HUKUM - HUKUM DASAR KIMIA

Basic Laws of Chemistry

The lesson introduces four basic chemical laws that form the foundation of chemical calculations, or stoichiometry.

1. Law of Conservation of Mass (Lavoisier)

In a chemical reaction carried out in a closed system, the total mass remains unchanged.

For example, if 8 g of magnesium reacts with 12 g of oxygen, the mass of magnesium oxide formed is:

[ 8\text{ g} + 12\text{ g} = 20\text{ g} ]

2. Law of Definite Proportions (Proust)

A given compound always contains its elements in a fixed mass ratio.

Examples:

• Carbon and oxygen in CO₂ have a mass ratio of 3:8.
• Hydrogen and oxygen in H₂O have a mass ratio of 1:8.

Worked example: Iron and sulfur combine in a mass ratio of 7:4. If 16 g of iron reacts with 8 g of sulfur:

• 8 g of sulfur requires 14 g of iron.
• Sulfur is fully used, leaving 2 g of iron unreacted.
• The iron sulfide formed has a mass of 22 g (14 g iron + 8 g sulfur).

3. Law of Multiple Proportions (Dalton)

If two elements form more than one compound, then, for a fixed mass of one element, the masses of the other element combine in a simple whole-number ratio.

For example, nitrogen and oxygen form N₂O, N₂O₃, and N₂O₄. When the nitrogen mass is fixed at 28 g, the oxygen masses are 16 g, 48 g, and 64 g. Their ratio is:

[ 16:48:64 = 1:3:4 ]

4. Law of Combining Volumes (Gay-Lussac)

At the same temperature and pressure, the volumes of reacting gases and gaseous products are in simple whole-number ratios. Gas reactions illustrate how these volume ratios can be expressed as small whole numbers.

Avogadro’s Hypothesis

At the same temperature and pressure, equal volumes of gases contain equal numbers of particles. Therefore, under these conditions, the volume ratio of gases is also their particle-number ratio.

For example, in the reaction

[ \mathrm{N_2 + 3H_2 \rightarrow 2NH_3} ]

the volume ratio is 1:3:2. If 1 L of N₂ contains (N) particles, then 3 L of H₂ contains (3N) particles and 2 L of NH₃ contains (2N) particles.

Speaker or Source Featured

• Mrs. Ria — chemistry teacher and presenter.