Salt is an ionic compound that is produced when a cation and an anion from an ac

Question

Salt is an ionic compound that is produced when a cation and an anion from an acid-base reaction combine, and solutions of salts can be neutral, acidic, or basic, depending on the acid-base properties of the constituent cations and anions.

Since strong acids and bases completely ionize in solution, the reaction essentially runs to completion forming the salt. Therefore, salts formed by reaction of a strong acid with a strong base are neutral. NaCl formed by the neutralization of NaOH and HCl is a neutral salt.

Salts formed by reaction of a strong acid with a weak base are acidic, and salts formed by reaction of a weak acid with a strong base are basic. This is due to the equilibrium the conjugate has with its acid or base. For example, sodium bicaronate NaHCO3 contains the conjugate of the weak acid H2CO3. The conjugate will reform the acid through the equilibrium

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

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

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

Part A

Given that

Ka for HCN is 4.9×1010 and

Kb for NH3 is 1.8×105,

calculate

Kb for CN and

Ka for NH4+.

Enter the Kb value for CN followed by the Ka value for NH4+, separated by a comma, using two significant figures.

Explanation / Answer

(a) Ka for HCN = 4.9 x 10^-10

Kb for CN- = Kw/Ka = 1 x 10^-14/4.9 x 10^-10 = 2.04 x 10^-5

(b) Kb for NH3 = 1.8x 10^-5

Ka for NH4+ = Kw/Kb = 1 x 10^-14/1.8 x 10^-5 = 5.55 x 10^-10

Kb , Ka = 2.04 x 10^-5 , 5.55 x 10^-10

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