Mastering Quantitative Chemistry for GCSE

Quantitative Chemistry Concepts

Conservation of Mass and Balanced Equations

In any chemical reaction, mass is always conserved. The total mass of reactants equals the total mass of products. This is represented by balanced chemical equations, where subscripts indicate the ratios of atoms involved.

Relative Formula Mass (Mr)

The relative formula mass of a compound is calculated by summing the relative atomic masses of its constituent atoms. It allows conversion between the mass of a substance and the amount in moles.

Example: Calculate Mr of CO2

Mr(CO2) = 12.0 (C) + 2 × 16.0 (O) = 44.0 g/mol

The Mole Concept

One mole of any substance contains 6.02 × 1023 particles or formula units. The mole links the mass of a substance to the amount present, allowing quantitative calculations.

Molar Calculations

Using moles, we can calculate:

• Masses of reactants and products from balanced equations
• Limiting reactants and theoretical yields
• Concentrations of solutions (g/dm3 and mol/dm3)

Problem: Calculate mass of CO2 produced from 0.5 mol of carbon

Given: C(s) + O2(g) → CO2(g)

Solution:

1. Balanced equation: C(s) + O2(g) → CO2(g)
2. Mole ratio: 1 mol C produces 1 mol CO2
3. Mass of CO2 = 0.5 mol × Mr(CO2) = 0.5 × 44.0 = 22.0 g

Percentage Yield and Atom Economy

Percentage yield compares the actual yield to the theoretical maximum. Atom economy measures how efficiently atoms in reactants form the desired product.

Gas Volume Calculations

Mole calculations can be extended to volumes of gases using the molar gas volume at room temperature and pressure (24 dm3 per mole of gas).