Which of these is least likely to act as a lewis base ?

Given the reaction between 2 gases represented by A₂ and B₂ to give the compound AB_(g),

A₂(g) + B_2(g) $$\rightleftharpoons$$ 2AB_(g)

At equilibrium, the concentration of
A₂ = 3.0 $$ \times $$ 10^$$-$$3 M, of B₂ = 4.2 $$ \times $$ 10^$$-$$3 M, of AB = 2.8 $$ \times $$ 10^$$-$$3 M
If the reaction takes place in a sealed vessel at 527^oC, then the value of K_c will be

Given that the equilibrium constant for the reaction,
2SO_2(g) + O_2(g) $$\rightleftharpoons$$ 2SO_3(g)
has a value of 278 at a particular temperature. What is the value of the equilibrium constant for the following reaction at the same temperature ?
SO_3(g) $$\rightleftharpoons$$ SO_2(g) + $${1 \over 2}$$ O_2(g)

pH of a saturated solution of Ba(OH)₂ is 12. The value of solubility product (K_sp) of Ba(OH)₂ is