Ionic Equilibrium: Acid & Base - Free MCQ Practice Test with solutions,

Electron rich species are called lewis base. Among the given, BF₃ is an electron deficient species, so have a capacity of electrons accepting instead of donating that's why it is least likely to act as a lewis base. It is a lewis acid. 

Acid ⇋ Conjugate Base  +  H⁺
By this definition,
HNO2 ⇋ NO₂^- + H⁺
CH₃NH₃+ ⇋ CH₃NH₂ + H⁺
C₆H₅COOH ⇋ C₆H₅COO^- + H⁺
H₃O⁺ ⇋ H₂O + H⁺
We can see that the conjugate base of H₃O+ is H₂O. But it is given that OH^- is the C.B. So option d is incorrect.

B₂H₆ is an electron deficient species. Soit will act as Lewis acid.

H₂CO₃ is formed by dissolution of CO₂ into water. This dissociates into H+ and HCO^3-. This is the first dissociation of H₂CO₃. Since HCO^3- has another H, it again dissociates to H⁺ and CO₃^2-. H+ obtained attacks the lone pair of H₂O to form H₃O⁺. So H₃O⁺, HCO^3-, CO₃^2-, H₂CO₃ are present in water when CO₂ is dissolved in  water.

Bronsted lowry acids are those acids which donate H⁺. In the reaction, A and D are giving H⁺. So, these both are bronsted lowry acid.

For a buffer, we need to have a weak base/acid and its salt. Here, we have a weak base and so we will need its salt. In option c, concentration of strong acid is less than base. So we will have both a weak base and its salt. Therefore, only option c gives us a buffer solution.

A buffer is an aqueous solution containing a weak acid and its conjugate base or a weak base and its conjugate acid. It is used to prevent any change in the pH of a solution, regardless of solute. Buffer solutions are used as a means of keeping pH at a nearly constant value in a wide variety of chemical applications.

When we move from left to right, electronegativity of central atom increases. Due to this nature, central atom has more tendency to break its bond with H and acquire negative charge. So, while moving from left to right, acidic strength increase.

Total number of compounds acting as Lewis base in the given example is 6. H⁺ and BF₃ are electron deficient so they can't act as Lewis base while FeCl₃ acts as Lewis acid so all the other compounds except these three are Lewis base.

The correct answer is 2
Here
we know [OH-][H+]=10^-14 at 25 degree Celsius
so pk_a+pk_b=14
hence answer is 2