Question

State and explain the law of multiple proportions. How is the law of multiple proportions different from the law of definite proportions?

Short answer

The law of multiple proportions states that the masses of one element that combine with a fixed mass of another element to form different compounds are in the ratio of small whole numbers. It differs from the law of definite proportions, which states that a chemical compound always contains the same proportion of elements by mass, irrespective of its source or how it was prepared.

Step-by-step solution

State the Law of Multiple Proportions

The law of multiple proportions states that when two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other are in the ratio of small whole numbers. For example, carbon and oxygen can combine to form both CO and CO2. If you compare the mass of oxygen that can combine with 1 gram of carbon to form CO with the mass that can combine with 1 gram of carbon to form CO2, the ratio will be a simple whole number.

Explain the Law of Multiple Proportions

To understand the law, consider element A combining with element B to form compound AB and also forming compound AB2. If you have a fixed mass of element A, say 1 gram, and it reacts with 0.5 grams of element B to form AB, when A combines with B to form AB2, it will react with a whole-number multiple of the 0.5 grams of B. So it could react with 1 gram of B, 1.5 grams of B, and so on. This illustrates the compounds have different properties and compositions.

Differentiate between the Laws of Multiple and Definite Proportions

The law of definite proportions states that a chemical compound always contains exactly the same proportion of elements by mass. This law explains that no matter the source of the compound or how it was prepared, it will always contain the same elements in the same mass ratio. On the other hand, the law of multiple proportions is about the ratios of masses of the second element that combine with a fixed mass of the first element, and these ratios can differ between different compounds formed by the same two elements.

Key concepts

Chemical Compound Composition

Understanding the composition of chemical compounds is fundamental in the study of chemistry. Fundamentally, a chemical compound is a substance made up of two or more different types of atoms that are bonded together in a specific ratio. This ratio is determined by the compound's chemical formula, which represents the types and numbers of atoms present.

For instance, water (H₂O) consists of two hydrogen atoms bonded to one oxygen atom. The subscripts in the formula indicate the number of each type of atom in a single molecule of the compound. The law of definite proportions underlines this concept by stating that no matter how a compound is created, it will always have the same elemental composition by mass. This means that each molecule of water will always contain the same proportion of hydrogen to oxygen by mass, regardless of its origin or method of preparation.

Chemical Laws in Stoichiometry

Stoichiometry is the branch of chemistry that deals with the quantitative relationships of the elements and compounds involved in chemical reactions. Key to stoichiometry is the understanding of chemical laws that govern these relationships. The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. As a result, the mass of the reactants must equal the mass of the products.

This principle works hand-in-hand with the law of definite proportions, which ensures specific mass ratios of elements within compounds, as well as the law of multiple proportions, which explains how these ratios change in different compounds made from the same elements. Together, these laws facilitate stoichiometric calculations, allowing chemists to predict the quantities of substances needed or produced in a given reaction.

Law of Definite Proportions

The law of definite proportions is also known as Proust's law. It states that a chemical compound, no matter how it is formed, always contains the same elements in the same proportion by mass. This empirical law underpins the predictability and uniformity of chemical compounds.

For example, carbon dioxide will always be composed of carbon and oxygen in a mass ratio of about 1:2.7, irrespective of whether it is produced by burning carbon in an excess of oxygen or by the decomposition of calcium carbonate. If a substance does not adhere to this law, it is likely not a pure compound. This law is essential in determining the purity of substances and plays a crucial role in materials science and pharmaceuticals, where exact proportions yield the desired chemical properties.

Stoichiometric Calculations

Stoichiometric calculations are mathematical processes used to determine the relative quantities of reactants and products in chemical reactions. These calculations are grounded on the balanced chemical equation representing the reaction, which must reflect the stoichiometric coefficients that adhere to the law of conservation of mass.

Such calculations often involve determining the molar ratios of reactants and products, converting grams to moles or vice versa, and using Avogadro's number for reactions involving gases at standard temperature and pressure. These calculations are crucial to nearly all quantitative aspects of chemistry from industrial synthesis to laboratory experiments and are taught extensively in introductory chemistry courses. Mastering stoichiometry allows chemists to optimize the yields of reactions and reduce waste, which is of paramount importance in both economic and environmental contexts.