In a first-order reaction A $$ \to $$ B, if k is rate constant and initial concentration of the reactant A is 0.5 M, then the half-life is

The reaction of hydrogen and iodine monochloride is given as :
H_2(g) + 2ICl_(g) $$ \to $$ 2HCl_(g) + I_2(g)
This reaction is of first order with respect to H_2(g) and ICl_(g),
following mechanisms were proposed.

Mechanism A :
     H_2(g) + 2ICl_(g) $$ \to $$ 2HCl_(g) + I_2(g)
Mechanism B :
     H_2(g) + ICl_(g) $$ \to $$ HCl_(g) + HI_(g) ; slow
     HI_(g) + ICl_(g) $$ \to $$ HCl_(g) + I_2(g) ; fast

Which of the above mechanism(s) can be consistent with the given information about the reaction?

Consider the following reactions :
(i) H⁺_(aq) + OH^$$-$$_(aq) = H₂O_(l),  $$\Delta $$H = $$-$$ X₁ kJ mol^$$-$$1
(ii) H_2(g) + 1/2O_2(g) = H₂O_(l),   $$\Delta $$H = $$-$$ X₂ kJ mol^$$-$$1
(iii) CO_2(g) + H_2(g) = CO_(g) + H₂O_(l),  $$\Delta $$H = $$-$$ X₃ kJ mol^$$-$$1
(iv) C₂H_2(g) + 5/2O_2(g) = 2CO_2(g) + H₂O_(l),    $$\Delta $$H = +X₄ kJ mol^$$-$$1

Enthalpy of formation of H₂O_(l) is

An element, X has the following isotopic composition:
²⁰⁰X : 90%  ¹⁹⁹X : 8.0%  ²⁰²X : 2.0%

The weighted average atomic mass of the naturally occuring element X is closest to