For each of the four situtaions below, please specify if a common-ion effect wil

Question

For each of the four situtaions below, please specify if a common-ion effect will be observed. If the answer is yes, please identify the common ion by formula.

a. Indicator HIn dissolved in strong acid solution

b. Addition of excess strong base after the equivalence point in a weak acid-strong base titration

c. Addition of Zn(OH)2 (s) to a basic aqueous solution

d. Addition of CaF2 (s) to 0.100 M NaCl (aq)

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Explanation / Answer

(a)

HIn is weak acid undergoes partial ionisation.

HIn (aq.) = H+ (aq.) + In- (aq.)

Additon of strong acid, produces more number of H+ ions.

HA (aq.) ------------> H+ (aq.) + A- (aq.)

SO, the common ion = H+

Due to the presence of common he ionisation of weak acid indicator will be suppressed.

(b)

Weak acid - strong base has the following equilibrium at equivalence point,

A- (aq.) + H2O (l) = HA (aq.) + OH- (aq.)

Added strong base undergoes complete ionisation as follows.

BOH (aq.) ------------> B+ (aq.) + OH- (aq.)

So, common ion = OH-

Due to this common ion, the hydrolysis of anion reaction will be suppressed.

(c)

Zn(OH)2 is a weak base undergoes partial ionisation.

Zn(OH)2 (s) = Zn2+ (aq.) + 2 OH- (aq.)

In the presence of basic solution, there will be OH- ions in common. so, ionisation fo Zn(OH)2 is suppressed.

(d) No common ions.

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