Law of Definite Proportions
States that a given chemical compound always contains the same elements in the exact same proportion by mass, irrespective of its source.
What is Law of Definite Proportions?
A substance formed when two or more chemical elements are chemically bonded together in a fixed ratio.
Key points
- Understand the principle of the Law of Definite Proportions.
- Differentiate between compounds and mixtures based on composition.
- Apply the law to calculate the mass ratio of elements in a compound.
- Appreciate its significance in chemical stoichiometry.
Common exam trap
Confusing compounds with mixtures (mixtures have variable compositions).
Definitions
- Term
Compound
- Meaning
A substance formed when two or more chemical elements are chemically bonded together in a fixed ratio.
- Term
Mass Ratio
- Meaning
The ratio of the masses of different elements present in a compound.
Learning objectives
Understand the principle of the Law of Definite Proportions.
Differentiate between compounds and mixtures based on composition.
Apply the law to calculate the mass ratio of elements in a compound.
Appreciate its significance in chemical stoichiometry.
Prerequisites
Concept of elements and compounds.
Understanding of atomic mass.
Basic knowledge of chemical formulas.
Common mistakes
Confusing compounds with mixtures (mixtures have variable compositions).
Assuming the law applies to elements (elements are pure substances, not compounds formed by combination).
Incorrectly calculating mass ratios based on atomic masses.
Keywords
Definite Proportions
Constant Composition
Joseph Proust
Stoichiometry
Mass Ratio
Chemical Compound
Practice preview
A sample of pure carbon dioxide (CO₂) is found to contain 27.27% carbon and 72.73% oxygen by mass. If another sample of CO₂ is prepared by burning 12g of carbon in excess oxygen, what would be the percentage of oxygen by…
easy
Water collected from different sources (river, well, sea) is analyzed. In all samples, the ratio of hydrogen to oxygen by mass is found to be 1:8. This observation supports which law?…
easy
A sample of magnesium oxide (MgO) is formed by the reaction of magnesium with oxygen. If the atomic mass of Mg is approximately 24 g/mol and O is approximately 16 g/mol, what is the percentage of magnesium by mass in MgO…
medium