Test: Common Ion Effect

If dilute HCl is added then concentration of H⁺ ions will increase so according to Lechatelier's principle equilibrium will shift in backward direction. So acetate ion concentration will decrease.

The correct answer is option C
[HA] = [OH–]
The reaction is: A– + H2O ⇌ HA + OH–

The correct answer is Option B.

NH4Cl is a strong electrolyte hence, dissociates completely, NH4OH is a weak electrolyte. Its dissociation is further suppressed by common ion NH4+ provided by NH4Cl in the solution.
NH4Claq  ⇌ NH4aq+ +Claq−
NH4OHaq ⇌ NH4aq++ OHaq−
This solution acts as a basic buffer and maintains its pH around 9.25. It is capable of resisting the change in pH in addition to a small amount of acid or alkali.
To Explain:
If a small amount of HCl is added to this solution, H+
ions of HCl get neutralized by OH− ions already present and more of NH4
​OH molecules get ionized to compensate for the loss of OH− ions. Thus pH practically, remains constant.
If a small amount of NaOH is added to this solution, OH− ions of NaOH combine with NH4+ ions already present in large numbers forming weakly ionized NH4OH. Thus, pH of solution remains practically unchanged.

K_sp = 4.0 x 10^-12 
For AB type salt, Solubility = K_sp^1/2
= (4.0 x 10^-12)^1/2
= 2 x 10^-6

Mixtures of CH₃COOH and CH₃COONa are called acidic buffer. Since the constitution of this buffer is a weak acid and its constituting salt, it is called Acidic buffer.

The correct answer is option D
Common ion effect is observed when a solution of weak electrolyte is mixed with a solution of strong electrolyte, which provides an ion common to that provided by weak electrolyte.
The NH4OH is a weak base and it does not ionise completely. Thus, due to presence of common ion NH4+ in NH4Cl, it suppresses the ionisation of weak base NH4OH in order to decrease the OH- concentration so that higher group cations will not get precipitated.
Thus the pair NH4​OH+NH4​Cl shows a common ion effect. 
Ammonium chloride suppresses the ionization of ammonium hydroxide
.

Kc = Product of Molar conc. of products/ Product of molar conc. of reactants 

If Kc is less this clearly means that the product of the molar conc. Of reactants is more or the reaction is less in forward direction and more in the backward direction.

A mixture of weak base and its salt with a strong acid serves as an basic buffer which resists changes in pH upon addition of small amount of acid or base.

The correct answer is option C
pH of blood remains constant because of the buffer system present in the blood. Acid-base buffers confer resistance to a change in the pH of a solution when hydrogen ions (protons) or hydroxide ions are added or removed. An acid-base buffer typically consists of a weak acid, and its conjugate base (salt).It used to neutralized the extra added protons or OH- in blood.The buffer for maintaining acid-base balance in the blood is the carbonic-acid-bicarbonate buffer.So pH of blood remains even after eating spicy food.

Given, pKa = 3.45
Concentration of HF = 0.1 M, concentration of KF = 0.300 M
For acidic buffer;
pH = pKa + log [salt of weak acid]/[weak acid]
= 3.45 + log0.3/0.1
= 3.45 + 0.48
= 3.93