### 8.45- why do you multiply?

Posted:

**Mon Jan 22, 2018 12:44 pm**The question is

4C(s) + S8(s) ---> 4CS2 (l) deltaH=358.8 KJ, and its asking how much heat is absorbed at constant P with 1.25 mol S8

My understanding of this is that the 358.8 KJ implies that it is for 1 mol of rxn even though it doesn't say KJ/mol, and so you have to use the 1.25 mol of S8 to find the heat. In the answers it has you multiply the deltaH by 1.25 mol, but I would think you would divide it:

(358.8 KJ/1 mol rxn)(1 mol rxn/1.25 mol S8), so that the mols of rxn cancel and you are left with the KJ per mol of S8. Can someone please explain to me why this logic is incorrect please, and how we would know when to multiply and when to divide in problems like these?

4C(s) + S8(s) ---> 4CS2 (l) deltaH=358.8 KJ, and its asking how much heat is absorbed at constant P with 1.25 mol S8

My understanding of this is that the 358.8 KJ implies that it is for 1 mol of rxn even though it doesn't say KJ/mol, and so you have to use the 1.25 mol of S8 to find the heat. In the answers it has you multiply the deltaH by 1.25 mol, but I would think you would divide it:

(358.8 KJ/1 mol rxn)(1 mol rxn/1.25 mol S8), so that the mols of rxn cancel and you are left with the KJ per mol of S8. Can someone please explain to me why this logic is incorrect please, and how we would know when to multiply and when to divide in problems like these?