Quote (razbagz @ Thu, Apr 30 2009, 12:45am)
ok then..
1) Helium balloons are used to carry scientific instruments high into the atmosphere. Suppose
that such a balloon is launched on a summer day when the temperature at ground level is
22.5°C and the barometer reading is 754 mm Hg. If the balloon’s volume is 1.00 x 106 L at
launch, what will the volume be (in litres) at a height of 37 km, where the pressure is 76.0 mm
Hg and the temperature is 240.0 K?
PV = nRT, they give you one scenario where T = 22.5 C = 295.5K, V = 1*10^6 L, P = 754 mmHg = .992 atm, and you know R = 0.0821 (I think, its been a while for me). So plug all the info that they give you, into PV = nRT, where n is the unknown and you should get n = 40889.42 moles (thus we now know that this balloon contains so many moles of helium in it)
Now they ask what the VOLUME will be at this new height, so again, using PV = nRT we will solve it. List the givens, P = 76 mmHg = .1atm, T = 240K, n = 40889.42 (as we found out), and R = .0821. So plug your info in, and you should get that volume = 8056851.317 L
2) Helium balloons reach high altitudes because the gas density is far less than the density of
the atmosphere (~1.18 kg m-3 at ground level). Calculate the density (in units of kg m-3) of the
helium gas in the balloon described above at 22.5°C and 754 mm Hg.
Answer
We know that density = mass/volume, mass = (moles * atomic mass of helium = 40889.42 * 4.002 = 163639.49 GRAMS ) [and be careful here, they want mass in KILOGRAMS, not grams, so divide the answer you get for mass by 1000, so mass = 163.639 Kg]. We found out that the volume = 8056851.317 L , but they want the answer in CUBIC METERS, so if you remember, 1 cubic meter = 1000 liters, so your volume = 8056.851 cubic meters of helium.
Now just go back to our original formula, D = M / V = 163.639 / 8056.851 = .02031 kg m^-3
This just requires some thinking, these questions aren't tough, maybe you should review your fundamentals because chemistry questions can get A LOT tougher than this.
Edit: Didn't bother using significant figures for this, so if your teacher requires that, just do it over again using sig figs. Sorry about that (I don't do chem anymore, so I fell out of the practice of using sig figs)
This post was edited by BovineDesi on Apr 29 2009 07:19pm