<p>How do you know if a gas is lighter than air? It came up on a Barron's test. (And is this something I need to know for the actual test?)</p>
<p>A gas is “lighter” than air if it is less dense than air. The density of air is typically 1.2 kg-m^-3</p>
<p>Wow. I wouldn’t have known that.</p>
<p>Haha No you usually don’t need to know this but it’s mostly common sense. Everything is usually lighter than air until you get into that heavy gas stuff like xenon.</p>
<p>Define “air” - is it Oxygen or Nitrogen or a bunch of gases? >_<</p>
<p>If:
Gas Atomic weight < “Air”‘s Atomic weight
Then:
It is lighter than air. james’ density thing is confusing, probably acceptable yes, but I don’t think one would be expected to know the density of air… I hope.</p>
<p>Well…air is usually a mixture so that could get a bit confusing.</p>
<p>If it was just talking about one gas, then it would be easier to compare.</p>
<p>I don’t see this as a question that could be asked. Air is a mixture of gases so there’s really no way to calculate its “molar mass.” I guess you could work from air being 78% N2, 21 O2, etc. but that’s way too tedious.</p>
<p>The gases were: NH3, NO, H2, SO3, and CO2. CO2 was the right answer</p>
<p>What was the exact question?</p>
<p>It was a lab question showing a set up. It’s a florence flask on a ring stand with water in it, with one tube going from the water, straight up with a sort of cup-like thing on top, and other only slightly in the flask that goes to a container. It asked what the set up could be used to prepare and collect. (FYI, this is Barron’s)</p>
<p>I only skimmed your description but I imagine this is collecting gases over water?</p>
<p>You can only collect a gas over water if it’s insoluble in water. CO2 is apparently the only gas of the choices that is insoluble in water.</p>
<p>i don’t think that’s what it was supposed to be. it was really, really weird. i can only assume that this would never actually be on the test.
i just took the CB practice test and got 690. I can probably get this up to like 710/720, but maybe not. That ends up being low in the percentile range, but will that hurt me in admissions?</p>