Finding Gas Mass Using Heat Exchange at Constant Pressure and Volume

[kdndk]

Homework Statement

The temperature of an ideal gas (M=28kg/mole) has increased 14K by adding 29J of heat at a constant pressure. To cool the gas back to the initaial temperature 20.7J of heat is extracted from the gas at a constant volume. Find the mass of the gas.

Homework Equations

ΔU=(3/2)nRΔT
n=number of moles
R=8.31
T=temperature

The Attempt at a Solution

none

 

[rude man]

1. ΔT = +14K, Q = 29J: can you use the 1st law to determine the number of moles n as a function of C_V, the heat capacity at constant volume?

2. ΔT = -14K, Q = -20.9J: can you use the 1st law again to determine C_V?
Then you'd have n and of course m = 28kg*n.