Question

Some of the substances commonly used in stomach antacids are MgO, Mg(OH)₂, and Al(OH)₃.

a. Write a balanced equation for the neutralization of hydrochloric acid by each of these substances.

b. Which of these substances will neutralize the greatest amount of 0.10 MHCl per gram?

Short answer

a.The balanced equations are

$$\begin{gathered} \mathrm{MgO}\left(\mathrm{s}\right)+2\mathrm{HCl}\left(\mathrm{aq}\right)\to {\mathrm{MgCl}}_2\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{I}\right) \\ \mathrm{Mg}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)+2\mathrm{HCl}\left(\mathrm{aq}\right)\to {\mathrm{MgCl}}_2\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{I}\right) \\ \mathrm{Al}{\left(\mathrm{OH}\right)}_3\left(\mathrm{s}\right)+3\mathrm{HCl}\left(\mathrm{aq}\right)\to {\mathrm{AlCl}}_3\left(\mathrm{aq}\right)+3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{I}\right) \end{gathered}$$

b.The substance that will neutralize HCl in the most significant amount is MgO.

Step-by-step solution

Balanced equation for the neutralization of HCl

The balanced equations for the neutralization of HCl by MgO, Mg(OH)₂, and Al(OH)₃ are:

$$\begin{gathered} \mathbf{MgO}\mathbf{\left(}\mathbf{s}\mathbf{\right)}\mathbf{+}\mathbf{2}\mathbf{HCl}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{\to }{\mathbf{MgCl}}_{\mathbf{2}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{+}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{\left(}\mathbf{I}\mathbf{\right)} \\ \mathbf{Mg}{\mathbf{\left(}\mathbf{OH}\mathbf{\right)}}_{\mathbf{2}}\mathbf{\left(}\mathbf{s}\mathbf{\right)}\mathbf{+}\mathbf{2}\mathbf{HCl}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{\to }{\mathbf{MgCl}}_{\mathbf{2}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{+}\mathbf{2}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{\left(}\mathbf{I}\mathbf{\right)} \\ \mathbf{Al}{\mathbf{\left(}\mathbf{OH}\mathbf{\right)}}_{\mathbf{3}}\mathbf{\left(}\mathbf{s}\mathbf{\right)}\mathbf{+}\mathbf{3}\mathbf{HCl}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{\to }{\mathbf{AlCl}}_{\mathbf{3}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{+}\mathbf{3}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{\left(}\mathbf{I}\mathbf{\right)} \end{gathered}$$

Determination of the substance that will neutralize the greatest amount

Calculate the number of HCl that can neutralize per gram of each compound.

$$\begin{gathered} \mathbf{MgO}\mathbf{=}\frac{\mathbf{2}\mathbf{}\mathbf{mol}\mathbf{}\mathbf{HCl}}{\mathbf{1}\mathbf{}\mathbf{mol}\mathbf{}\mathbf{MgO}}\mathbf{\times }\frac{\mathbf{1}\mathbf{}\mathbf{mol}\mathbf{}\mathbf{MgO}}{\mathbf{40}\mathbf{.}\mathbf{31}\mathbf{}\mathbf{g}\mathbf{/}\mathbf{mol}} \\ \mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{=}\frac{\mathbf{4}\mathbf{.}\mathbf{962}\mathbf{\times }{\mathbf{10}}^{\mathbf{-}\mathbf{2}}\mathbf{}\mathbf{mol}\mathbf{}\mathbf{HCl}}{\mathbf{1}\mathbf{}\mathbf{g}\mathbf{}\mathbf{compound}} \\ \mathbf{Mg}{\mathbf{\left(}\mathbf{OH}\mathbf{\right)}}_{\mathbf{2}}\mathbf{=}\frac{2\mathrm{mol}\mathrm{HCl}}{1\mathrm{mol}\mathrm{Mg}\left(\mathrm{OH}\right)} \end{gathered}$$₂×1molMg(OH)258.33g/mol=3.429×10-2molHCl1gcompoundAl(OH)3=2molHCl1molAl(OH)3×1molAl(OH)378.00g/mol=3.846×10-2molHCl1gcompound

Thus, the substance that will neutralize HCl in the most significant amount is MgO.