33. With one exception, the standard heats of formation of Na(s), O2(g), Brz(D, CO(g),
Fe(s), and He(g) are identical.
What is the exception?
Explain.

The answer is (3) 7.64 days. The remaining mass of radon-222 is one fourth of the original mass, so two half lives have elapsed (because one half time one half equals one fourth). Each half life of radon-222 is about 3.82 days, so the total time elapsed is 3.82 * 2 = 7.64 days