I need help with some thermochemistry questions, PLEASE, PLEASE HELP!

Question

I've spent all day on the following questions and can't seem to get them right. If anyone would be willing to provide a break down with the answer that would be greatly appreciated. I just want to be done so that I can review (it's an online assessment that counts), because I'm obviously not as great at it as I thought. :((( Please help me

 

1. A 0.01552-mol sample of an organic compound was burned in oxygen in a bomb calorimeter. The temperature of the calorimeter increased from 24.9oC to 29.3oC. If the heat capacity of the calorimeter is 4.88 kJ (oC)-1, then what is the constant volume heat of combustion of this compound, in kilojoules per mole? (Remember to include a "+" or "-" sign in your answer, as appropriate.) --> I tried the answers 1.38 10^3 and 1.26E3 and both were wrong :( 

 

2.The enthalpy change for the reaction below at 25oC is -1,249 kJ (per mole of C6H14). What is the internal energy change for the reaction at 25oC? 

6 CO(g) + 13 H2(g) --> C6H14(l) + 6 H2O(l) --> I thought this was pretty straight forward but I got stuck trying to calculate W first?

 

3.What is w when 1.91 kg of H2O(l), initially at 25.0oC, is converted into water vapour at 155oC against a constant external pressure of 1.00 atm? Assume that the vapour behaves ideally and that the density of liquid water is 1.00 g/mL. Enter your answer in acceptable SI units, accurate to 3 sig figs... also +/- --> I got really stuck on this one

 

4. What is w when a gas is compressed from 36.1 L to 25.1 L using a constant external pressure of 729 mmHg? (Remember to include a "+" or "-" sign as appropriate.)
Enter your answer in acceptable SI units, accurate to 3 significant figures. Use an acceptable SI abbreviation for units! --> I tried -/+ 10.5 J and both were wrong :(

Answer 1

1. Q = mcΔT =  4.88KJ/oC x (29.3 - 24.9)oC = 21.472KJ

Therefore heat of combustion in KJ/mol = -21.472 KJ/0.01552 mol = -1383.5KJ/mol

This is an exothermic reaction- heat is given off, so ΔH is negative.