Question

Among the following pairs of compounds, the one that illustrates the law of multiple proportions is: (a) \(\mathrm{Cu}\) and \(\mathrm{CuSO}_{4}\) (b) \(\mathrm{CuO}\) and \(\mathrm{Cu}_{2} \mathrm{O}\) (c) \(\mathrm{H}_{2} \mathrm{~S}\) and \(\mathrm{SO}_{2}\) (d) \(\mathrm{NH}_{3}\) and \(\mathrm{NCl}_{3}\)

Short answer

(b) CuO and Cu2O illustrate the law of multiple proportions.

Step-by-step solution

Identify the Concept

The law of multiple proportions states that when two elements combine in more than one way to form different compounds, the ratios of the masses of one element that combine with a fixed mass of the other element are ratios of small whole numbers.

Analyze Compound Pair (a)

In this pair, \(Cu\) is an element and \(CuSO_4\) is a compound consisting of copper, sulfur, and oxygen. These do not form compounds with differing compositions based on the same elements, but instead, \(CuSO_4\) is a unique compound.

Analyze Compound Pair (b)

Both \(CuO\) and \(Cu_2O\) are compounds of copper and oxygen. In \(CuO\), 79.9% of the mass is copper, while in \(Cu_2O\), 88.8% of the mass is copper. The mass ratios of oxygen in both show small whole number proportions.

Analyze Compound Pair (c)

This pair consists of \(H_2S\) and \(SO_2\) which are compounds of different elements. They don't demonstrate multiple proportions as they don't form different compounds from the same set of two elements.

Analyze Compound Pair (d)

This pair includes \(NH_3\) and \(NCl_3\), which are compounds that contain nitrogen but pair each time with a different second element (hydrogen in one and chlorine in the other). This doesn't fit the definition since the compounds involve two different elements each time.

Conclusion Based on Analysis

The pair that illustrates the law of multiple proportions is \(CuO\) and \(Cu_2O\), as they contain the same elements (copper and oxygen) in different mass ratios that are in whole numbers.

Key concepts

Chemical Compounds

Chemical compounds are substances formed from two or more elements combined in specific ratios. They maintain a unique set of properties different from the individual elements they consist of. These compounds can be simple like water (H₂O) or complex like Copper(II) sulfate (CuSO₄).
Each compound has a chemical formula that expresses the types and numbers of atoms present in the smallest unit of the substance.

• The elements in a compound are held together by chemical bonds, which can be ionic, covalent, or metallic.
• The proportion of elements in a compound is always fixed. This is because they form in specific ratios as dictated by their valency and bonding characteristics.
• An example is water, which always consists of two hydrogen atoms for every oxygen atom, hence the formula H₂O cannot change.

Understanding these fundamentals is essential for grasping more complex chemistry concepts such as the law of multiple proportions, which allows us to predict the ratios of elements in various compounds.

Mass Ratios

Mass ratios in chemistry describe how much of one substance is present compared to another within a compound. This concept is central to understanding chemical formulas and reactions. By comparing mass ratios, chemists can discern the composition of different compounds.
In the context of the law of multiple proportions, mass ratios become significant when two elements combine to form more than one compound.

• For instance, in Copper(I) oxide (Cu₂O) and Copper(II) oxide (CuO), the ratios of copper to oxygen differ. The mass of oxygen that combines with copper is in a small whole-number ratio.
• This means that if you have 79.9% copper in CuO versus 88.8% in Cu₂O, the amount of oxygen differs, presenting a clear whole-number relationship.
• These ratios help us understand chemical compositions and reactions because they provide insights into the stoichiometry of compounds.

By mastering mass ratios, students become adept at predicting how substances interact and form new compounds in chemical reactions.

Chemistry Laws

Chemistry is guided by fundamental laws that explain how chemical reactions occur. One such important law is the law of multiple proportions articulated by John Dalton. This law is crucial for understanding the formation of compounds.
It states that if two elements form more than one compound, the masses of one element that combine with a fixed mass of the other element will do so in ratios of small whole numbers.

• This can be seen when copper combines with oxygen to form CuO and Cu₂O. Here, the difference in oxygen mass that pairs with a fixed mass of copper is a simple ratio.
• The law of multiple proportions helps chemists predict possible compounds that can form from known elements.
• It also aids in validating chemical formulas and understanding the nature of chemical bonds.

Laws like these not only establish a foundation for theoretical chemistry but also provide tools for practical applications in lab work and industrial processes. By understanding these laws, learners can better navigate the complexities of chemical synthesis and analysis.