the density of a noble gas is 2.71 g/L at 3.00atm and O degrees C. What is the gas.?

Answer 1

The noble gases are Helium, Neon, Argon, Xenon and Krypton and Radon. When the pressure is three times normal the gas weighs three times as much, so you can call 2.71 g/L approx equal to 0.9 g/L. Raising the temp to STP is going to knock it down to about 0.8 g/L, so look em up on Wikipedia and find a match. I doubt seriously it is Xenon, Krypton or Radon.

If you want more accuracy, use PV=nRT to convert. The variables are pressure, volume, moles, the gas constant and temp (in K). Do a wikipedia search on Gas Laws and Kelvins.