Suppose you need a tank of oxygen that is 95% pure. Describe a process by which you could obtain such a gas, starting with air.

A tank of oxygen that is 95% pure.

The phase diagram for nitrogen and oxygen is given below:

Consider the experimental phase diagram for nitrogen and oxygen at atmospheric pressure in Figure 5.31 of the book. Starting with air (which is a mixture of nitrogen 79 percent N2 and oxygen 21 percent O2), we lower the temperature until we reach a temperature of 81.5 K, at which point the oxygen will start to liquefy, and to find the percentage of oxygen in the liquid, we draw a horizontal line from the upper curve to the lower curve until it intersects with it at this point x= 0.5, which means the oxygen in the liquid is 95 percent pure. Then, while keeping the temperature constant, we pump the gas down the liquid, then let the temperature rise again, increasing the proportion of oxygen in each cycle, and so on.