If K_a is the ionisation constant of weak acid (HA) and K_b is the ionisation constant of its conjugate base(A^-) then K_a K_b = K_w
\({ K }_{ b }({ F }^{ - })=\frac { { K }_{ w } }{ { K }_{ a }(HF) } =\frac { 1\times { 10 }^{ -14 } }{ 6.8\times { 10 }^{ -4 } } =1.47\times { 10 }^{ -11 }\)
\({ K }_{ b }(HCOO^{ - })=\frac { { K }_{ w } }{ { K }_{ a }(HCOOH) } =\frac { 1\times { 10 }^{ -14 } }{ 1.8\times { 10 }^{ -4 } } =5.56\times { 10 }^{ -11 }\)
\({ K }_{ b }(Cn^{ - })=\frac { { K }_{ w } }{ { K }_{ a }(HCn) } =\frac { 1\times { 10 }^{ -14 } }{ 4.8\times { 10 }^{ -4 } } =2.08\times { 10 }^{ -6 }\)

Prediction of the following stages of a reaction by comparing the value \({ K }_{ c }\) and \({ Q }_{ c }\) are If \({ Q }_{ c }\)<\({ K }_{ c }\), the reaction will proceed in the direction of the products (forward reaction)

pH = -log[H⁺] = -log [3.8\(\times \)10^-3]
= - [log[3.8] + log[10^-3]
= - [(0.58) + (-3.0)] = - [-2.42] = 2.42
Therefore, the pH of the soft drink is 2.42 and it can be inferred that it is acidic.

Although BF₃ does not have a proton but acts as Lewis acid as it is an electron deficient compound. It reacts with NH₃ by accepting the lone pair of electrons from NH₃ and complete its octet. the reaction can be represented by 
BF₃+:NH₃⟶BF₃\(\leftarrow\)NH₃
Lewis electronic theory of acids and bases can explain it. Boron in BF₃ is sp² hybridised where N in NH₃ is sp³ hybridised.