Overview
This lecture explains the law of multiple proportions, demonstrates its application using several compound examples, and shows how to use mass ratios to illustrate the law.
The Law of Multiple Proportions
- The law states that when two elements form multiple compounds, the mass ratio of one element combined with a fixed mass of the other can be reduced to small whole numbers.
- To apply this law, use the same two elements forming different compounds and compare their combining mass ratios.
Example: Carbon Monoxide and Carbon Dioxide
- Carbon monoxide (CO): 12 g of carbon combines with 16 g of oxygen.
- Convert to a per-gram basis: 1 g of carbon combines with 1.33 g of oxygen (16/12).
- Carbon dioxide (CO₂): 12 g of carbon combines with 32 g of oxygen.
- Per-gram basis: 1 g of carbon combines with 2.67 g of oxygen (32/12).
- The ratio of oxygen masses (2.67/1.33) simplifies to 2, a small whole number.
Example: Nitrogen and Oxygen Compounds
- Compounds with 1 g nitrogen combine with 1.142 g, 2.284 g, and 2.855 g oxygen, respectively (labelled A, B, C).
- B to A: 2.284 / 1.142 = 2 (ratio is 2:1).
- C to A: 2.855 / 1.142 ≈ 2.5. Multiply by 2 to get 5:2.
- C to B: 2.855 / 2.284 ≈ 1.25. Multiply by 4 to get 5:4.
Example: Sulfur and Oxygen Compounds
- Compound A: 50 g sulfur, 50 g oxygen in 100 g total.
- Compound B: 40 g sulfur, 60 g oxygen in 100 g total.
- Normalize to 1 g sulfur: Compound A has 1 g oxygen, Compound B has 1.5 g oxygen.
- Oxygen ratio (B to A): 1.5/1 = 1.5. Multiply by 2 to get 3:2.
How to Apply the Law
- Set the mass of one element to 1 g in all compounds.
- Compute the mass of the second element per gram of the first.
- Find ratios between these masses and reduce to the smallest whole numbers.
Key Terms & Definitions
- Law of Multiple Proportions — If two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in small whole number ratios.
- Atomic Mass — The mass of one atom of an element, usually measured in atomic mass units (amu).
Action Items / Next Steps
- Practice applying the law to other compound examples as homework.
- Review textbook sections on the law of multiple proportions.