Ch 202 Problem 10.53

Magnesium can be used as a "getter" in evacuated enclosures, to react with the last traces of oxygen. (The magnesium is usually heated by passing an electric current through a wire or ribbon of the metal). If an enclosure of 0.382 L has a partial pressure of O2 of 3.5 x 10-6 torr at 27oC, what mass of magnesium will react according to the following equation?

2Mg(s) + O2(g) ----> 2MgO(s)

The trick here is to identify the species for which we can immediately calculate moles. Here, we have P, V, and T for O2; from the ideal gas law, we can find moles O2. Once we have found moles O2, we convert moles O2 ---> moles magnesium, and then convert to grams magnesium. Here goes:

get the pressure of O2 in atm:

get the temperature in Kelvins

solve the ideal gas law for n (moles oxygen), and plug away:

now, from the balanced reaction, we can convert mol O2 ---> mol Mg ---> g Mg:

grams Mg

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DAC 7/14/08