# Homework Help: Find Heat Energy Needed to Expand Gas to Triple Volume

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1. Mar 20, 2016

### Munir M

1. The problem statement, all variables and given/known data
When 0.40 mol of oxygen(O2) gas is heated at constant pressure starting at 0 degrees C, how much energy must be added to the gas as heat to triple its volume? (The molecules rotate but do not oscillate)

2. Relevant equations
pV=nRT
p1V1/T1=p2V2/T2
Q=mcdT
Value of Cp for Oxygen I used is 0.919 kJ/kg K
3. The attempt at a solution
I used the equation pV=nRT to get the value of pV at 273K which was pV=907.5. I tripled that value to get 2722.5. Then, using p1V1/T1=p2V2/T2, I plugged in the values to get the final temperature(819K). From that I used Q=mcdT which gave me the answer of 3211kJ. The answer in the back was 6.4 kJ.

2. Mar 20, 2016

### MexChemE

You must have missed a conversion factor somewhere, your temperatures are correct. For an isobaric process and constant heat capacity:
$$Q= \Delta H = m C_P \Delta T$$
$$Q = 0.4 \ mol \left(0.032 \frac{kg}{mol} \right) \left( 0.919 \frac{kJ}{kg \cdot K} \right) (819.45 \ K - 273.15 \ K) = 6.426 \ kJ$$

3. Mar 20, 2016

### Munir M

My mistake was using gmol-1 instead of kgmol-1 for the molar mass. Thanks!!