A Level Chemistry Revision "Calculating Empirical Formula Part 2"

Introduction

This tutorial provides a step-by-step guide to calculating the empirical and molecular formulas of a compound, building on concepts from A Level Chemistry. Understanding how to derive these formulas is crucial for analyzing chemical compounds and is relevant for students preparing for exams.

Step 1: Understanding Empirical Formula

• The empirical formula represents the simplest whole-number ratio of atoms in a compound.
• To find the empirical formula:
  1. Determine the mass of each element present in grams.
  2. Convert these masses to moles by dividing by the atomic mass of each element.
  3. Divide all the mole values by the smallest number of moles calculated to get the simplest ratio.

Practical Tip

• Always ensure you're using the correct atomic masses from the periodic table, as inaccuracies can lead to incorrect ratios.

Step 2: Understanding Molecular Formula

• The molecular formula indicates the actual number of atoms of each element in a molecule.
• To determine the molecular formula:
  1. Calculate the empirical formula mass (the sum of the atomic masses in the empirical formula).
  2. Find the relative molecular mass of the compound (often given in chemical problems).
  3. Divide the relative molecular mass by the empirical formula mass to find a multiplier (n).
  4. Multiply the subscripts in the empirical formula by n to get the molecular formula.

Example

• If the empirical formula is CH2 and the empirical formula mass is 14 g/mol while the molecular mass is 28 g/mol:
  • n = 28 g/mol ÷ 14 g/mol = 2
  • Therefore, the molecular formula is C2H4.

Step 3: Practice Question

• Now it's time to apply what you've learned. Use the following scenario:
  • Given a compound with an empirical formula of NO2 and a molecular mass of 92 g/mol, determine the molecular formula.

Calculation Steps

1. Calculate the empirical formula mass of NO2:
   • N: 14 g/mol
   • O: 16 g/mol × 2 = 32 g/mol
   • Total = 14 + 32 = 46 g/mol
2. Find n:
   • n = 92 g/mol ÷ 46 g/mol = 2
3. Multiply the subscripts in the empirical formula by n:
   • Molecular formula = N2O4

Conclusion

In this tutorial, we covered how to calculate both empirical and molecular formulas, emphasizing the importance of understanding atomic masses and conversion to moles. Practice these steps with various compounds to solidify your understanding. For further practice and resources, consider reviewing additional A Level Chemistry materials available online.