Law of Definite Proportions

Definition and Historical Context

The Law of Definite Proportions, also known as the Law of Constant Composition, states that a chemical compound always contains exactly the same proportion of elements by mass. This law was first formulated by the French chemist Joseph L. Proust in 1799.

• Mass ratio: The ratio of the masses of elements in a compound.

• Different samples of a pure chemical compound always contain the same proportions of elements by mass.

Calculating Mass Ratios

To determine the mass ratio in a compound, place the element with the larger mass on top in the ratio.

• Example: For carbon dioxide (CO2):

C: 12.01 g/mol O: 16.00 g/mol (per atom, 2 atoms per molecule)

Example: Comparing Compounds

Two unknown compounds are examined:

• Compound A: 2.0 g H, 32.0 g O

• Compound B: 15.0 g H, 120.0 g O

Calculate the mass ratio for each:

• Compound A: g O/g H

• Compound B: g O/g H

Conclusion: The mass ratios are different, so A and B are not the same compound.

Proportions and Use of Ratios

Determining Unknown Masses

If you know the mass ratio and the mass of one element, you can determine the unknown amount of the other element using a proportion.

Example: Calculating Mass from Law of Definite Proportions

A compound contains only calcium and fluorine. A sample is determined to contain 2.00 g Ca and 1.90 g F. According to the Law of Definite Proportions, how much calcium should another sample contain if it possesses 2.85 g of fluorine?

• Set up the proportion:

Solve for x:

g Ca

Answer: 3.00 g Ca

Practice Problem

A 7.74 g sample of HCN is found to contain 0.287 g H and 4.01 g N. Find the mass of carbon in a sample of HCN with a mass of 3.43 g.

• First, calculate the mass of C in the original sample:

• Set up the proportion for the new sample:

Solve for x:

g C

Answer: 1.53 g C

Summary Table: Mass Ratios in Compounds