Chapter 22.3, Problem 17R

To determine

The number of molecules of $\text{HCl}$ needed to form four molecules of ${\text{H}}_2\text{O}$ .

Explanation of Solution

Introduction:

The chemical equation for the reaction:

  ${\text{2HCl(aq)+Ca(OH)}}_2(\text{aq})\to {\text{CaCl}}_2(\text{aq)}+2{\text{H}}_2\text{O(l)}$

The chemical equation for the reaction:

  ${\text{2HCl+Ca(OH)}}_2\to {\text{CaCl}}_2+2{\text{H}}_2\text{O}$

From the equation, the reactants are $\text{HCl}$ and ${\text{Ca(OH)}}_2$ which react in the ratio $2:1$ .

The products formed are ${\text{CaCl}}_2$ and ${\text{H}}_2\text{O}$ , in the ratio $\text{1:2}$ .

For every two molecules of $\text{HCl}$ reacting with ${\text{Ca(OH)}}_2$ , two molecules of ${\text{H}}_2\text{O}$ are formed. So the ratio between $\text{HCl}$ and ${\text{H}}_2\text{O}$ is $\text{1:1}$ .

Therefore, for the formation of four molecules of ${\text{H}}_2\text{O}$ , four molecules of $\text{HCl}$ are required.

Conclusion: Four molecules of $\text{HCl}$ are needed to form four molecules of ${\text{H}}_2\text{O}$ .