Ch.2 - Atoms, Molecules, and Ions

Brown - Chemistry: The Central Science 14th Edition

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Brown 14th Edition

Ch.2 - Atoms, Molecules, and Ions

Problem 46a

Chapter 2, Problem 46a

Ball-and-stick representations of benzene, a colorless liquid often used in organic chemistry reactions, and acetylene, a gas used as a fuel for high-temperature welding, are shown below. (a) Determine the molecular formula of each.

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Identify the number of carbon (C) atoms and hydrogen (H) atoms in the ball-and-stick model of pentane. Each black ball represents a carbon atom, and each white ball represents a hydrogen atom.

Count the carbon atoms in the pentane structure. There are 5 carbon atoms.

Count the hydrogen atoms in the pentane structure. There are 12 hydrogen atoms.

Write the molecular formula for pentane based on the counted atoms: C5H12.

Repeat the process for toluene: count the carbon and hydrogen atoms. Toluene has 7 carbon atoms and 8 hydrogen atoms, with one of the carbons being part of a methyl group attached to a benzene ring. The molecular formula for toluene is C7H8.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Molecular Formula

The molecular formula of a compound indicates the number and types of atoms present in a molecule. It is expressed as a series of chemical symbols with subscripts denoting the quantity of each type of atom. For example, benzene has the molecular formula C6H6, indicating it contains six carbon atoms and six hydrogen atoms.

Structural Representation

Ball-and-stick models are a common way to represent molecular structures, where balls represent atoms and sticks represent bonds between them. This representation helps visualize the geometry and connectivity of atoms in a molecule, which is crucial for understanding chemical properties and reactions. For instance, the structure of toluene shows a benzene ring with a methyl group attached.

Hydrocarbons

Hydrocarbons are organic compounds composed solely of carbon and hydrogen atoms. They can be classified into aliphatic (like pentane) and aromatic (like benzene and toluene) compounds. Understanding the classification and structure of hydrocarbons is essential for predicting their chemical behavior and reactivity in organic chemistry.

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Related Practice

Textbook Question

The structural formulas of the compounds n-butane and isobutane are shown below. (c) Which formulas—empirical, molecular, or structural—allow you determine these are different compounds?

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Textbook Question

The structural formulas of the compounds n-butane and isobutane are shown below. (b) Determine the empirical formula of each.

Textbook Question

What are the molecular and empirical formulas for each of the following compounds? Write the molecular formula for the following compound.

Textbook Question

Two substances have the same molecular and empirical formulas. Does this mean that they must be the same compound?

Textbook Question

Write the empirical formula corresponding to each of the following molecular formulas: (a) Al₂Br₆