
Concept explainers
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.
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
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
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.
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.
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