Help please which substance is not a compound​

Determine the number of moles of air present in 1.35 l at 750 torr and 17.0°c. which equation should you use? n=pv/rt what is the number of moles present? ⇒ 0.056 mol a sample of n2 gas occupying 800.0 ml at 20.0°c is chilled on ice to 0.00°c. if the pressure also drops from 1.50 atm to 1.20 atm, what is the final volume of the gas? which equation should you use? v2= p1v1t2/p2t1 what is the final volume of the gas? ⇒ 932 ml these are the answers