Which of the following species has an enthalpy change of formation = 0 kJ/mol?

Below was a question in the 2012 Australian Chemistry Olympiads:



> 
> The enthalpy change of formation ($\Delta\_\mathrm{f}H^\circ$) for a species at
>  $\pu{298 K}$ is defined as the enthalpy change that accompanies the
>  formation of one of the following of one mole of a substance from its
>  constituent elements in their standard states. Which of the following
>  species has $\Delta\_\mathrm{f}H^\circ = \pu{0 kJ/mol}$?
> 
> 
> **a.** $\ce{H2O(l)}$   
>  **b.** $\ce{Na(g)}$   
>  **c.** $\ce{Na(s)}$   
>  **d.**
>  $\ce{CO2(g)}$   
>  **e.** $\ce{O3(g)}$   
> 
> 
> 
> 


[Original](https://i.stack.imgur.com/4iuGf.png)


Solutions stated the answer is **b**, but isn't the standard state of $\ce{Na}$ solid?




The correct answer is indeed **c.** $\ce{Na(s)}$


Since for the heat of formation to be zero, the molecule should be the must abundant chemical formula of the element.


$\ce{Na}$ is a metal and is predominantly found in solid state (I dont think it exists as gas anywhere in the atmosphere of earth).


$\Delta\_\mathrm{f}H^\circ$ is a quantity which is taken relative to the heat of formation of the most abundant naturally occuring form of an element.


 6

Of course it must be a typo. Correct answer is "**c**".


The $\Delta\_\mathrm{f}H^\circ$ of $\ce{Na(s)}$ is $\pu{0 J}$ and the $\Delta\_\mathrm{f}H^\circ$ of $\ce{Na(g)}$ is $\pu{107.8 kJ}$.


 5