3. Stoichiometry

Question 1

State the formulae of the elements and compounds named in the subject content

Answer 1

(e.g. Ammonia: 1 nitrogen and 3 hydrogen atoms, so NH₃)

Question 2

Define the molecular formula of a compound

Answer 2

The number and type of different atoms in one molecule

Question 3

Construct word equations and symbol equations to show how reactants form products, including state symbol

Answer 3

• Word equations: reactants → products
• Symbol equations: The chemical formula
• State symbols:
  1. (s) = solid
  2. (l) = liquid
  3. (g) = gas
  4. (aq) = aqueous solution

Question 4

Construct symbol equations with state symbols, including ionic equations

Answer 4

Example: S (s) + O₂ (g) → SO₂ (g)

Question 5

State the rule of balancing symbol equation and give an example

Answer 5

• In the left and right side of the equation, there must an equal number of atoms of each element
• Example: 2Fe₂O₃ (aq) + 3C (s) ⟶ 4Fe (s) + 3CO₂ (g)

Question 6

Describe relative atomic mass

Answer 6

The average mass of the isotopes of an element compared to 1/12th of the mass of an atom of ¹²C

Question 7

Define relative molecular mass

Answer 7

The sum of the relative atomic masses (mass no.)

Question 8

What’s the key to calculate reacting masses in simple proportions

Answer 8

• Total Mass of Reactants = Total Mass of Products
  Example: 2Ca + O₂ → 2CaO
• 2 x 40 = 80 units of mass of calcium
• 2 x 16 = 32 units of mass of oxygen (O2 molecule, 16 + 16 = 32)
• 2 x (40 + 16) OR 80 + 32 = 112 units of mass of CaO