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Did I do this right?? PV=nRT, solving for moles to get molar mass?
mass of flask w/dry cap = 55.041g
water bath temp = 99.5 degrees C
mass of flask and condensed vapor = 56.039g
flask filled to top with water = 270.9g
barometric pressure = 752mmHg
water density = 0.99829 g/mL
FIND MOLAR MASS OF UNKNOWN SAMPLE.
-The answer I got seems to be to big to me but I thought I did the steps right. Please check for me, thanks in advance. It really isn't as much as it looks :-)
1. I needed mmHg in atm...
1mmHg = 0.00131578atm
752mmHG(0.00131578atm/1mmHG) = 0.9895atm
2.) I needed to find volume
m/d=v
55.041g/0.9982 g/mL = 55.140mL ---> Liters 55.140mL(0.001L/1mL) = 5.514x10^(-2)
3. Temp needs to be in K not C
99.5 + 273.15 = 372.65 degrees K
4. n=PV/RT plugging it all in to get moles...
n=(0.9895 x (5.514 x 10^-2))/(0.08206 x 372.65) = 0.00178 mol
5. MM = (grams of gas/moles of gas)
grams of gas = 56.039 - 55.041 = 0.998g
0.998g/0.00178mol = 559.36 <------ This is my molar mass answer. Doesn't it look to large because I am not able to tell what the sample is by this number?
Oh, I don't think it'll make much of a difference but I didn't mean to put a 9 on end of water density.. It's just 0.9982 g/mL
Ohhh I see, thanks guys!!! You're great!
1 Answer
- Dr WLv 77 years agoFavorite Answer
let's see..
.. volume flask = mass H2O x (1 / density H2O)
..... .... .... ... ... = (270.9g - 55.041g) x (1mL / 0.99829g) x (1L/1000mL) = 0.2162L
.. mass sample = 56.039g - 55.041g = 0.998g
.. pressure sample = 752mmHg x (1atm / 760mmHg) = 0.9895atm
.. T = 99.5°C = 372.65K
and then from these 2 equations
.. PV = nRT
.. n = mass / mw
subbing and rearranging
.. PV = (mass / mw) RT
.. mw = mass x RT/(PV)
solving
.. mw = (0.998g) x (0.08206 Latm/molK) x (372.65K) / (0.2162L x 0.9895atm)
.. mw = 143g/mol
**********
let's walk through the logic.
.. (1) you start with an empty flask and weight it
.. (2) you add water to the top and weigh it again
.. (3) the difference is mass of water
.. (4) from the density of the water, and the mass of water, you calculate
... .. .VOLUME of the water and therefore VOLUME of the flask!
.. (5) you place a sample of liquid in the flask and vaporize it
...(6) the vapor has volume of the flask, atmospheric pressure
.... .. (it's open to the atmosphere right?) and
.. ... .whatever temperature it took to vaporize it.
...(7) then we cool the flask and condense the vapor. The idea being the vapor
... .. .is more dense than air and displaces the air. So we condense it, we
.. .. ..allow air back into the flask. We need this because the original
.. .. . "empty flask" was actually a flask filled with air.
anyway..
.. mass H2O = (mass H2O + flask) - mass flask
...volume flask = volume H2O = mass H2O x (1 / density H2O)
...mass gas sample = (mass flask + condensed gas + "returned air")
... ......... ... ... ... .... .. .- (mass flask + air originally)
...and we know the rest
***********
check your calcs again
this..
2.) I needed to find volume
m/d=v
55.041g/0.9982 g/mL = 55.140mL ---> Liters 55.140mL(0.001L/1mL) = 5.514x10^(-2)
should be this
2.) I needed to find volume
m/d=v
(270.9 - 55.041)g/0.9982 g/mL = ____mL