AP Chem Unit 8

Pure water autoionizes as shown in the equation below. Based on this information, which of the following is correct? H2O(l) + H2O(l) ⇌ H3O+(aq) + OH−(aq) Kw = 7.0 × 10^-14 at 55°C

• The autoionization equilibrium for pure water favors the formation of reactants at 55°C compared to 25°C.

• The autoionization equilibrium for pure water produces the same amount of OH− ions at 55°C and 25°C.

• At 55°C, pH = -log(√Kw) for pure water.

• At 55°C, pH = -log(Kw) for pure water.

Tags

NGSS.HS-PS1-5

Based on the information below, which of the following is true for a sample of pure water at 25°C? Kw = [H3O+] [OH−] = 1.0 × 10^-14 at 25°C

• [H3O+] = 7.0 M

• [OH−] = 1.0 × 10^-14 M

• pH = 10^7

• pOH = 7.00

The endothermic autoionization of pure water is represented by the chemical equation shown below. The pH of pure water is measured to be 7.00 at 25.0°C and 6.02 at 100.0°C. Which of the following statements best explains these observations? 2H2O(l) ⇌ H3O+(aq) + OH−(aq) ΔH° = +56 kJ/mol

• At the higher temperature water dissociates less, [H3O+] < [OH−], and the water becomes basic.

• At the higher temperature water dissociates less, [H3O+] = [OH−], and the water remains neutral.

• At the higher temperature water dissociates more, [H3O+] > [OH−], and the water becomes acidic.

• At the higher temperature water dissociates more, [H3O+] = [OH−], and the water remains neutral.

Tags

NGSS.HS-PS1-5

Which of the following gives the best estimate for the pH of a 5×10^-4 M Sr(OH)2(aq) solution at 25°C?

• pH ≈ 3.0 because Sr(OH)2 is a strong acid.

• pH ≈ 5.0 because Sr(OH)2 is a weak acid.

• pH ≈ 9.0 because Sr(OH)2 is a weak base.

• pH ≈ 11.0 because Sr(OH)2 is a strong base.

Which of the following gives the best estimate for the pH of a 1 × 10^-5 M HClO4(aq) solution at 25°C?

• pH ≈ 1.0 because HClO4 is a strong acid.

• pH ≈ 5.0 because HClO4 is a strong acid.

• pH ≈ 7.0 because ClO4- is a strong base.

• pH ≈ 9.0 because ClO4- is a strong base.

Which of the following is the correct mathematical relationship to use to calculate the pH of a 0.10M aqueous HBr solution?

• pH = [H₃O⁺] = 0.10

• pH = -log(1.0 × 10^-1)

• pH = 7.00 - (0.10)

• pH = log(1.0 × 10^-1)

The equilibrium for the acid ionization of HCOOH is represented by the equation in the image and the table gives the percent ionization for HCOOH at different initial concentrations of the weak acid at 25°C. Based on the information, which of the following is true for a 0.125M aqueous solution of HCOOH?

• It has a larger percent ionization, a lower [H₃O⁺]eq, and a lower pH than a 0.150M HCOOH solution does.

• It has a larger percent ionization, a lower [H₃O⁺]eq, and a higher pH than a 0.150M HCOOH solution does.

• It has a lower percent ionization, a larger [H₃O⁺]eq, and a higher pH than a 0.100M HCOOH solution does.

• It has a lower percent ionization, a larger [H₃O⁺]eq, and a higher pH than a 0.100M HCOOH solution.

Tags

NGSS.HS-PS1-5

The reaction between NH3 and water is represented in the image. A solution that is initially 0.200M in NH3 has a pH of 11.28. Which of the following correctly predicts the pH of a solution for which [NH3]initial = 0.100M, and why?

• The pH will be lower than 11.28 because decreasing the initial concentration of NH3 increases the equilibrium concentration of the conjugate acid NH4+.

• The pH will be lower than 11.28 because the equilibrium concentration of OH- ions decreases when the initial concentration of the base decreases.

• The pH will be higher than 11.28 because decreasing the initial concentration of NH3 decreases the equilibrium concentration of the conjugate acid NH4+.

• The pH will be higher than 11.28 because the equilibrium concentration of OH- ions increases when the initial concentration of the base decreases.

Tags

NGSS.HS-PS1-5

The acid ionization equilibrium for HClO at 25°C is shown below. A 0.100M aqueous solution of this acid has a pH of about 4.2. If a solution with an initial concentration of HClO of 0.200M is allowed to reach equilibrium at the same temperature, which of the following correctly predicts its pH, and why?

• The pH will be higher than 4.2 because increasing the concentration of the weak acid decreases the percent ionization.

• The pH will be higher than 4.2 because, for weak acid solutions, the pH is directly proportional to the initial concentration of the weak acid.

• The pH will be lower than 4.2 because increasing the concentration of the weak acid produces more H3O+ to establish equilibrium.

• The pH will be lower than 4.2 because the Kₐ of the weak acid is inversely proportional to the initial concentration of the weak acid.

Tags

NGSS.HS-PS1-5

The weak acid CH3COOH has a pKa of 4.76. A solution is prepared by mixing 500.0 mL of 0.150M CH3COOH(aq) and 0.0200mol of NaOHₐₚ. Which of the following can be used to calculate the pH of the solution?

• pH = 4.76 + log(0.0200 / 0.150) = 3.88

• pH = 4.76 + log(0.0200 / 0.0200) = 4.19

• pH = 4.76 + log(0.0750 / 0.0200) = 4.32

• pH = 4.76 + log(0.0950 / 0.0750) = 4.86