# Homework Help: Finding total internal energy

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1. Sep 23, 2016

### john mcgrain

1. The problem statement, all variables and given/known data
There is a monatomic gas held at a constant pressure of P = 1.48-atm, it also has a molar mass M = 16-g/mol and density ρ =1.9 × 10-3-g·cm-3. Find the total internal energy of 1-mol of this gas.

2. Relevant equations
U = Q + W
E = nCvT
PV = nRT

3. The attempt at a solution
I have tried E = 3/2nRdT but I am unsure as to how I could use molar mass and density to find anything that can be subbed in to the equations. I tried dividing M and ρ cm-3/mol but i'm not sure were to go from there.

2. Sep 23, 2016

### TSny

Welcome to PF
What does "d" stand for here?

OK. Watch the units, they're not quite correct here. With the units corrected, what useful information is this giving you?

3. Sep 24, 2016

### john mcgrain

The d is for delta. As in delta T change in temperature. When dividing M and ρ it seems as though grams cancels out and you're left with 1/cm-3*mol. If i instead divide ρ by M we are left with cm-3*mol which is V x n. I don't see where this can be put into the formula though, is there a formula i'm missing?

4. Sep 24, 2016

### TSny

Why do you want to use a temperature change? Are you trying to determine a change in energy?

OK. Note the negative power on cm. What would the units look like if you rewrote it with the cm part in the numerator?

5. Sep 24, 2016

### Staff: Mentor

Have you considered using the ideal gas law to get the temperature? From the ideal gas law, how is mass density $\rho$ related to P, M, R, and T?