AP Acid/Base Equilibria

A student was testing the pH of two different acid solutions and found them both to have a pH of 3.25. Which of the following must be true?

• Both solutions must be weak acids.

• Both solutions must have the same molarity.

• Both solutions must have the same percent dissociation.

• Both solutions must have the same hydronium ion concentration.

The autoionization of water is an endothermic process. Which statement is correct concerning the pH of pure water at 100^oC?

• The pH would be 7 because the [H₃O⁺] = [OH^-].

• The pH would be 7 because pure water has no H₃O⁺ ions or OH^- ions.

• The pH would be greater than 7 because the reaction shifts to produce more H₃O⁺ and OH^-.

• The pH would be less than 7 because the reaction shifts to produce more H₃O⁺ and OH^-.

Phosphoric acid, H₃PO₄, dissociates stepwise as shown below.

H₃PO₄ <-> H⁺ + H₂PO₄^-

H₂PO₄^- <-> H⁺ + HPO₄^2-

HPO₄^2- <-> H⁺ + PO₄^3-

Which of the following is an acid followed by its conjugate base?

• HPO₄^2-, H₂PO₄^-

• H₂PO₄^-, PO₄^3-

• H₃PO₄, HPO₄^2-

• H₂PO₄^-, HPO₄^2-

Phosphoric acid, H₃PO₄, dissociates stepwise as shown below.

H₃PO₄ <-> H⁺ + H₂PO₄^-

H₂PO₄^- <-> H⁺ + HPO₄^2-

HPO₄^2- <-> H⁺ + PO₄^3-

In a 0.1 M solution of H₃PO₄, which species will be in the greatest concentration?

• H₃PO₄

• H₂PO₄^-

• HPO₄^2-

• PO₄^3-

Phosphoric acid, H₃PO₄, dissociates stepwise as shown below.

H₃PO₄ <-> H⁺ + H₂PO₄^-

H₂PO₄^- <-> H⁺ + HPO₄^2-

HPO₄^2- <-> H⁺ + PO₄^3-

Which of the following changes would increase the phosphate ion concentration in 100 mL of 0.5 M phosphoric acid?

• Add 10 mL of 0.5 M HCl.

• Add 10 mL of 0.5 M H₃PO₄.

• Add 10 mL of 0.5 M NaH₂PO₄.

• Add a catalyst.

A solution is prepared by adding 0.10 mole of potassium fluoride, KF, to 1.00 L of water. Which pH is closest to that of the solution?

• 2.00

• 7.00

• 9.00

• 13.00

A 0.010 M solution of which of these acids would have the highest pH?

• HF (K_a = 7.2 x 10^-4)

• HNO₂ (K_a = 4.0 x 10^-4)

• HCO₂H (K_a = 1.8 x 10^-4)

• HC₃H₅O₃ (K_a = 1.4 x 10^-4)

Given 0.10 M solutions of the four weak acids below all with 0.10 M concentration, which will have the greatest percent ionization?

HF (K_a = 7.2 x 10^-4)

HNO₂ (K_a = 4.0 x 10^-4)

HCO₂H (K_a = 1.8 x 10^-4)

HC₃H₅O₃ (K_a = 1.4 x 10^-4)

• HF because its K_a is the largest.

• HF because its molar mass is the smallest.

• HC₃H₅O₃ because its K_a is the smallest.

• HC₃H₅O₃ because its molar mass is the greatest.

As the number of oxygen atoms increases in any series of oxygen acids, such as HXO, HXO₂, HXO₃, ..., which of the following is generally true?

• The acid strength increases with an increasing oxidation state of the central atom because it causes each oxygen atom to become more electronegative.

• The acid strength increases with an increasing oxidation state of the central atom because the formal charge of the central atom increases.

• The acid strength increases with greater numbers of oxygen atoms because the hybridization of the central atom increases, thus making it easier to ionize the hydrogen.

• The acid strength increases with greater numbers of oxygen atoms because it causes the strength of the bond between oxygen and hydrogen to increase.

Which of the following acids along with their conjugate base, would make the most effective buffer to maintain the pH of a solution at 4.95?

• acetic acid (K_a = 1.8 x 10^-5)

• benzoic acid (K_a = 6.4 x 10^-5)

• hydrofluoric acid (K_a = 7.2 x 10^-4)

• lactic acid (K_a = 1.4 x 10^-4)