 Okay, let's try this next one. We'll calculate the density of, or the mass of a sample of air. Okay, so gases normally will have very low, low, low densities. And that's usually why they're on top of everything else, because they have a very low density. Remember that experiment that we did earlier. But anyways, the problem says air has a density of 0.00013 grams per milliliter. Okay, so notice very low density. What is the mass of 6 liters? So we know the volume is 6 liters, 6.0 liters. What is the mass we're looking for? Of 6 liters of a sample of air. Well, remember, again, this is not the way we want to write the density if we're trying to use it in a problem. So let's just go ahead and rewrite it real quick for 1 milliliter. And notice, liters and milliliters won't cancel out with each other. So we're going to have to convert this to liters or this to milliliters, whichever way you prefer. So let's just convert this one to liters, because, you know, usually gas densities are actually given to you in liters. So what do we know? There's 1,000 milliliters per 1 liter. So if we do that, remember from our last lecture that milliliters will cancel out of the two-dimensional analysis. So all we've got to do is multiply that by 1,000. So 1, 2, 3. So we've got 1.3 grams per 1 liter is the density. So we know of this sample 2, and we're looking for the mass. So if we got liters here and we got liters here, if we cancel those out, we'll get the mass here. So we didn't even really have to, here, let's just do that before I go on to, so this is how you get the density. Your density equals 1.3 grams, 6 liters, or this is how you get the density. You get the mass this way. Cancel, cancel like that. And then you just do 1.3 times 6. And that gives you 7.8, 6 liters of air, 7.8 grams. And you could have done it that way, so let's do it that way too. We can manipulate that formula, remember. So we're looking for mass. So we want to isolate that variable side by v, and if we do that, we've got to multiply that side by v. So cancel, cancel, so we write, which is what we calculated here, 1.3, if you just remember, all those leading zeros are insignificant. So again, you don't really have to remember that formula if you remember canceling units. That's what the whole point of learning how to cancel units is. So that's actually what we learned that now, I promise you. Okay, cool.