Honors Chemistry I
Gas Laws Problem Set #3
Solution Set
1. If an unknown gas diffuses at a rate 4.25 times slower than that of neon, what is the molecular weight of
this gas?
Rate X / Rate Ne =
MWNeon / MWUnknown
1 / 4.25 =
20.2 g/mol / MWUnknown
M.Wt of X = 365 g/mol
o
2. Four grams of a gas measured at 35 C is contained in a 2.00-liter container and found to have a pressure
of 775 mm Hg. What is the molecular weight of this gas?
MW = gRT / PV
MW = [(4.00 g)(0.0821 liter-atm./mol-K)(308)] / [(775 mm / 760 mm)(2.00 liters)]
= 49.6 g/mol
3. A mixture of three gases - neon, oxygen, and chlorine - is found to exert a pressure of 200. kPa. If there
are 40.0 grams of each of the gases, what is the partial pressure of the most abundant gas (by volume)?
PA = PTotal XA
40.0 g of Ne = 1.98 moles; (most abundant according to Avogadro’s Law)
40.0 g of O2 = 1.25 moles;
40.0 g of Cl2 = 0.563 moles
PNe = (200. kPa)[1.98 mol / (1.98 mol + 1.25 mol + 0.563 mol)] = 104 kPa
20
4. If there are 3.44 x 10 molecules of xenon contained in a 500.0-ml container, and the pressure is found
to be 13.0 psi, what is the absolute temperature of the gas? PV = nRT
20
23
T = PV / nR = [(13.0 psi / 14.7 psi)(0.500 ml)] / [(3.44 x 10 /6.02 x 10 )(0.0821)
T = 9425 K = 9430 K
o
5. When 400.0 ml of a gas is collected over water at a temperature of 30 C, the pressure is found to be 1.20
atm. What would be the volume of this gas (dry) if the room temperature were doubled and the pressure
was increased by 1.00 atm.? (You will need to check a water vapor table.)
V1P1 / T1 = V2P2 / T2
Vapor Pressure @ 30oC = 31.82 mm
V2 = [(400.0 ml)(912 mm - 31.8 mm)] / 303 K] / [(333 K) / (1672 mm)]
= 231 ml
-26. If CO effuses through a pinhole at a rate of 400. ml/sec, at what rate will ammonia effuse?
(Assume the same temperature for both gases.)
Rate CO / Rate NH3 =
400 ml/sec / Rate NH3 =
MWNH3 / MWCO
17.0 g/mol / 28.0 g/mol
Rate of NH3 = 513 ml/sec
7. What is the density of chlorine gas at STP?
71.0 g/mol x 1 mol / 22.4 liters = 3.17 g / liter
8. How many grams of O2 will be available in a 10.00-liter reaction vessel if the pressure of the gas is 900.
o
mm Hg and the temperature is 50 C?
MW = gRT / PV
g = [(32.0 g/mol)(900. mm / 760 mm)(10.00 liters)] / [(0.0821 liter-atm./mol-K)(323)]
= 14.3 grams
9. What is the true, or molecular, formula of an organic compound given the following information:
The rate of effusion of this unknown gas is 241 ml per minute. Under the same conditions, oxygen gas
effuses at a rate of 635 ml per minute. The compound contains 48.65% carbon, 8.11% hydrogen, and the
rest is oxygen.
Rate X / Rate O2 = (241 ml/min) / (635 ml/min) =
(32.0 g/mol)/ MWUnknown
M.Wt of X = 222 g/mol
C 48.65 g / 12.0 g/mol
= 4.05 mol
C:H:O
=
H
8.11 g / 1.0 g/mol
= 8.11 mol
O
43.24 g / 16.0 g/mol
= 2.70 mol
4:05/2.70 : 8.11/2.70 : 2.70/2.70 = 1.50 : 3.00 : 1.00 = 3: 6: 2
C3H6O2
10. What is the partial pressure of neon gas in a 3-gas system if there are 20.0 grams of argon, chlorine, and
o
neon in the sample. The volume of the container is 500.0 ml and the temperature is 35.5 C.
P = nRT / V = [(20.0 g / 20.2 g / mol)(0.0821 liter-atm./mol-K)(308.5 K)] / (0.5000 liters)
P = 50.2 atm.