What is the number of moles?

Question

The vapor pressure of  CCl3F at 300 K is 856 torr .  If 12.0 g of CCl3F is enclosed in a 1.2 L container, calculate the number of moles of CCl3F in the gas phase.

Answer 1

If we neglect the volume of liquid CCl3F, we can use the ideal gas equation to determine the number of moles of vapor.

PV = nRT n = PV / RT The value for R should be 62.4 L-torr /mole-K

n = 856 torr x 1.2 L / 62.4 x 300 K = 0.055 moles

Answer 2

Hello, Chris,

I'm not comfortable with my answer, but perhaps you'll see something helpful. I'm not clear what we should be doing with ther information describing the vapor pressure at 300K. I'll focus on the question of moles of gas in the 1.2 Liter container.

Use the ideal gas law, PV=nRT, where n is the moles and R the gas constant. The conditions provides say nothing about the pressure or temperature. Are we supposed to use the 300K and 856 torr? I just don't see the direct connection, and would normally assume, since nothing was specified, other than volume, that it is at standard temperature and pressure. I had typed the complete calculation for STP, but my message suddenly disappeared, so I'll just summarize: At 1 atm, 0C and 1.2 liters, I got 0.0535 moles of CCl₃F. 12.0 grams of CCl₃F is (12.0g/137.4g/mole) = 0.087 moles CCl₃F, so there is enough CCl₃F.

I just don't know how that relates to the information at the start of the problem. Are we supposed to use those pressure and temperature conditions? I don't know. If yes, then change those pesky torr to atm (1 torr = (1/760)atm) and recalutate, using R = 0.08206 L*atm*K^-1mol^-1. I get n = 0.0549 moles.

Sorry, I don't know what to do with the two calculations for moles. Perhaps someone else will help.

Bob