Formulae
Section: 3. Stoichiometry | Syllabus: Cambridge AS Level Physics 9702
What is a Chemical Formula? Chemical Formula A representation of a substance using chemical symbols and numbers to show which elements are present and how many atoms of each element are in one unit of the substance.
Chemical formulae are a shorthand way of representing compounds and elements. They tell us exactly what a substance is made of. Uses chemical symbols from the periodic table Numbers show how many atoms of each element If no number is shown, it means 1 atom Subscript numbers apply to the element immediately before them Example H₂O means 2 hydrogen atoms and 1 oxygen atom.
CO₂ means 1 carbon atom and 2 oxygen atoms. Types of Chemical Formulae 1. Molecular Formula Molecular Formula Shows the actual number of atoms of each element in one molecule of a substance. Examples: H₂O - water (2 hydrogen, 1 oxygen) C₆H₁₂O₆ - glucose (6 carbon, 12 hydrogen, 6 oxygen) H₂SO₄ - sulfuric acid (2 hydrogen, 1 sulfur, 4 oxygen) 2.
Empirical Formula Empirical Formula Shows the simplest whole number ratio of atoms of each element in a compound. Examples: Glucose: Molecular = C₆H₁₂O₆, Empirical = CH₂O (ratio 1:2:1) Ethane: Molecular = C₂H₆, Empirical = CH₃ (ratio 1:3) Water: Molecular = H₂O, Empirical = H₂O (already simplest) Key Difference Molecular formula shows actual numbers.
Empirical formula shows the simplest ratio. For some compounds (like H₂O), they're the same! Reading Chemical Formulae Simple Formulae Formula Name What it means H₂ Hydrogen 2 hydrogen atoms bonded together O₂ Oxygen 2 oxygen atoms bonded together NaCl Sodium chloride 1 sodium atom, 1 chlorine atom MgO Magnesium oxide 1 magnesium atom, 1 oxygen atom Complex Formulae with Brackets Brackets are used when a group of atoms appears multiple times: Ca(OH)₂ - The subscript 2 applies to everything in the bracket This means: 1 Ca, 2 O, 2 H (not 1 O and 1 H!) Cu(NO₃)₂ - 1 Cu, 2 N, 6 O (the 2 multiplies the NO₃ group) Ca(OH)₂ = 1 calcium + 2(OH groups) = 1 Ca + 2 O + 2 H Al₂(SO₄)₃ = 2 Al + 3(SO₄ groups) = 2 Al + 3 S + 12 O Counting Atoms in Formulae Method for Counting Atoms Work through the formula from left to right If there's a subscript number after an element, multiply If there are brackets, multiply everything inside by the subscript after the bracket Add up all atoms of the same element Worked Examples Example 1: H₂SO₄ H₂ = 2 hydrogen atoms S = 1 sulfur atom O₄ = 4 oxygen atoms Total: 7 atoms (2 H, 1 S, 4 O) Example 2: Mg(NO₃)₂ Mg = 1 magnesium atom (NO₃)₂ = 2 × (1 nitrogen + 3 oxygen) N = 2 nitrogen atoms O = 6 oxygen atoms Total: 9 atoms (1 Mg, 2 N, 6 O) Example 3: Al₂(SO₄)₃ Al₂ = 2 aluminium atoms (SO₄)₃ = 3 × (1 sulfur + 4 oxygen) S = 3 sulfur atoms O = 12 oxygen atoms Total: 17 atoms (2 Al, 3 S, 12 O) State Symbols State Symbols Letters written in brackets after a chemical formula to show the physical state of a substance.
Symbol State Meaning (s) Solid The substance is in solid form (l) Liquid The substance is in liquid form (g) Gas The substance is in gaseous form (aq) Aqueous The substance is dissolved in water Examples with state symbols: H₂O(l) - liquid water H₂O(g) - water vapour/steam NaCl(s) - solid sodium chloride NaCl(aq) - sodium chloride dissolved in water CO₂(g) - carbon dioxide gas Important (aq) means "aqueous" - dissolved in water.
It does NOT mean the substance itself is water! Common Element Symbols Elements You Must Know Symbol Element Symbol Element H Hydrogen O Oxygen C Carbon N Nitrogen Na Sodium K Potassium Mg Magnesium Ca Calcium Cl Chlorine Br Bromine Fe Iron Cu Copper Zn Zinc Al Aluminium S Sulfur P Phosphorus Watch Out!
Some symbols don't match the English name: Na (sodium), K (potassium), Fe (iron), Cu (copper). These come from Latin names. Diatomic Molecules Diatomic Molecules Elements that exist naturally as molecules containing two atoms bonded together.
Seven elements exist as diatomic molecules. Remember them with: "Have No Fear Of Ice Cold Beer" Element Formula State at Room Temp Hydrogen H₂ Gas Nitrogen N₂ Gas Fluorine F₂ Gas Oxygen O₂ Gas Iodine I₂ Solid Chlorine Cl₂ Gas Bromine Br₂ Liquid Important When writing equations, always use the diatomic form (e.g., O₂, not O) for these seven elements in their elemental state!
Common Compound Formulae Acids HCl - Hydrochloric acid H₂SO₄ - Sulfuric acid HNO₃ - Nitric acid CH₃COOH - Ethanoic acid Bases and Alkalis NaOH - Sodium hydroxide KOH - Potassium hydroxide Ca(OH)₂ - Calcium hydroxide NH₃ - Ammonia Other Important Compounds H₂O - Water CO₂ - Carbon dioxide NH₄Cl - Ammonium chloride CaCO₃ - Calcium carbonate Common Misconceptions Misconception 1: "The subscript applies to all elements before it" Correction: The subscript only applies to the element or bracket immediately before it.
In H₂SO₄, the 4 only applies to oxygen, not to sulfur or hydrogen. Misconception 2: "CO and CO₂ are the same thing" Correction: CO is carbon monoxide (1 C, 1 O) - toxic gas. CO₂ is carbon dioxide (1 C, 2 O) - what we breathe out.
The subscript makes a huge difference! Misconception…
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