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11 Dec 2019

Salts formed by reaction of a strong acid with a weak base areacidic, and salts formed by reaction of a weak acid with a strongbase are basic. This is due to the equilibrium the conjugate haswith its acid or base. For example, sodium bicaronate NaHCO3contains the conjugate of the weak acid H2CO3. The conjugate willreform the acid through the equilibrium

HCO3?(aq)+H2O(aq)?H2CO3(aq)+HO?(aq)

and, thus, acts as a weak base. A solution of NaHCO3yields abasic solution.

If a salt is derived from a weak acid and a weak base, the pH ofthe solution depends on the relative acid strength (Ka) ofthe cation and base strength (Kb) of the anion. IfKa>Kb, the solution of the salt is acidic. IfKa<Kb, it is basic. If Ka?Kb, it is neutral.

Given that

  • Ka for HCN is 4.9

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Hubert Koch
Hubert KochLv2
13 Dec 2019

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Related questions

For conjugate acid-base pairs, how are Ka and Kb related?

Consider the reaction of acetic acid in water:

CH3CO2H (aq) + H2O (l) ⇌ CH3CO2− (aq) + H3O+ (aq)             where Ka = 1.8 × 10−5

  1. Which two bases are competing for the proton?
  2. Which is the stronger base?
  3. In light of your answer to part b. why do we classify the acetate ion (CH3CO2−) as a weak base? Use an appropriate reaction to justify your answer.
  4. In general, as base strength increases, conjugate acid strength decreases. Explain why the conjugate acid of the weak base NH3 is a weak acid.

 

To summarize, the conjugate base of a weak acid is a weak base and the conjugate acid of a weak base is a weak acid (weak gives you weak). Assuming Ka for a monoprotic strong acid is 1× 106, calculate Kb for the conjugate base of this strong acid.

Why do conjugate bases of strong acids have no basic properties in the water?

List the conjugate bases of the six common strong acids.

To tie it all together, some instructors have students think of Li+, K+, Rb+, Cs+, Ca2+, Sr2+, and Ba2+ as the conjugate acids of the strong bases LiOH, KOH, RbOH, CsOH, Ca(OH)2, Sr(OH)2, and Ba(OH)2. Although not technically correct, the conjugate acid strength of these cations is similar to the conjugate base strength of the strong acids. That is, these cations have no acidic properties in water; similarly, the conjugate bases of strong acids have no basic properties (strong gives you worthless). Fill in the blanks with the correct response. The conjugate base of a weak acid is a_____base. The conjugate acid of a weak base is a_____acid. The conjugate base of a strong acid is a_____base. The conjugate acid of a strong base is a_____ acid. ( Hint: Weak gives you weak and strong gives you worthless.)

A salt consisting of the conjugate base of a ____________ and conjugate acid of a ____________ will be neutral since neither will react with water. The salt consisting of the conjugate base of a ____________ and the conjugate acid of a ____________ will be acidic since the conjugate ____________ of a ____________ is acidic. A salt consisting of the conjugate base of a ____________ and the conjugate acid of a ____________ will be basic since the conjugate ____________ of a ____________ is basic. A salt consisting of the conjugate base of a ____________ and conjugate acid of a ____________ can be either acidic or basic depending on the relative Ka and Kb of the respective ions.

The word box includes: Weak base, base, strong base, acid, strong acid, weak acid

Consider a salt of a weak base and a weak acid such as ammonium cyanide. Both the NH4+ and CN− ions interact with water in an aqueous solution, but the net reaction can be considered as a proton transfer from NH4+ to CN−.

NH4+ (aq) + CN− (aq) ⇌ NH3 (aq) + HCN (aq)

  1. Show that the equilibrium constant for this reaction, K act, is 1. K net = Ka Kb Kw where Ka is the ionization constant for the weak acid NH4+ and Kb is the constant for the weak base CN−  
  2. Calculate K net values fur each of the following, NH4CN, NH4CH3CO2, and NH4K Which salt has the largest value of K act, and why?
  3. Predict whether a solution of each of the compounds in (b) is acidic or basic. Explain how you made this prediction (Yon do not need a calculation to make this prediction.)

 

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