If a chemistry student collected 825 mL of gas at 25 degrees C and 7.90*10^2torr, how many moles did she collect?
PV=nRT
Put P in atmospheres. torr/760 = atm.
V is volume (in LITERS.)
solve for n, the number of mols.
R is a constant of 0.08206 L*atm/mol*K
T is temperature in Kelvin. K = 273 + C.
Post your work if you gt stuck.
So I have this so far:
PV=n(24.45388)
but i don't what to do from here.
No, that isn't what you have. You didn't follow the instructions in my response. Don't make this harder than it is. It is a simple matter of plugging in the numbers.
PV=nRT
P is pressure in atmospheres. torr/760 = atm. So P=7.90x10^2 torr.
P in atm = 7.90x10^2/760 = ?? atm. Plug that in for P.
V = volume in liters. 825 mL = 0.825 L. Plug that in for V.
solve for n.
R is the gas constant of 0.08206. Plug that in.
T is temperature in Kelvin. Kelvin = 273 + 25 = ?? K. Plug that in for T.
NOW, solve for n.
If there is something you don't understand about this please reply with an exact description of what you don't understand or why you are unsure of the next step.
So when I plug in the numbers for PV=nRT I get
1.039*(0.825)=n(24.45388)then multiply P*v and get 0.857175=n(24.45388) Do i have this right now because I am not sure how to find n now?
Yes, you have it right so far. A couple of notes, though. You don't need to carry that many places. 24.45 is ok if you want to carry 4 and you do in 1.039. (Technically, 0.825 is three places and that's all we can have in the others BUT I often carry one more place, then round at the end). On that same note, then, 0.8572 would be the number I would use for 1.039*0.825.
Here is what you do to finish. When we have 0.8572=n(24.45), and we wish to solve for n, just say to yourself, what must I do to get my numbers on one side and the unknown on the other. Well, I see we could simply divide the right side by 24.45. Remember to do the same thing to both sides. So we will divide both sides by 24.45 and it will look like this.
0.8572/24.45 = n(24.45)/24.45
Then 0.8572/24.45 = ??
I hope this helps. Post again if you don't undertand.
So I get that n=0.035059305 but I have to write my answer in significant figures so would the new answer be n=.3505? or did I do that wrong?Thanks
If I do that on my calculator all at once instead of writing down each multiplication and each division, I arrive at an answer of 0.035069. You may have as many significant digits in the answer as the least significant number in the calculations. That means you may have two (from the 25o C) so the answer should be rounded to 0.035 mols.
I would not show each separate calculation. I would make it look like this.
PV=nRT
(790/760)(0.825)= n(0.08206)(298)
n=(790/760)*0.825/0.08207*298
n=0.035
Check my thinking. Check my work carefully.