A 6.58g sample of compound was placed in a 12.6 L container. At 25.0*C, the pressure was found to be 0.75 atm. what is this compound?

Use PV = nRT and solve for n = number of moles. Then n = grams/molar mass and solve for molar mass. I assume this is a relatively common compound and you are to recognize the name of the compound from the molar mass.