Finding molar enthalpy of formation with unknown reactant

[cscott]

I have a chemical equation and the molar enthalpies of formation for its products, along with the total change in enthalpy for the reaction. With this I can easily find the total enthalpy of the reactants. In this case, there are two reactants and I need the molar enthalpy of formation of one of them. How can I achieve this knowing nothing about the second reactant?

 

[maverick280857]

I have a chemical equation and the molar enthalpies of formation for its products, along with the total change in enthalpy for the reaction. With this I can easily find the total enthalpy of the reactants. In this case, there are two reactants and I need the molar enthalpy of formation of one of them. How can I achieve this knowing nothing about the second reactant?

Only if you are computing the standard molar enthalpy of formation...if one of the reactants is in its most stable allotropic form, its enthalpy of formation is zero (because it exists that way in the standard state anyway so you don't have to form it...look up the textbook for a list of allotropes which are assigned a standard molar enthalpy of formation equal to zero).