Ch.16 - Acid-Base Equilibria

Problem 104

Chapter 16, Problem 104

Calculate the pH of a solution made by adding 2.50 g of lithium oxide 1Li2O2 to enough water to make 1.500 L of solution.

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Determine the molar mass of lithium oxide (Li2O). Calculate it by adding the atomic masses of lithium (Li) and oxygen (O) from the periodic table.

Convert the mass of lithium oxide (2.50 g) to moles using the molar mass calculated in the previous step.

Write the balanced chemical equation for the reaction of lithium oxide with water. Li2O reacts with water to form lithium hydroxide (LiOH).

Calculate the concentration of lithium hydroxide (LiOH) in the solution by dividing the moles of LiOH by the volume of the solution in liters (1.500 L).

Use the concentration of LiOH to find the pH. Since LiOH is a strong base, it dissociates completely in water, and the concentration of OH- ions is equal to the concentration of LiOH. Calculate the pOH and then use the relation pH + pOH = 14 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.

pH Scale

The pH scale measures the acidity or basicity of a solution, ranging from 0 to 14. A pH of 7 is considered neutral, while values below 7 indicate acidity and values above 7 indicate basicity. The pH is calculated as the negative logarithm of the hydrogen ion concentration in the solution, which is crucial for understanding the solution's chemical behavior.

Lithium Oxide (Li2O) Dissociation

Lithium oxide is a basic oxide that reacts with water to form lithium hydroxide (LiOH). This reaction increases the concentration of hydroxide ions (OH-) in the solution, which in turn affects the pH. Understanding this dissociation is essential for calculating the resulting pH after the oxide is dissolved in water.

Molarity and Concentration Calculations

Molarity is a measure of concentration defined as the number of moles of solute per liter of solution. To calculate the pH, one must first determine the number of moles of lithium oxide in the given mass and then find the concentration of hydroxide ions produced in the solution. This step is critical for accurately determining the pH of the resulting solution.

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Textbook Question

Salts containing the phosphate ion are added to municipal water supplies to prevent the corrosion of lead pipes. (a) Based on the pKa values for phosphoric acid 1pKa1 = 7.5 * 10 - 3, pKa2 = 6.2 * 10 - 8, pKa3 = 4.2 * 10 - 132 what is the Kb value for the PO43 - ion?

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Textbook Question

Which, if any, of the following statements are true? (a) The stronger the base, the smaller the pKb. (b) The stronger the base, the larger the pKb. (c) The stronger the base, the smaller the Kb. (d) The stronger the base, the larger the Kb. (e) The stronger the base, the smaller the pKa of its conjugate acid. (f) The stronger the base, the larger the pKa of its conjugate acid.

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Textbook Question

Predict how each molecule or ion would act, in the Brønsted-Lowry sense, in aqueous solution by writing 'acid,' 'base,' 'both,' or 'neither' on the line provided. (b) Prozac

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Open Question

What is the pH of a solution that is 2.5 * 10^-9 M in NaOH? Does your answer make sense? What assumption do we normally make that is not valid in this case?

Open Question

Oxalic acid H2C2O4 is a diprotic acid. By using data in Appendix D as needed, determine whether each of the following statements is true: (a) H2C2O4 can serve as both a Brønsted–Lowry acid and a Brønsted–Lowry base. (b) C2O4²⁻ is the conjugate base of HC2O4⁻. (c) An aqueous solution of the strong electrolyte KHC2O4 will have a pH of 6 or 7.

Open Question

Succinic acid (H2C4H6O4), which we will denote H2Suc, is a biologically relevant diprotic acid with the structure shown below. At 25 °C, the acid-dissociation constants for succinic acid are Ka1 = 6.9 * 10^-5 and Ka2 = 2.5 * 10^-6. (a) Determine the pH of a 0.32 M solution of H2Suc at 25 °C, assuming that only the first dissociation is relevant. (b) Determine the molar concentration of Suc2- in the solution in part (a). (c) Is the assumption you made in part (a) justified by the result from part (b)?