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Question:

The species which by definition has zero standard molar enthalpy of formation at 298 K is:

Br2(g)

Cl2(g)

CH4(g)

H2O(g)

Solution:

The standard molar enthalpy of formation of an element in its standard state is taken as zero. Since the standard state of Cl2 is gas, so it's ΔfH = 0. Bromine and water exist in the liquid state at 298 K. Methane is not an elemental species. Hence, the correct option is B.