Composition of Pure Water — Formula, Elements and Mass Percentage

Molecular formula: H₂O

Atoms per molecule: • 2 hydrogen atoms • 1 oxygen atom • Total: 3 atoms per molecule

Atom ratio: H : O = 2 : 1

Mass composition: • Molar mass of H₂O = 2(1) + 16 = 18 g/mol • Mass of H in H₂O = 2 g/mol • Mass of O in H₂O = 16 g/mol

Percentage by mass: • % H = (2/18) × 100 = 11.11% • % O = (16/18) × 100 = 88.89%

Mass ratio H : O = 2 : 16 = 1 : 8

Therefore: • In 9 g of water: 1 g of H and 8 g of O • In 18 g of water: 2 g of H and 16 g of O

The composition of water can be verified by electrolysis:

2H₂O → 2H₂ + O₂ (electrolysis)

Volume ratio: • H₂ : O₂ = 2 : 1 (by volume, at the same temperature and pressure) • This matches the atom ratio in H₂O (2H : 1O)

In electrolysis experiment: • Twice as much hydrogen is collected at the cathode as oxygen at the anode • Confirming H₂O has H and O in 2:1 ratio by volume/moles

This experiment verifies:

1. Water contains H and O in 2:1 mole ratio
2. Water is a compound of hydrogen and oxygen
3. Law of constant composition applies to water — always same ratio

Note: Pure water is required for electrolysis. A few drops of H₂SO₄ or NaOH are added to increase conductivity (ionic conductors), but only water is electrolysed.

Physical properties: • Molecular formula: H₂O • Molar mass: 18 g/mol • Melting point: 0°C (273.15 K) • Boiling point: 100°C (373.15 K) at 1 atm • Density: 1 g/mL (1 g/cm³) at 4°C (maximum density) • Colourless, odourless, tasteless • pH: 7 (neutral) • Kw = 1.0 × 10⁻¹⁴ at 25°C

Chemical properties: • Amphoteric — acts as acid or base • Universal solvent — dissolves many ionic and polar compounds • Reacts with reactive metals: 2Na + 2H₂O → 2NaOH + H₂↑ • Reacts with basic oxides: CaO + H₂O → Ca(OH)₂ • Reacts with acidic oxides: CO₂ + H₂O → H₂CO₃

Pure water vs tap water: Pure water contains only H₂O; tap water contains dissolved minerals, salts, and gases.