Chapter 7, Problem 8STP

Interpretation Introduction

Interpretation:

The law of conservation of mass must be explained. Its application to balancing a chemical equation must be discussed.

Concept Introduction:

The law of conservation of mass states that mass can’t be created or destroyed.

A balanced chemical equation represents a chemical reaction in which the number of each atom is the same on both sides of the equation.

Answer to Problem 8STP

According to the law of conservation of mass, neither mass can be created nor it can be destroyed. One substance can be converted to another in a chemical reaction keeping the total mass constant.

In a chemical reaction, new substances are generated with different formulas and properties. But the total mass of the reactants must be equal to the total mass of the products. To do so, all the atoms must be balanced on both sides of the equation.

Explanation of Solution

An unbalanced and a balanced chemical equation are given below.

  $\begin{array}{l}{\text{Cl}}_{\text{2}}\text{(g)+KBr(aq)}\to {\text{Br}}_{\text{2}}\text{(l)+KCl(aq)}.\\ {\text{Cl}}_{\text{2}}\text{(g)+2KBr(aq)}\to {\text{Br}}_{\text{2}}\text{(l)+2KCl(aq)}\end{array}$

From the reaction, it can be seen that the number of Cl atoms is 2 on the reactant side, but it is 1 on the product side. To balance the 2 Cl atoms, 2 moles of KCl are required on the product side. Then to balance 2 K atoms on the product side, 2 moles of KBr are required on the reactant side. The number of Cl, K and Br atoms are equal on both sides of the balanced equation. Now, the law of conservation of mass can be applied. In the unbalanced equation, the number of each atom was not the same which was violating the law of conservation of mass.