Question

How do I calculate Energy (kcal/g) if I know the Chemical formula, the kcal/mol, and the molar mass?

Answer 1

The question is a bit open-ended...

Explanation:

Thermochemical data are usually quoted `kJ*mol^-1`...and what do we mean by `mol^-1`...we mean `"per mole of reaction as written"`

And so for a simple combustion reaction....we might write...

`CH_4(g) + 2O_2(g) rarr 2CO_2(g)+2H_2O(l)+890*kJ`

(of course I did not know the value off the top of my head!)

And so HERE `DeltaH_"combustion"^@"(methane)"=-890*kJ*mol^-1`

By which we MEAN PRECISELY THAT the `"COMPLETE COMBUSTION"` of one MOLE of methane with two mole of dioxygen gives TWO MOLE of carbon dioxide gas, and TWO MOLE of liquid water PLUS AN ENERGY of `890*kJ`. Do you see from where I am coming?

Now of course, since we know the molar masses of products and reactants, I could convert that `kJ*mol^-1` into `kJ*g^-1` of fuel, or if I am a real masochist into `kcal*mol^-1`, or `kcal*g^-1`.

So the take home message is that `kJ*mol^-1` implies a `"MOLE QUANTITY"` of a specific reaction of reference. You have to be sufficiently flexible to recognize WTF that reaction is, and write it out.