What mass (in grams) of carbon dioxide gas would occupy a volume of 3.25 L at 2.50 atm and 40.0oC?

Use the ideal gas law: pV=nRT

You're given p, V, and T, and R is a constant (R=0.0821 \frac{L \cdot atm}{K \cdot mol}). All you have to do is solve for n to get the number of moles, then multiply by the molecular weight to get the mass in grams.

Just make sure you convert the temperature to Kelvin.

One small detail, pressure is most of the time denoted by capital P. :)

Small p is usually used for momentum.

But here, it's understood that p is pressure.

I knew it! I first wrote it with a capital P, but then couldn't remember if R was traditionally small or capital, so I went to the Wikipedia page (http://en.wikipedia.org/wiki/Ideal_gas_law), and they used small p and capital R. :)