 Let's try, let's try this one. Again, it says hydrogen is used in inflate weather balloons because it's much less dense than air. Calculate the density in grams per liter of gaseous hydrogen at 25 degrees Celsius and 1 atm of pressure. Okay, so well I was nice enough already to calculate for you guys the Kelvin temperature, okay, so that reminds you hopefully that this has to, all your temperatures for these gas law problems have to be in Kelvin, okay. So another way to remember that is to remember the, well let's write down the density formula first, okay, so first thing we'll do is write that down. So d equals mp over rt. So remember that, okay, if you can. And remember r here, that's the ideal gas constant, okay, so that will help us or remind us what units we need the other variables to be in. So r is going to be given to you, in this case, the units are liters atm mole Kelvin, okay, so that's why we switched that degree Celsius to Kelvin because we're going to cancel that out eventually. We want only units of grams per liter. We have the pressure in the atm and well the other thing we need is the molar mass, okay, so molar mass and the problem says that it's nice enough to remind you that gaseous hydrogen is diatomic. So if we look up at the periodic table we find that the molar mass of diatomic gas is 2.016 grams per mole, okay, so that's going to get our mole canceling out of there, okay, so again if we look at this equation we should realize that canceling all of this stuff out should give us grams per mole in the end. So let's plug and chug now and just prove to ourselves that this happened. So the molar mass of hydrogen 2.016 grams per mole times the pressure, so 1 atm by r, so 0.0821 liter atm per mole. Multiply that by r Kelvin temperature 298 Kelvin, okay, so if you've done it right all of your units should cancel out so let's make sure they do, so atm and atm cancel there, moles and moles cancel there, Kelvin, Kelvin cancel there and we're left with grams early, okay, so that's wonderful because that's only one, so now let's just plug all these values into the calculator and see if we get an answer, that makes sense. 2.016, okay, so the density for a gas you would expect, especially a gas that's less than dense than air, to be very, very low, okay, so the density I got is 0.0 and how many sig figs, so 3, so 824 grams per liter, so that's the density of hydrogen gas, as you might expect very, very low, okay, so are there any questions on doing something like that? Wonderful, so I think the main thing to do in this one is to remember the density formula, okay.