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Ch.17 - Acids and Bases
All textbooksTro 5th EditionCh.17 - Acids and BasesProblem 79a
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Chapter 17, Problem 79a

Find the pH of each mixture of acids. a. 0.115 M in HBr and 0.125 M in HCHO2

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Step 1: Identify the acids involved in the mixture. HBr is a strong acid, and HCHO2 (formic acid) is a weak acid.
Step 2: Calculate the concentration of hydrogen ions [H+] contributed by the strong acid, HBr. Since HBr is a strong acid, it dissociates completely in solution. Therefore, [H+] from HBr is equal to its concentration, 0.115 M.
Step 3: Consider the contribution of hydrogen ions from the weak acid, HCHO2. Since HCHO2 is a weak acid, it does not dissociate completely. However, in the presence of a strong acid, its contribution to [H+] is negligible compared to the strong acid.
Step 4: Determine the total [H+] in the solution. Since the contribution from HCHO2 is negligible, the total [H+] is approximately equal to the [H+] from HBr, which is 0.115 M.
Step 5: Calculate the pH of the solution using the formula pH = -\log[H+]. Substitute the total [H+] into the formula to find the pH.

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Key Concepts

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

Strong vs. Weak Acids

Strong acids, like HBr, completely dissociate in solution, releasing all their hydrogen ions (H+). In contrast, weak acids, such as formic acid (HCHO2), only partially dissociate, establishing an equilibrium between the undissociated acid and its ions. Understanding the difference between these types of acids is crucial for calculating the pH of mixtures.
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Weak Acid-Strong Base Titration Curve

pH Calculation

pH is a measure of the hydrogen ion concentration in a solution, calculated using the formula pH = -log[H+]. For strong acids, the pH can be directly derived from their molarity, while for weak acids, the pH requires the use of the acid dissociation constant (Ka) to determine the concentration of H+ ions at equilibrium.
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pH Calculation Example

Acid-Base Mixtures

When mixing strong and weak acids, the strong acid will dominate the pH due to its complete dissociation. However, the presence of the weak acid can affect the overall pH depending on its concentration and dissociation. Analyzing the contributions of both acids is essential for accurately determining the final pH of the mixture.
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Related Practice
Textbook Question

Calculate the percent ionization of a formic acid solution having the given concentration. c. 0.100 M

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Calculate the percent ionization of a formic acid solution having the given concentration. d. 0.0500 M

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A 0.148 M solution of a monoprotic acid has a percent ionization of 1.55%. Determine the acid ionization constant (Ka) for the acid.

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Open Question

Find the pH of each mixture of acids. b. 0.150 M in HNO2 and 0.085 M in HNO3 c. 0.185 M in HCHO2 and 0.225 M in HC2H3O2 d. 0.050 M in acetic acid and 0.050 M in hydrocyanic acid

Open Question

Find the pH of each mixture of acids. a. 0.075 M in HNO3 and 0.175 M in HC7H5O2 b. 0.020 M in HBr and 0.015 M in HClO4 c. 0.095 M in HF and 0.225 M in HC6H5O d. 0.100 M in formic acid and 0.050 M in hypochlorous acid

Textbook Question

For each strong base solution, determine [OH–], [H3O+], pH, and pOH. a. 0.15 M NaOH b. 1.5×10–3 M Ca(OH)2 c. 4.8×10–4 M Sr(OH)2 d. 8.7×10–5 M KOH

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