Ch.3 - Molecules and Compounds

Tro - Chemistry: A Molecular Approach 6th Edition

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Tro 6th Edition

Ch.3 - Molecules and Compounds

Problem 73a

Chapter 3, Problem 73a

How many molecules are in each sample? a. 55.9 g Cl2

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Determine the molar mass of \( \text{Cl}_2 \). Since chlorine (Cl) has an atomic mass of approximately 35.5 g/mol, the molar mass of \( \text{Cl}_2 \) is \( 2 \times 35.5 \text{ g/mol} \).

Calculate the number of moles of \( \text{Cl}_2 \) in the sample using the formula: \( \text{moles} = \frac{\text{mass}}{\text{molar mass}} \). Substitute the given mass (55.9 g) and the molar mass of \( \text{Cl}_2 \) to find the moles.

Use Avogadro's number to convert moles to molecules. Avogadro's number is \( 6.022 \times 10^{23} \text{ molecules/mol} \).

Multiply the number of moles of \( \text{Cl}_2 \) by Avogadro's number to find the total number of molecules in the sample.

Ensure all units are consistent and check calculations for accuracy to confirm the number of molecules.

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Key Concepts

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

Molar Mass

Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). For chlorine gas (Cl2), the molar mass is approximately 70.9 g/mol, calculated by adding the atomic mass of two chlorine atoms. Understanding molar mass is essential for converting between grams and moles in stoichiometric calculations.

Avogadro's Number

Avogadro's number, approximately 6.022 x 10^23, is the number of particles (atoms, molecules, etc.) in one mole of a substance. This constant allows chemists to relate the macroscopic scale of substances (grams) to the microscopic scale (molecules). It is crucial for determining the number of molecules present in a given mass of a substance.

Stoichiometry

Stoichiometry is the branch of chemistry that deals with the quantitative relationships between the reactants and products in a chemical reaction. It involves using molar mass and Avogadro's number to convert between grams, moles, and molecules. Mastery of stoichiometry is vital for solving problems related to the amounts of substances involved in chemical reactions.

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