A sample of helium gas occupies 12.9 L at 315 K and 1.20 atm.

This sample contains how many moles of helium?

There are 0,6 moles of helium.

Explanation:

We use the ideal gas formula, we use the gas constant R = 0.082 l atm / K mol and we solve for n (number of moles) of the formula:

PV=nRT - -> n = PV/RT

n = 1,20 atm x 12,9 L / 0.082 l atm / K mol x 315 K = 0,599303135 moles