Ch.4 - Reactions in Aqueous Solution

Brown - Chemistry: The Central Science 15th Edition

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Brown 15th Edition

Ch.4 - Reactions in Aqueous Solution

Problem 32

Chapter 4, Problem 32

Which of the following solutions is the most basic? a. 0.6𝑀 NH3 b. 0.150 M KOH c. 0.100𝑀Ba(OH)2

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Identify the nature of each compound: NH3 is a weak base, KOH is a strong base, and Ba(OH)2 is a strong base.

For strong bases like KOH and Ba(OH)2, determine the concentration of hydroxide ions (OH⁻) they produce. KOH dissociates completely to give 0.150 M OH⁻, while Ba(OH)2 dissociates to give 0.200 M OH⁻ (since it produces two OH⁻ ions per formula unit).

For the weak base NH3, use the base dissociation constant (Kb) to estimate the concentration of OH⁻ ions it produces. This requires setting up an equilibrium expression and solving for OH⁻ concentration.

Compare the OH⁻ concentrations from each solution: higher OH⁻ concentration indicates a more basic solution.

Determine which solution has the highest OH⁻ concentration to identify the most basic solution.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

pH and Basicity

pH is a measure of the hydrogen ion concentration in a solution, indicating its acidity or basicity. A pH greater than 7 signifies a basic solution. Basicity refers to the ability of a substance to accept protons (H+) or donate electron pairs, with stronger bases resulting in higher pH values.

Concentration of Hydroxide Ions

The basicity of a solution is often determined by the concentration of hydroxide ions (OH-). Strong bases, such as KOH and Ba(OH)2, dissociate completely in water, increasing the OH- concentration. The more hydroxide ions present, the more basic the solution will be.

Dissociation of Bases

Different bases dissociate in water to varying extents. For example, KOH is a strong base that fully dissociates into K+ and OH-, while Ba(OH)2 also dissociates completely, producing two moles of OH- per mole of Ba(OH)2. NH3, on the other hand, is a weak base that does not fully dissociate, making its contribution to basicity less significant compared to strong bases.

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Acid-Base Dissociation Example

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You know that an unlabeled bottle contains an aqueous solution of one of the following: AgNO3,CaCl2, or Al2(SO4)3. You take a portion of the solution and add an aqueous solution of Ba(NO3)2 to it, and observe that a white solid precipitates Then you take another portion of the unlabeled solution and add an aqueous solution of NaCl to it; nothing appears to happen. What is the most likely identity of the solution in the unlabeled bottle: silver nitrate, calcium chloride, aluminum sulfate?

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