Question

At 27 degrees Celsius and 750 mm Hg, a sample of hydrogen occupies 5.00 L. The space it occupies at STP = 4.49 L.

Should the 4.49 L be rounded to 4.5 L? If not, is there a specific rule that explains rounding for this type of problem?

Answer 1

In multiplication and division, which is all you have in this problem, the rule is that you may have as many significant figures in the answer as you have in any of the numbers. So where did the 4.49 come from?
5.00 x (750/760) x (273./300.) = 4.49. You're allowed 3 s.f., you have 3 s.f., you don't need to round anything.
Here is a very good site that explains about significant figures.
http://www.chemteam.info/SigFigs/SigFigs.html