is the mass of a molecule compared to one-twelfth of the mass of a carbon-12 atom. To calculate it:

1. Identify the chemical formula of the molecule.
2. Look up the atomic masses of each element from the periodic table.
3. Multiply the atomic mass of each element by the number of times it appears in the formula.
4. Sum all the values to find the relative molecular mass.

Example Calculation: For water (H₂O):

• Hydrogen (H) has an atomic mass of approximately 1.01 g/mol.
• Oxygen (O) has an atomic mass of approximately 16.00 g/mol.

Calculation:

• (2 × 1.01) + (1 × 16.00) = 2.02 + 16.00 = 18.02 g/mol

Tip: Always check the periodic table for the most accurate atomic masses, as they can vary slightly between sources.

Step 2: Understanding Relative Formula Mass

Relative formula mass (Mr) is similar to relative molecular mass but is used for ionic compounds. The steps are akin to those for molecular mass:

1. Identify the formula of the ionic compound.
2. Find the atomic masses for each constituent element.
3. Multiply each atomic mass by the number of atoms in the formula.
4. Add these values to calculate the relative formula mass.

Example Calculation: For sodium chloride (NaCl):

• Sodium (Na) has an atomic mass of approximately 22.99 g/mol.
• Chlorine (Cl) has an atomic mass of approximately 35.45 g/mol.

Calculation:

• (1 × 22.99) + (1 × 35.45) = 22.99 + 35.45 = 58.44 g/mol

Common Pitfall: Many students forget to account for the number of atoms of each element in the formula. Always double-check your counts.

Step 3: Practice Problems

To reinforce your understanding, try calculating the relative molecular mass and relative formula mass for the following compounds:

1. Carbon dioxide (CO₂)
2. Magnesium oxide (MgO)
3. Glucose (C₆H₁₂O₆)

Tip: Work through each step methodically and verify your atomic mass values.

Conclusion

In this tutorial, you learned how to calculate relative molecular mass and relative formula mass. These calculations are fundamental in chemistry, especially for stoichiometry and reaction equations. To further solidify your knowledge, practice with various compounds and ensure you understand the periodic table's role in these calculations. Consider checking the UK A Level Chemistry specifications for additional context and requirements.