AQA GCSE FOUNDATION Chemistry: Empirical and Molecular Formulas

AQA GCSE Foundation Chemistry: Empirical and Molecular Formulas

Introduction

This tutorial focuses on determining empirical and molecular formulas from experimental data. These formulas are crucial in understanding the composition of chemical compounds.

Empirical Formula: The simplest whole number ratio of atoms of each element in a compound. Molecular Formula: The actual number of atoms of each element in a molecule.

Determining Empirical Formulas

1. Experimental Data: You'll be given the mass of each element present in a compound.
2. Convert Mass to Moles: Divide the mass of each element by its molar mass (found on the periodic table).
3. Find the Simplest Ratio: Divide all the calculated moles by the smallest number of moles. This gives you the simplest ratio of atoms.
4. Write the Empirical Formula: Use the whole number ratio to write the empirical formula.

Example:

A compound contains 2.4 g of carbon and 0.6 g of hydrogen. Find its empirical formula.

• Moles:
  • Carbon: 2.4 g / 12 g/mol = 0.2 mol
  • Hydrogen: 0.6 g / 1 g/mol = 0.6 mol
• Simplest Ratio: Divide both by 0.2 (the smallest number of moles):
  • Carbon: 0.2 mol / 0.2 mol = 1
  • Hydrogen: 0.6 mol / 0.2 mol = 3
• Empirical Formula: CH3

Determining Molecular Formulas

1. Empirical Formula: You need the empirical formula of the compound.
2. Molecular Mass: You'll be given the molecular mass of the compound.
3. Calculate the Empirical Formula Mass: Add the atomic masses of all the atoms in the empirical formula.
4. Find the Multiplier: Divide the molecular mass by the empirical formula mass. This gives you the multiplier to convert the empirical formula to the molecular formula.
5. Write the Molecular Formula: Multiply the subscripts in the empirical formula by the multiplier.

Example:

The empirical formula of a compound is CH2O. Its molecular mass is 180 g/mol. Find its molecular formula.

• Empirical Formula Mass: 12 g/mol + 2 g/mol + 16 g/mol = 30 g/mol
• Multiplier: 180 g/mol / 30 g/mol = 6
• Molecular Formula: (CH2O)6 = C6H12O6

Key Points

• Percent Composition: If you're given the percentage composition of the compound, convert the percentages to grams (assume 100 g of the compound) and then follow the same steps.
• Practice: Practice solving problems with different compounds and scenarios to solidify your understanding.

Conclusion

By following these steps, you can confidently determine the empirical and molecular formulas of compounds, essential for understanding chemical reactions and properties. Remember to use the periodic table to find the molar mass of each element and carefully carry out the calculations.