What is the mass of a sample of O2 gas?

Question

Which has a pressure of 740. mmHg, at a temperature of 25 degree Celsius, in a volume of 250. mL? 

I tried figuring this out myself by putting (740)(o.25)/ (0.0821)(298) but I am not getting the correct answer which I am pretty sure is 0.318. I asked this question before but there is one tutor out there who keeps giving out the wrong information to work out the problems.

              

Answer 1

You were correct in using the formula PV=nRT, but there are a couple of mistakes from the equation you wrote after plugging in the values. You put the value of R as 0.0821, whose units are L*atm*K^-1*mol^-1. You were correct in converting 25^oC to Kelvin (except I would be more specific and use 298.15 instead of 298) and 250 mL to 0.25L. However, you forgot to convert the pressure from mmHg to atm, which is 740/760. By plugging in these values, we are left with the variable "n", which represents moles. For this problem, n=moles of O2. Thus, the equation is n=(0.974)(0.25)/(0.0821)(298.15)=0.00995 moles of O2. Remember that moles is not the same as mass, and the problem is asking for mass of O2. You can convert moles to mass using the molar mass of O2, which is 32. Thus, the mass of O2 gas=0.00995*32=0.318.

 

Your final answer is 0.318 g.

 

If you have any further questions, feel free to respond! I'd be happy to help.

 

Jackie

Answer 2

Are you asking for number of moles. Your question as written asks for pressure, but the pressure is already given. Assuming you want moles, tgen use

PV = nRT

P = 740 mm x 1atm/760 mm = 0.974 atm

V = 250 ml = 0.250 L

n = ?

R = 0.0821 Latm/Kmol

T = 25C +273 = 298K

n = PV/RT = (0.974)(0.25)/(.0821)(298) = .00995 moles. 

You can’t use mm Hg pressure with R = 0.0821 because that has units of atmospheres in it. So convert mm to atm and use 0.0821 or leave the P in mm Hg and use R = 62.63 mm Hg-L/K-mol 

Answer 3

Here is the same answer as Jackie and J.R.S., but working in SI units:

 

n = PV/RT

 

R = 8.314 J/(mol K)

T = (25 + 273)K = 298 K

V = 0.250 L = 0.000250 cubic meters

P = 740(133.3) Pa ≅ 98640 Pa

 

n = (98640)(0.000250)/[8.314(298)] moles = 0.009953 moles

 

molecular weight of O2 ≅ 32.00

 

m = 0.009953(32.00) g ≅ 0.318 g