Hot Stellar Moles T o reinforce elemental concepts of chemistry, i t helps to see how they work under unusual conditions. As David Clark has pointed out, things are both familiar and exotic in the hot plasma inside the sun.' I have beguiled my introductory classes with the following question. Astrophysicists can predict the behavior of stars quite accurately using a model in which their chemistry is deliehtfullv simole. Thev believe that stars contain iust three elements: "hvdroeen" . .. and "helium".. olus a trace of "everything else" In the very hot interiurufastar, the molar masrof"hydrogen" iin.5.g m d and that of"everythmg else" is ahuut :!glmol. Explain these w i r d molar masses, and predict the molar mass of stellar "helium".
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The key t o this conundrum is, of course, that a t very high temperatures atoms are completely ionized. So 1 g of hydrogen exists as 1mol of protons and 1mol of electrons. Its molar mass is therefore (1g)/(2 mol) = 0.5 g/mol. "Everything else" means mostly elements in the hump of the abundance curve. When an element of atomic mass A ionizes, it produces Z 1particles. Since far the first two or three dozen elements A is roughly 22,the molar mass of "everything else" is (22 g)/(Z 1)mol or close to 2 glmol. The molar mass of "helium" is calculated thus: 4 g; 1mol nuclei and 2 ma1 electrons; average molar mass (4 g)/(3 mol) = 1.3 elmol. In addition toshowing that fundamental chemiral principlesapply in unexpected places, thrr pnmdes n very nice lead in tu the calculatiun oiaveragr molar massr* oi isoropic mixtures and partially ionized tlectndpes.
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'Clark, D. 6 . J. Chem. Educ. 1989, 66,826. John 5. Martln The Univerrily of Alberta Edmonton, AB Canada T6G 262
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