Chapter U4.21, Problem 6E

Interpretation Introduction

Interpretation: The reason for the pH of a solution of Mg(OH)₂ to remain above 7 when 1 mole of NaOH is added to the solution needs to be explained.

Concept Introduction: The pH of an aqueous solution is defined as

pH = -log [H⁺]

Here, [H⁺] is the concentration of hydrogen ion solution.

The pOH of an aqueous solution is the negative logarithm of the hydroxide, OH- ion concentration, i.e,

pOH = -log [OH^-]

Here, [OH^-] is hydrogen ion concentration in the solution.

An acid-base neutralization reaction is the reaction of H+ with OH- ions to produce undissociated water, H₂O molecules as per the equation below.

  $H^{+}\left(aq\right)+O{H}^{-}\left(aq\right)\to H_{2}O$

The reaction above is quantitative and consumes equal amounts of H+ and OH- ions.

It is known that for solutions rich in H+ (i.e, solutions containing excess H⁺), the pH is less than 7. Such solutions are known as acidic solutions. Similarly, for solutions containing excess of OH^-, the pH is greater than 7 and such solutions are known as basic solutions.

Answer to Problem 6E

The pH of the solution of Mg(OH)₂ remains above 7 upon the addition of 1 mole of NaOH due to the presence of excess OH- ions in the solution.

Explanation of Solution

Magnesium hydroxide, Mg(OH)₂ ionizes in an aqueous solution as

  $Mg{\left(OH\right)}_2\left(aq\right)\to M{g}^{2+}\left(aq\right)+2O{H}^{-}\left(aq\right)$

Since the solution contains hydroxide, OH- ions, hence, the solution is basic and therefore, the pH of the solution is higher than 7.

Sodium hydroxide, NaOH ionizes in the solution as

  $NaOH\left(aq\right)\to N{a}^{+}\left(aq\right)+O{H}^{-}\left(aq\right)$

The addition of 1 mole of NaOH to the solution of Mg(OH)₂ introduces additional hydroxide, OH- ions in the solution. Therefore, the hydroxide ion concentration increases making the solution more basic and thus, the pH of the solution increases. In fact, the pH still remains above 7 and acquires a higher value than when only Mg(OH)₂ was present.

Conclusion

The addition of NaOH to the solution of Mg(OH)₂ increases the OH- ion concentration in the solution and therefore, the pH remains above 7.