Multiple Choice
Which of the following substances is less soluble in hot water than in cold water?
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6. Chemical Quantities & Aqueous Reactions
Solubility Rules
Multiple Choice
According to general solubility rules, what is the solubility of Mg(OH)2 in pure water?
A
Mg(OH)2 is highly soluble in pure water.
B
Mg(OH)2 is moderately soluble in pure water.
C
Mg(OH)2 is mostly insoluble in pure water.
D
Mg(OH)2 is completely soluble in pure water.
Verified step by step guidance1
Recall the general solubility rules for hydroxides: most hydroxides (OH⁻ compounds) are insoluble in water, except those of alkali metals (Group 1) and some alkaline earth metals like Ca(OH)₂, Sr(OH)₂, and Ba(OH)₂, which are moderately soluble.
Identify that magnesium hydroxide, Mg(OH)₂, is a hydroxide of an alkaline earth metal (magnesium is in Group 2), but unlike Ca(OH)₂, it has limited solubility in water.
Understand that 'mostly insoluble' means that only a small amount of Mg(OH)₂ dissolves in pure water, establishing an equilibrium between solid Mg(OH)₂ and its ions in solution.
Write the dissolution equilibrium for Mg(OH)₂: \(\mathrm{Mg(OH)_2 (s) \rightleftharpoons Mg^{2+} (aq) + 2 OH^- (aq)}\) and recognize that the solubility product constant, \(K_{sp}\), quantifies its solubility.
Conclude that based on the solubility rules and the low \(K_{sp}\) value for Mg(OH)₂, it is mostly insoluble in pure water, meaning only a small concentration of Mg²⁺ and OH⁻ ions are present in solution.
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