Does anyone know any examples of how a teacher could make a very diffuclut mol problem involving densitiy? i got a big test tomorrow and im trying to prepare for it

Here is an example off the top of my head:

 

How many moles are there in 100.0 mL of of pure hexane, C6H14? (density of hexane = 0.655g/cm3). If you can answer something like that I think you should be alright.

You could also do something such as "You have 10.7 liters of SF6 at 45 Degrees Fahrenheit. If the sulfur hexafluoride was decomposed into its elements, how many grams of sulfur and how many grams of fluorine gas would you have. In addition, how many moles would this be?"

 

This would be quite difficult as you have to deal with gas density, the fact that sulfur and fluorine aren't simple one atom molecules, and converting Fahrenheit to Kelvin.

guys its a little late, but thanks anyway. i think i got pwned on an uber hard question. the question gave us the dimesnisons of a room and we had to find how much gas fills it, it was super hard....

Remember, if you're using the Ideal Gas Law, ANY gas takes up 22.4 liters of space if it is ONE MOLE of the gas. Doesn't matter if it's an uber heavy gas, or something like hydrogen. One mole of gas at STP takes up 22.4 Liters of space according to the Ideal Gas Law.

Actually, we can use a real example of a question like this.

 

Let's say we have a room that is 8 feet high, by 10 feet long, by 14 feet wide. There are 0.3048 meters in one foot, so converting to metrics your room is 2.4384x3.048x4.2672 meters. This is equal to 31.7149 cubic meters.

 

0.001 cubic meters makes up one liter of volume, so your room has a total volume of 31,714.868 liters. That's a lot of volume. So how many grams of hydrogen at one atmosphere would it take to fill that room up if the hydrogen was an ideal gas and the temperature was 80 degrees Fahrenheit?

 

The first step here is to write out the ideal gas law. PV=nRT. You have the volume (31714.868L), you have the Pressure (1 Atmosphere), you can find the ideal gas constant (R=0.0820574587 L · atm · K-1 · mol-1), and you have the temperature, though it needs to be converted into Kelvin (80F).

 

To convert to Kelvin, you first convert to Celcius and then add 273.15. So to start, you subtract 32 from your Fahreneheit value.

 

80-32 = 48

 

Now take this value and multiply by 5/9.

 

48(5/9) = 26.67

 

Now take this value and add it to 273.15

 

26.67+273.15 = 299.82 K.

 

You now have all the values you need to calculate the number of moles of H2 gas needed to fill your room!

 

(1)(31714.868) = n(0.0820574587)(299.82)

 

n = # of moles = 1289.09 moles of gas!

 

One mole of Hydrogen is 2 grams, so you would need 2.57818 kilograms of hydrogen gas to fill up your room.

WHY COULDN'T YOU POST THIS EARLIER??????????????????????????????????????????????????????????????????????????????????????????? . You would have prevented me from getting pwned on a 15 pointer.......

hey I have on that is kinda hard (I forget how to do it)

what is the volume of 2.2 mols of chlorine gas (2Cl) the answer is 49 just let you know and I keep getting it wrong.

hey I have on that is kinda hard (I forget how to do it)

what is the volume of 2.2 mols of chlorine gas (2Cl) the answer is 49 just let you know and I keep getting it wrong.

Hint: All gases take up 22.4 L / mol, regardless of the gas (at least assuming this is at STP)

 

Also, chlorine gas is Cl2, not 2Cl