For which element in each of the following pairs has the standard heat of formation of zero? (i) `[O_(2)(g),O_(3)(g)]`
(ii) [`Cl_(2)(g),Cl(g)]`
(iii) `[S("s, rhombic"),S(s", monoclinic")]` ?

Explanation

The enthalpy of formation of an element in its standard state is zero. At `25^(@)C`, the standard standard state of oxygen chlorine and sulphur are `O_(2)(g),Cl_(2)(g) and ` S (s, rhombic). So, at `25^(@)C`, the standard heats of formation of `O_(2)(g),Cl_(2)(g) and S`(s, rhombic) will be zero.