Dry air can be cooled and then fractionally distilled to separate its components. What is the minimum volume of dry air needed to prepare a 25 dm3 sample of oxygen using this method? (Assume that all volumes are measured at room temperature and pressure, and that all gases are ideal.)

Answer:

18) D

Air has 21% oxygen. We need to prepare 25 dm3 of oxygen.
So we know that 21% = 25 dm3
1% = 25/21
100% = 25/21x 100 = 119 dm3