Calculation using Enthalpy change in combustion and Internal energy?

Question

If I have the standard enthalpy of combustion of a known compound, the atmospheric pressure and temperature, how do I calculate the standard internal energy?

I think I have to use the following formula:

DeltaH = DeltaU + PdeltaV

I read somewhere to work out PdeltaV its:

[(no. of moles of products - no of moles of reactants) * RT] / atmospheric pressure

Is that formula right and if it is what do I do with P as that is pressure too?

I am really confused!

Answer 1

You are actually spot on.

H = U + PV

delta H = delta U + delta (PV)

PV = nRT

So at constant temperature, delta (PV) = RT delta n

just like your formula says.

So the RT delta n bit includes the effect of pressure.

Hope this helps.