# Homework Help: Thermodynamics Chemistry

1. Nov 17, 2014

### KTiaam

1. The problem statement, all variables and given/known data
How much energy is required to decompose 765 g of PCl3, according to the reaction below? The molar mass of PCl3 is 137.32 g/mol and may be useful.

4 PCl3(g) → P4(s) + 6 Cl2(g) ΔH°rxn = +1207 kJ

3. The attempt at a solution

765 g PCl3 x 4 mol PCl3/137.32 g PCl3 = 22.28 mol PCl3

22.28 mol PCL3 x 1207 kJ/4 mol PCl3 = 6.72 x 103 kJ

Which is incorrect...

I converted from grams to mols then i converted mols to kJ, which did not give me the correct answer.
Im probably using a wrong sign somewhere or completely approaching the problem correctly.

Thank you for your help

2. Nov 17, 2014

### Bystander

The highlighted operators are incorrect. Pay attention to number of moles in the equation for enthalpy.

3. Nov 17, 2014

### KTiaam

the number of moles is 4? I got 4 from the balanced equation?

could you explain more on what I'm doing wrong?

4. Nov 17, 2014

### Bystander

Step at a time: PCL3 is 137.32 gram per mole (gram mole, gram molecular weight), also written 137.32 g/mol. How many moles in 765 g?