Chemistry Chapter 14: Acids and Bases Q&A

The equilibrium constant for the reaction A- + H+ <--> HA is called

1/Ka The equilibrium constant associated with the dissociation of an acid is defined as Ka, so the equilibrium constant for the reverse reaction is 1/Ka.

The pH of a solution at 25ºC in which [OH-] = 2.1 * 10^-8 M is:

6.3 Since the pOH for [OH-] = 2.1 * 10^-8 M is 7.7 and pH + pOH = 14.00, the pH is 14.00 - 7.7 = 6.3.

What volume of water must be added to 100.0 mL of an aqueous NaOH solution to change the pH from 12.00 to 11.00?

900.0 mL For every pH unit, the concentration of H+ (or OH-) changes by a factor of 10. To lower the pH of a base by one pH unit, therefore, the total volume must increase by a factor of 10. Adding 900.0 mL to an initial solution of 100.0 mL will result in a solution with a volume of 1000.0 mL.

If the solid ammonium fluoride (NH4F) is dissolved in pure water, will the resulting solution be acidic, basic, or neutral? Note: For NH3, Kb = 1.8*10^-5; for HF, Ka = 7.2*10^-4.

Acidic Since the Ka value for HF is greater than the Kb value for NH3, the Kb value for F- will be less than the Ka value for NH3, making NH4F acidic.

Identify the Brønsted acids and bases in the following equation (A = Brønsted acid, B = Brønsted base): HPO42- + HSO4- <--> H2PO4- + SO42-

B A A B Species that donate protons (based on having a proton as a reactant and lacking a proton as a product) will be Brønsted acids. Species that accept protons (based on attaining a proton as a product) will be Brønsted bases.

What are the major species in an aqueous solution of HCl?

H2O, H+, Cl- A strong acid dissociates completely in water.

What are the major species in an aqueous solution of HF? The Ka value for HF is 7.2 × 10-4.

H2O, HF A weak acid dissociates to only a slight extent in water.

Consider the reaction of an acid in water: HA(aq) + H2O <--> H3O+(aq) + A-(aq) If A- is a better base than H2O, is HA a weak acid or a strong acid?

HA is a weak acid. Since equilibrium lies to the left, the acid HA is a weak acid. Also, the conjugate base (A-) is a relatively strong base, making HA a weak acid.

Consider the reaction of an acid in water: HA(aq) + H2O <--> H3O+(aq) + A-(aq) If A- is a better base than H2O, is the value of Ka greater than or less than 1?

Less than 1. HA is a weak acid, so the value for Ka is less than 1.

Consider the reaction of an acid in water: HA(aq) + H2O <--> H3O+(aq) + A-(aq) If A- is a better base than H2O, which way will equilibrium lie?

To the left. If A- is a better base than H2O, the reverse reaction dominates and equilibrium lies to the left.

Consider two separate solutions of equal concentration. The first solution contains sodium hydroxide and the second solution contains barium hydroxide. Which of the following statements is true? a)The sodium hydroxide solution has the lower pH. b)The barium hydroxide solution has the lower pH. c)The pH values of both solutions are equal.

a) The sodium hydroxide solution has the lower pH. Since both bases are strong and of equal concentration, the barium hydroxide, which has two hydroxide ions to contribute, will be more basic and thus will have a higher pH.

Which of the following graphs best represents a plot of [H+] (y-axis) versus [HA]o (x-axis) for an aqueous weak acid at constant temperature? see slide 32

b) As the concentration of a weak acid increases, the concentration of [H+] increases as well, although this relationship is not linear [so choice (a) is not correct].

HA and HB are both weak acids in water, but HA is a stronger acid than HB. Which of the following statements is correct? a)A- is a stronger base than B-, which is a stronger base than H2O, which is a stronger base than Cl-. b)B- is a stronger base than A-, which is a stronger base than H2O, which is a stronger base than Cl-. c)B- is a stronger base than A-, which is a stronger base than Cl-, which is a stronger base than H2O. d)Cl- is a stronger base than A-, which is a stronger base than B-, which is a stronger base than H2O.

b) B- is a stronger base than A-, which is a stronger base than H2O, which is a stronger base than Cl-. The stronger the acid, the weaker the conjugate base. Since HCl is a strong acid, the conjugate base (Cl-) will be a weaker base than water.

If the solid sodium cyanide (NaCN) is dissolved in pure water, will the resulting solution be acidic, basic, or neutral?

b) Basic Since NaCN is a strong electrolyte, it completely dissociates in water. The CN- can react with water to produce HCN and OH-, which results in a basic solution.

The pH of a solution is raised from 3 to 5. Which of the following statements is false? a)The pOH decreases from 11 to 9. b)The [H+] decreases by a factor of 20. c)The final [OH-] (at pH = 5) is 10^-9 M. d)The initial [H+] (at pH = 3) is 10^-3 M. e)The initial solution could be 0.001 M HNO3.

b) The [H+] decreases by a factor of 20. As the pH of a solution increases from 3 to 5, the [H+] decreases by a factor of 100.

Which of the following graphs best represents a plot of [H+] (y-axis) versus [OH-] (x-axis) for an aqueous solution at a constant temperature of 25°C?

d) [H+][OH-] = 1.0 × 10-14 at 25°C. Thus [H+] and [OH-] are inversely proportional.

The conjugate acid and conjugate base of bicarbonate ion (HCO3-) are, respectively, a)H3O+ and OH-. b)H3O+ and CO32-. c)H2CO3 and OH-. d)H2CO3 and CO32-. e)CO32- and OH-.

d) H2CO3 and CO32-. A conjugate acid has an additional proton (H+). The conjugate base is what remains after the proton is lost.

You have 100.0 mL of a solution of hydrochloric acid that has a pH of 3.00. You add 100.0 mL of water to this solution. What is the resulting pH of the solution? a)pH = 5.00 (the average of 3.00 and 7.00). b)pH = 10.00 (3.00 + 7.00 = 10.00). c)pH = 3.00 (water is neutral and does not affect the pH). d)None of these is correct, but the pH must be greater than 3.00. e)None of these is correct, but the pH must be less than 3.00.

d) None of these is correct, but the pH must be greater than 3.00. For every pH unit, the acidity level changes by a factor of 10. Doubling the volume will lower the [H+] by a factor of 2, which will increase the pH.

As water is heated, its pH decreases. This means that: a)the water is no longer neutral. b)[H+] > [OH-]. c)[OH-] > [H+]. d)Two of these are correct. e)None of these is correct.

e) None of these is correct As water is heated, the value of Kw describing the relationship between water, hydrogen ions, and hydroxide ions changes.

The value of Ka for 4.00 M weak acid HA is _____ the value of Ka for 8.00 M weak acid HA.

equal to The value of Ka for a given acid is not dependent on the concentration of the acid (only on the temperature).

The pH for 4.00 M weak acid HA is _____ the pH for 8.00 M weak acid HA.

greater than As a solution becomes more acidic, its pH becomes lower. Thus, as the concentration of the acid increases, the pH will decrease; conversely, as the concentration of the acid decreases, the pH will increase.

The percent ionization for 4.00 M weak acid HA is _____ the percent ionization for 8.00 M weak acid HA.

greater than As the concentration of a weak acid decreases, the percent dissociation increases.

HF and HCN are both weak acids, but HF is a stronger acid than HCN. Consider the following reaction: HF + CN- <--> F- + HCN The value of K for this reaction is:

greater than 1. HF is a stronger acid than HCN, so the forward reaction dominates. Therefore K is greater than 1.