Chemistry scenario (empirical formula)?

Question

Sodium azide, a compound containing only sodium (Na) and nitrogen (N), is used as a propellant for inflating airbags in some automobiles. A sample of sodium azide was weighed into a crucible. The crucible was covered and heated until only Na remained. The crucible was then cooled and reweighed. The following data were collected.

mass of crucible and cover, g: 28.887

mass of crucible, cover, and sodium azide, g: 30.354

mass of crucible, cover, and Na, g: 29.406

Determine the empirical formula of sodium azide.

I determined the correct empirical formula, but I did not use the 29.406 g for the mass of crucible, cover, and Na in my calculations. Do I need that information? If so, how do I do all of it?

Answer 1

Yes, you need this information.

To solve the problem, we start by finding the mass of sodium azide: subtract the mass of crucible and cover from that of crucible, cover, and azide.

Next, find the mass of Na produced (subtract mass crucible and cover from mass crucible, cover, and Na).  We know that the total mass of compounds present should not have changed, so we know that the mass of nitrogen produced must be the difference between the mass of sodium azide used and the mass of Na measured - subtract them.

Next, find out how many moles of Na and N were produced (the N will be made as N2, but if we use the molar mass for N instead, rather than that for N2 and then multiply by two, the answer will be the same).  Divide the mass by the respective molar mass.

To make an empirical formula, divide both molar amounts by one: since you know the answer, you probably know that it is easier to divide the moles of Na and the moles of N both by the moles of Na.  The quotient for Na will be 1 (duh) and that for N will be the number of nitrogens per sodium.  You should get NaN3 as your formula.

I cannot think of a way to do this problem without using the mass of sodium.

Also, this is a perfect example of really, really bad experimental design.  First off, much of the sodium metal will be produced as a vapor rather than as a liquid or solid.  Secondly, sodium metal is extremely reactive and will easily react with oxygen and water vapor in the air.  The crucible will not be airtight (if it was, the nitrogen couldn't get out), and some of these gases will seep in and react with the Na, increasing its apparent weight.

Answer 2

I must add my support to Zircaliu. This is not the way to teach students how to do a laboratory experiment. The results would bear no relationship at all with reality.