AP #7 - Gases !?

Question

A piece of solid carbon dioxide, with a mass of 8.2 g, is placed in a 4.0 L otherwise empty container at 27°C.

(a) What is the pressure in the container after all the carbon dioxide vaporizes?
__________atm
(b) If 8.2 g solid carbon dioxide were placed in the same container but it already contained air at 740 torr, what would be the partial pressure of carbon dioxide?
__________atm
(c) What would be the total pressure in the container after the carbon dioxide vaporizes?
__________atm



Thanksss you guys !(:

Answer 1

a]
Calculate the number of moles of CO2 [8.2/44]
Use PV = nRT to find pressure
b] Pretty much the same as a], though air does include a small amount of CO2 [370 ppm]- if your teacher is picky it would be the pressure in a] above plus 370 ppm times 740 torr.
c] 740 torr plus the answer to a]

Answer 2

First compute teh number of CO2 molecules by dividing the mass of a molecule (mass of carbon atom plus 2x mass of oxygen atom) into the mass of the block of CO2.

The use pV = nkT where n = number of molecules, V is 4 L, T = 27+273 deg K and k = 1.38x10^-23 J/K

SOlve for p ---> p = nkT/V. Plug in and get p in Pascals. 100,000 Pascals = 1 atm so you can do the conversion to atms.

Part b is the same answer - assuming the air molecules do not occupy much volume.

Total pressure in part c. First convert Torr to Pascal. Now total pressure is

pV = (nair+nco2)*kT where nair = number of air molecules, nco2= number of CO2 molecules, V = 4 L and T = 27 + 273 K

But nco2*kT = pco2V where you have pco2 = partial pressure. Similarly nair*kT = pair*V so

p = pair+ pco2 where pair is 740 Torr