Chapter 3, Problem 3.72P

(a)

Interpretation Introduction

Interpretation:

The number of moles of ${\text{Cr}}_2{\text{O}}_3$ which produces $421\text{ g}$ of  ${\text{Cr}}_2{\text{S}}_3$ is to be determined.

Concept introduction:

In a balanced chemical equation, the total mass of reactants and products are equal in a balanced chemical equation, thus, it obeyed the law of conservation of mass. Also, the amounts of substances in a balanced chemical reaction are stoichiometrically equivalent to each other.

Stoichiometry gives the quantitative relationship between the reactants and products of the chemical reaction. It helps in the calculation of the percentage yield of the product of any chemical reaction.

Following are the steps to calculate the moles of a chemical substance when the mass of other substance present in the chemical reaction is given.

Step 1: Divide the given mass of the substance by its molar mass to calculate the moles of that compound. The expression to calculate the amount of a chemical substance in moles is as follows:

$\text{Amount}\left(\text{mol}\right)=\frac{\text{Mass }\left(\text{g}\right)}{\text{Molar mass}\left(\text{g/mol}\right)}$ (1)

Step 2: Determine the molar ratio between the substance whose mass is given and the substance whose amount is to be calculated from the balanced chemical equation. Multiply the calculated moles in step 1 by the molar ratio to obtain the amount of the substance.

(b)

Interpretation Introduction

Interpretation:

The mass of ${\text{Cr}}_2{\text{O}}_3$ in grams which produces $421\text{ g}$ of  ${\text{Cr}}_2{\text{S}}_3$ is to be determined.

Concept introduction:

In a balanced chemical equation, the total mass of reactants and products are equal in a balanced chemical equation, thus, it obeyed the law of conservation of mass. Also, the amounts of substances in a balanced chemical reaction are stoichiometrically equivalent to each other.

Stoichiometry gives the quantitative relationship between the reactants and products of the chemical reaction. It helps in the calculation of the percentage yield of the product of any chemical reaction.

The expression to calculate the mass from the moles of a substance is as follows:

$\text{Mass}\left(\text{g}\right)=\left(\text{Amount}\left(\text{mol}\right)\right)\left(\text{Molar mass}\left(\text{g/mol}\right)\right)$ (2)