BackLaw of Definite Proportions and Mass Ratios
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Law of Definite Proportions
Definition and Historical Context
The Law of Definite Proportions states that a chemical compound always contains exactly the same proportion of elements by mass, regardless of the sample's source or size. This law was first formulated by Joseph L. Proust in 1799 and is also known as the Law of Constant Composition.
Key Point: The mass ratio of elements in a compound is always constant.
Key Point: This law is fundamental to understanding chemical formulas and the composition of substances.
Calculating Mass Ratios
To determine the mass ratio of elements in a compound, divide the mass of one element by the mass of the other element in the compound.
Example: For carbon dioxide (CO2):
This means that for every 1 g of carbon, there are 2.67 g of oxygen in carbon dioxide.
Application Example
Example: Two unknown compounds are examined. Compound A contains 12.0 g of hydrogen and 32.0 g of oxygen. Compound B contains 11.0 g of hydrogen and 88.0 g of oxygen. Do Compounds A and B represent the same compound?
Calculate the mass ratio for each:
Compound A: Compound B:
Since the ratios are different, the compounds are not the same.
Proportions and Sample Calculations
When given the mass of one element in a compound, you can use the known mass ratio to determine the mass of the other element.
Example: A compound contains sodium and chlorine. If a sample contains 2.00 g of sodium, how much chlorine should it contain if the mass ratio (Cl:Na) is 1.54:1?
Practice Problem
A 7.14 g sample of HCN is found to contain 0.287 g of H and 4.41 g of N. Find the mass of carbon in a sample of HCN with a mass of 1.53 g.
First, determine the mass of C in the original sample:
Set up a proportion to solve for the mass of C in the 1.53 g sample:
Summary Table: Law of Definite Proportions Applications
Compound | Element 1 (g) | Element 2 (g) | Mass Ratio | Same Compound? |
|---|---|---|---|---|
Compound A | 12.0 (H) | 32.0 (O) | 2.67 | — |
Compound B | 11.0 (H) | 88.0 (O) | 8.00 | No |
Additional info: The Law of Definite Proportions is foundational for understanding chemical formulas and stoichiometry, and is closely related to the Law of Multiple Proportions, which describes how elements can combine in different ratios to form different compounds.
