Ch 1 · Formulae · O Level Chemistry 5070
Topic 1 of 3 in Stoichiometry — O Level Chemistry 5070 — 48 lessons.
Introduction
Every calculation, equation and explanation in chemistry starts with a correct formula. If you write MgCl instead of MgCl_2, every mark that follows is lost. This lesson gives you the formulae you must know by heart, the rules for working out the ones you do not, and the method for turning a word equation into a balanced symbol equation with state symbols.
Key ideas — Formulae you must know, and the molecular formula
The molecular formula of a compound is the number and type of different atoms in one molecule. Water, H_2O, has two hydrogen atoms and one oxygen atom in each molecule.
🧑🏫 Teacher's explanation
Think of a chemical formula like a recipe for a single tiny unit of a substance. If a recipe for a sandwich calls for 2 slices of bread and 1 slice of cheese, the 'formula' for that sandwich is B_2C_1. In chemistry, we don't use 'slices'; we use atoms. The molecular formula tells you exactly what is inside one single, discrete molecule.
📖 Definitions
🔍 Worked example — Easy
Identify the number of atoms of each element in one molecule of sulfuric acid, H_2SO_4.
🔍 Worked example — Standard
Deduce the formula of an ionic compound composed of Mg^2+ ions and Cl^- ions.
🔍 Worked example — Exam-style
A compound contains 24g of Magnesium and 32g of Oxygen. Calculate the empirical formula given A_r(Mg) = 24 and A_r(O) = 16.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Deducing formulae from diagrams and from ion charges
From a model or diagram, count the atoms of each type in one molecule. A ball-and-stick model with one nitrogen sphere joined to three hydrogen spheres is NH_3; a diagram showing two nitrogen atoms and four oxygen atoms is N_2O_4.
🧑🏫 Teacher's explanation
Think of chemical formulae as a recipe or a shopping list for a molecule. If a recipe calls for 2 eggs and 1 cup of flour, the 'formula' is 2E + 1F. In chemistry, we use symbols to represent these ingredients. When looking at a diagram (like a ball-and-stick model), you simply count how many of each colored sphere are present.
📖 Definitions
🔍 Worked example — Easy
A model of a molecule shows 2 Carbon atoms (black spheres) and 6 Hydrogen atoms (white spheres). Deduce the molecular formula.
🔍 Worked example — Standard
Deduce the formula for the ionic compound formed between Magnesium ions, Mg^2+, and Chloride ions, Cl^-.
🔍 Worked example — Exam-style
Deduce the formula for Aluminum Sulfate, formed from Al^3+ ions and SO_4^2- ions.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Word equations and balanced symbol equations
A word equation names the reactants on the left and the products on the right:
🧑🏫 Teacher's explanation
Think of a chemical equation like a recipe for a cake. If a recipe says you need 2 eggs and 1 cup of flour to make 1 cake, you cannot suddenly decide to use 3 eggs and 1 cup of flour and expect the same result; the proportions must stay consistent. In chemistry, these 'proportions' are the number of atoms in a molecule.
📖 Definitions
🔍 Worked example — Easy
Write a balanced symbol equation for the reaction between solid magnesium and aqueous hydrochloric acid to produce aqueous magnesium chloride and hydrogen gas.
🔍 Worked example — Standard
Balance the following equation: Fe(s) + H_2SO_4(aq) arrow FeSO_4(aq) + H_2(g)
🔍 Worked example — Exam-style
Balance the combustion of methane: CH_4(g) + O_2(g) arrow CO_2(g) + H_2O(g)
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Empirical formulae and ionic equations
empirical formula. The empirical formula is the simplest whole number ratio of the different atoms or ions in a compound. Ethane has molecular formula C_2H_6 but empirical formula CH_3. Ionic compounds form giant lattices rather than molecules, so the formula of an ionic compound, deduced from the ion charges as in section 2, is always an empirical formula.
🧑🏫 Teacher's explanation
Think of a recipe for a sandwich. A molecular formula is like saying 'I have 2 slices of bread and 3 slices of ham' (C_2H_6). The empirical formula is the simplest ratio of that recipe, like saying 'for every 1 slice of bread, I need 1.5 slices of ham', which we simplify to the whole number ratio: 2 slices of bread to 3 slices of ham (C_2H_3). It's the 'core' ratio that cannot be simplified further.
📖 Definitions
🔍 Worked example — Easy
A compound contains 80% Carbon and 20% Hydrogen by mass. Calculate its empirical formula. (A_r: C=12, H=1)
🔍 Worked example — Standard
Write the ionic equation for the reaction between aqueous silver nitrate and aqueous sodium chloride to form solid silver chloride. (Note: AgNO_3 and NaCl are soluble).
🔍 Worked example — Exam-style
A compound consists of Mg^2+ ions and $ ext{SO}_4^{2-}$ ions. Deduce the formula of the ionic compound.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Question. Aluminium reacts with dilute sulfuric acid to form aluminium sulfate solution and hydrogen. (a) Deduce the formula of aluminium sulfate. [1] (b) Write a balanced symbol equation, with state symbols, for the reaction. [3] (c) Aqueous barium chloride is added to the aluminium sulfate solution and a white precipitate of barium sulfate forms. Write the ionic equation for this reaction. [2]
🔍 Worked example — Easy
Determine the formula of a compound composed of magnesium ions, Mg^2+, and chloride ions, Cl^-, to form a neutral compound.
🔍 Worked example — Standard
Write a balanced symbol equation with state symbols for the reaction between solid magnesium and dilute hydrochloric acid to produce magnesium chloride solution and hydrogen gas: Mg(s) + HCl(aq) arrow MgCl_2(aq) + H_2(g)
🔍 Worked example — Exam-style
Aqueous barium chloride is added to aluminium sulfate solution. Write the ionic equation for the formation of the white precipitate of barium sulfate.
⚠️ Common errors
Common mistakes
Practice
Answers (a) K_2O [B1]; (b) Fe(NO_3)_2 [B1]; (c) (NH_4)_2CO_3 [B1]. SO_3 [B1]; the number and type of different atoms in one molecule [B1]. CaCO_3(s) + 2HCl(aq) arrow CaCl_2(aq) + H_2O(l) + CO_2(g): formulae [M1], balancing [A1], all state symbols correct [B1]. The simplest whole number ratio of the different atoms or ions in a compound [B1]; CH_2O [B1]. H^+(aq) + OH^-(aq) arrow H_2O(l): species [M1], state symbols [A1].