The law of definite proportions, also known as Proust's law or the law of constant composition, states that a pure chemical compound always contains the same proportion of elements by mass, regardless of the source of the sample. This principle is fundamental in chemistry, as it allows scientists to determine the composition of compounds based on mass ratios.
To illustrate this concept, consider carbon dioxide (CO2), which consists of one carbon atom and two oxygen atoms. The atomic mass of carbon is approximately 12.01 grams per mole, while oxygen has an atomic mass of about 16 grams per mole. Therefore, the total mass contributed by carbon in CO2 is 12.01 grams, and the total mass contributed by oxygen is 32 grams (2 × 16 grams). When calculating the mass ratio, the larger mass (oxygen) is placed on top, resulting in a mass ratio of:
Mass Ratio = \(\frac{32 \text{ grams (O)}}{12.01 \text{ grams (C)}} \approx 2.66\)
This ratio indicates that there are approximately 2.66 grams of oxygen for every gram of carbon in CO2. According to the law of definite proportions, if two samples of CO2 were taken from different locations, such as New York City and London, they would yield the same mass ratio of 2.66, confirming that both samples are indeed the same compound.
Understanding the law of definite proportions is crucial for performing calculations related to chemical composition. By applying this law, chemists can compare known samples with unknown ones to determine their identity based on consistent mass ratios. This principle not only reinforces the concept of chemical identity but also serves as a foundation for further explorations in stoichiometry and chemical reactions.