[dvaesedum]

Hello guys! It sucks that my first post is a request for help, but that's how I stumbled upon this website.

I have these 2 questions in my homework due tomorrow that I can't understand at all, any tips would be greatly appreciated!

1.   The solubility of N2 in blood at 37 oC and at a partial pressure of 0.80 atm is 5.6 X 10-4 mol/L. A deep-sea diver breathes compressed air with the partial pressure of N2 equal to 4.0 atm. Assume that the total volume of the blood in the body is 5.0 L. Calculate the amount of N2 gas released (in liters at 37oC and 1 atm) when the diver returns to the surface of the water, where the partial pressure of N2 is 0.80 atm.

and

2. How many grams of sucrose (C12H22O11) must be added to 552 g of water to give a solution with a vapor pressure 2.0 mmHg less than that of pure water at 20oC? (The vapor pressure of water at 20oC is 17.5 mmHg.)   

Thank you in advance!

[dvaesedum]

Fair enough:

1) I multiplied the solubility by 4 atm and 5 liters to find how much N2 is in the blood

((5.6x10^-4 mol/L)/.80atm)x4.0atmx5.0L=.014mol

the atm and L should cancel out if I am not wrong

then, using the moles in the blood I found how many litres will be released at 37degC and 1atm by using PV=nRT

V=nRT/V=[.014mol x (.0821 L*atm/mol*k) x 310K)]/1atm=.36L N2

I think I was doing it right but this is the part of the problem that confuses me a lot:

Calculate the amount of N2 gas released (in liters at 37oC and 1 atm) when the diver returns to the surface of the water, where the partial pressure of N2 is 0.80 atm.   

I hope I did the first part right, then the second part should be easy to get with pv=nrt but at which pressure? I looked everywhere in the book, nothing like this can be found.


2) I think I got this one right

552g H2O= 30.6 mol H2o (I did the work, I promise)

17.5mmHg-2.0mmHg=15.5mmHg

Using Raoult's law Psolv=Xsolv x Psolv

15.5mmHg=Xsolv x 17.5mmHg

Xsolv= .886 = mole fraction of solvent (water)

.886= 30.6 mol water/(30.6 + mol sucrose)

30.6+ x= 30.6/.886=3.94 mol sucrose which then I will turn into grams

[dvaesedum]

Any ideas on the first one? I really need a pointer in the right direction. Should I use S=k x Pgas?

[Borek]

Seems to me like you have correctly calculated amount of nitrogen dissolved at depth. But it is not identical with amount of nitrogen that would be released - you forgot that some of the nitrogen will be still dissolved!

To be sure you did first part OK you can make it in steps - first, calculate Henry's constant for nitrogen & blood using data given for surface, then use the constant to calculate concentration at depth. You did it in one step, which is not wrong, just it is easier to make a mistake.

Didn't check the second one too precisely, just skimmed, and the approach looks correctly to me.

[dvaesedum]

Seems to me like you have correctly calculated amount of nitrogen dissolved at depth. But it is not identical with amount of nitrogen that would be released - you forgot that some of the nitrogen will be still dissolved!

To be sure you did first part OK you can make it in steps - first, calculate Henry's constant for nitrogen & blood using data given for surface, then use the constant to calculate concentration at depth. You did it in one step, which is not wrong, just it is easier to make a mistake.

Didn't check the second one too precisely, just skimmed, and the approach looks correctly to me.

Ok, thank you!

Now, when I plug everything in, s=kp I get

5.6x10^-4 mol/L=k x .80 atm for the blood and k=.0007 (mol/L)/atm

what do I do next for the underwater pressure, and the surface pressure?? That's what's bugging me now...

[dvaesedum]

Borek, I called up a friend, and she told me that she calculated k which ends up being in mol/l/atm

you multiply that by 4 atm to get rid of the atm (i forgot the reason), and then you multiply that by 5l to find the concentration of n2 in the blood underwater which is .011 mol in sf because of the 5.0 (has 2 sf)

pivner, and then it gives you .28l because of the .011 mol's 2 sf.

Thank you, I hope I get an A on this assignment!

I am looking forward to contributing to this forum in the future.