Study Notes
Overview
Quantitative chemistry is the backbone of many chemical concepts. It's how we measure, calculate, and predict the amounts of substances involved in reactions. For your WJEC GCSE, this topic (2.3) is crucial as it underpins much of your chemical understanding and is a favorite of examiners for testing your application (AO2) skills. You'll learn how to work out the mass of products, determine the simplest formula of a compound, and understand the concept of the mole – a chemist's most important unit. Mastering these skills is not just about passing the exam; it's about thinking like a chemist and understanding the precise, mathematical nature of the science. Expect to see these calculations in a variety of contexts, from simple formula mass questions to complex, multi-step problems involving reacting masses and limiting reactants, especially in the Higher tier.
Key Concepts
Concept 1: Relative Formula Mass (Mr)
The relative formula mass (Mr) of a compound is the sum of the relative atomic masses (Ar) of all the atoms in its chemical formula. You find the Ar values on the periodic table. It is a fundamental skill that you will use in almost every calculation in this topic.
Example: To calculate the Mr of calcium carbonate (CaCO₃):
- Find the Ar of each element: Ca = 40, C = 12, O = 16.
- Count the atoms of each element: 1 Calcium, 1 Carbon, 3 Oxygen.
- Calculate: Mr = 40 + 12 + (16 × 3) = 40 + 12 + 48 = 100.
Examiner Tip: A very common mistake is to use the large balancing numbers from a chemical equation when calculating Mr. You must ignore these and only use the small subscript numbers within the chemical formula itself.
Concept 2: The Mole
The mole is a unit of measurement for the amount of a substance. One mole of any substance contains the same number of particles: 6.02 x 10²³ particles, known as Avogadro's constant. For your exam, the most important thing is the relationship between moles, mass, and Mr. This is the key to unlocking almost all reacting mass calculations.
Concept 3: Percentage Composition
This calculation tells you the percentage by mass of each element within a compound. It's useful for determining the purity of a substance or identifying a compound from its elemental makeup.
Concept 4: Empirical and Molecular Formulae (Higher Tier)
The empirical formula is the simplest whole-number ratio of atoms of each element in a compound. The molecular formula gives the actual number of atoms of each element in a molecule. For example, for ethene, the molecular formula is C₂H₄, but the empirical formula is CH₂.
Mathematical/Scientific Relationships
- Relative Formula Mass (Mr): Sum of Ar values for all atoms in a formula. (Must memorise)
- Moles (n):
n = mass (m) / Mr(Given on formula sheet) - Mass (m):
m = moles (n) × Mr(Derived from the moles formula) - Percentage Composition:
(% by mass) = (Ar × number of atoms / Mr) × 100(Must memorise) - Concentration (g/dm³):
Concentration = mass (g) / volume (dm³)(Given on formula sheet)
Unit Conversions
- Mass: 1 kilogram (kg) = 1000 grams (g); 1 gram (g) = 1000 milligrams (mg)
- Volume: 1 decimetre cubed (dm³) = 1000 centimetres cubed (cm³). To convert from cm³ to dm³, you must divide by 1000. This is a very common source of error in concentration calculations.