Problem 17 - A Complete Resource Book in Chemistry for JEE Main

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 \(\mathrm{NCl}_{3}\)

Step-by-Step Solution

Answer

(b) CuO and Cu2O illustrate the Law of Multiple Proportions.

Step 1: Understand the Law of Multiple Proportions

This law states that when two elements form more than one compound, the ratios of the masses of the second element that combine with a fixed mass of the first element are in the ratio of small whole numbers.

Step 2: Analyze the Pairs of Compounds

Review each pair to determine if they show the law of multiple proportions: - (a) Cu and CuSO4: These are not compounds with varying ratios of the same two elements. - (b) CuO and Cu2O: Both are compounds made from copper and oxygen. - (c) H2S and SO2: These compounds involve different elemental combinations. - (d) NH3 and NCl3: Nitrogen forms compounds with hydrogen and chlorine, not adjusting ratios of the same elements.

Step 3: Compare Relevant Pair

For option (b), CuO and Cu2O: - CuO contains copper and oxygen in a 1:1 ratio. - Cu2O contains copper and oxygen in a 2:1 ratio. This means the ratio of oxygen in CuO to Cu2O with a fixed mass of copper is 1:0.5, which simplifies to the small whole number ratio of 2:1.

Step 4: Identify the Correct Pair

CuO and Cu2O illustrate the Law of Multiple Proportions because they contain copper and oxygen, with different ratios indicating variable proportions of oxygen with a fixed amount of copper. These ratios can be reduced to small whole numbers, a key feature of the law.

Key Concepts

Stoichiometry

Stoichiometry might sound like a big word, but it's a simple concept. It's all about measuring and figuring out the amounts of substances involved in reactions. Think of it as a recipe for a chemical reaction. If you're baking a cake, you need specific amounts of flour, sugar, and eggs. Similarly, stoichiometry helps you determine how much of each chemical is needed or produced in a reaction. A key part of stoichiometry is the mole concept. The mole is a unit used in chemistry to count molecules or atoms, just like a dozen counts eggs. For example, 1 mole of carbon atoms equals about 6.022 x 10^23 atoms. Understanding this can help predict the results of chemical reactions, ensuring you get the right amounts of each substance. Stoichiometry also involves using balanced chemical equations to find the proportions of reactants and products. By looking at these equations, you can tell how many moles of each substance are involved and figure out the ratios they react or are produced in.

Chemical Compounds

Chemical compounds are everywhere, making up all the different substances we see and use every day. A chemical compound forms when two or more different elements bond together. Each compound has a unique combination of elements that gives it distinctive properties. Some common examples of chemical compounds include water (H2O), carbon dioxide (CO2), and table salt (NaCl). The properties of compounds are often very different from the elements that form them. For instance, sodium is a highly reactive metal and chlorine is a poisonous gas, but together they form sodium chloride, a stable substance safe to use as table salt. Chemical compounds are classified into various types, including ionic compounds, covalent compounds, and metallic compounds, each with unique bonding properties and behaviors. Understanding these compounds is essential in the study of chemistry as they form the basis of countless reactions and interactions.

Chemical Bonding

Chemical bonding is the force that holds atoms together in a compound and determines how they unite to form molecules. There are three main types of chemical bonds: ionic, covalent, and metallic bonds. - **Ionic Bonds**: These occur when electrons are transferred from one atom to another, typically between a metal and a nonmetal. For example, in sodium chloride, sodium gives up an electron to chlorine, forming an ionic bond. - **Covalent Bonds**: Involve the sharing of electron pairs between atoms, usually between nonmetals. Water (H2O) and carbon dioxide (CO2) are examples where atoms share electrons to achieve stability. - **Metallic Bonds**: Found in metals, these involve a sea of shared free electrons among a lattice of metal atoms, giving metals their unique properties such as conductivity and malleability. Understanding chemical bonding is crucial as it explains how compounds are formed. The type of bond affects the physical and chemical properties of the substance, impacting everything from melting points to electrical conductivity.