Ch.17 - Additional Aspects of Aqueous Equilibria

Problem 14c

Chapter 17, Problem 14c

Consider the equilibrium B1aq2 + H2O1l2 Δ HB+1aq2 + OH-1aq2. Suppose that a salt of HB+1aq2 is added to a solution of B1aq2 at equilibrium. (c) Will the pH of the solution increase, decrease, or stay the same?

Verified step by step guidance

Step 1: Understand the equilibrium reaction. The reaction B1aq2 + H2O1l2 Δ HB+1aq2 + OH-1aq2 is an acid-base reaction where B1aq2 acts as a base and H2O1l2 acts as an acid. The products are the conjugate acid HB+1aq2 and the conjugate base OH-1aq2.

Step 2: Identify the change in the system. The problem states that a salt of HB+1aq2 is added to the solution. This means that the concentration of HB+1aq2 in the solution increases.

Step 3: Apply Le Chatelier's Principle. According to Le Chatelier's Principle, when a change is applied to a system at equilibrium, the system will shift to counteract that change. In this case, the increase in HB+1aq2 will cause the equilibrium to shift to the left to reduce the concentration of HB+1aq2.

Step 4: Determine the effect on OH-1aq2. As the equilibrium shifts to the left, the concentration of OH-1aq2 will decrease because it is a product of the reaction and the reaction is moving in the reverse direction.

Step 5: Relate the change in OH-1aq2 to pH. The pH of a solution is related to the concentration of OH-1aq2 in the solution. As the concentration of OH-1aq2 decreases, the pH of the solution will also decrease. Therefore, the pH of the solution will decrease when a salt of HB+1aq2 is added to the solution.

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Key Concepts

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

Le Chatelier's Principle

Le Chatelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the system will adjust to counteract the change and restore a new equilibrium. In this case, adding a salt of HB+ will increase the concentration of HB+, shifting the equilibrium to the left to reduce the concentration of HB+ and OH-.

Acid-Base Equilibrium

Acid-base equilibrium involves the balance between acids and bases in a solution, which affects the pH. In this equilibrium, HB+ acts as an acid, donating protons (H+) to the solution, while B- acts as a base. The addition of HB+ will influence the concentration of H+ ions, thereby affecting the pH.

pH Scale

The pH scale measures the acidity or basicity of a solution, ranging from 0 (very acidic) to 14 (very basic), with 7 being neutral. A decrease in pH indicates an increase in H+ concentration, while an increase in pH indicates a decrease in H+ concentration. Understanding how the addition of HB+ affects H+ concentration is crucial for predicting changes in pH.

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Related Practice

Textbook Question

Ca1OH22 has a Ksp of 6.5 * 10-6. (b) If 50 mL of the solution from part (a) is added to each of the beakers shown here, in which beakers, if any, will a precipitate form? In those cases where a precipitate forms, what is its identity? [Section 17.6]

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Textbook Question

Three cations, Ni²⁺ , Cu²⁺ , and Ag⁺, are separated using two different precipitating agents. Based on Figure 17.23, what two precipitating agents could be used? Using these agents, indicate which of the cations is A, which is B, and which is C.[Section 17.7]

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Textbook Question

Which of these statements about the common-ion effect is most correct? (a) The solubility of a salt MA is decreased in a solution that already contains either M+ or A-. (b) Common ions alter the equilibrium constant for the reaction of an ionic solid with water. (c) The common-ion effect does not apply to unusual ions like SO32 - . (d) The solubility of a salt MA is affected equally by the addition of either A- or a noncommon ion.

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Open Question

Use information from Appendix D to calculate the pH of (a) a solution that is 0.060 M in potassium propionate (C2H5COOK or KC3H5O2) and 0.085 M in propionic acid (C2H5COOH or HC3H5O2). (b) a solution that is 0.075 M in trimethylamine (CH3)3N and 0.10 M in trimethylammonium chloride (CH3)3NHCl. (c) a solution that is made by mixing 50.0 mL of 0.15 M acetic acid and 50.0 mL of 0.20 M sodium acetate.

Open Question

Use information from Appendix D to calculate the pH of: (a) a solution that is 0.250 M in sodium formate (HCOONa) and 0.100 M in formic acid (HCOOH); (b) a solution that is 0.510 M in pyridine (C5H5N) and 0.450 M in pyridinium chloride (C5H5NHCl); (c) a solution that is made by combining 55 mL of 0.050 M hydrofluoric acid with 125 mL of 0.10 M sodium fluoride.

Textbook Question

(a) Calculate the percent ionization of 0.0075 M butanoic acid 1Ka = 1.5 * 10-52.

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