Chem 400
Nuss
Lecture Problems (Set #5)
Gases
1. A sample of HCl at P = 67.5 mm Hg with a volume of 256 mL is transferred to a new
flask.; P = 23.6 mm Hg. What is the volume of the new flask? (Ans: 732 mL)
2. A sample of propane is in a 3.50 L container at 25 C. It’s pressure is 735 mm Hg. If
the gas is transferred to a 15.0 L container, also at 25 C, what is the pressure in the
larger container? (Ans: 172 mm Hg)
3. A 25.0 mL sample of a gas is enclosed in a gas tight syringe at 22 C. If the syringe is
immersed in an ice bath at 0C, what is the new gas volume, assuming that the
pressure is constant? (Ans: 22.9 mL)
4. Gaseous silane ignites spontaneously in air according to the equation:
SiH4(g) + 2 O2(g)

SiO2(s) + 2 H2O(g)
If 5.2 L of SiH4(g) are treated with O2(g), how many L of O2(g) are required for
complete reaction? How many L of H2O(g) are produced? (Ans: 10.4L O2 required;
10.4 L H2) produced)
5. A sample of H2 gas has a volume of 5.86 L at 0 C exerts a pressure of 1140 torr.
How many moles of H2 are in this sample? (Ans: 0.392 moles)
6. A sample of CO2 gas in flask A with a volume of 265 mL has a pressure of 136.5 mm
Hg at 22.5 C. the sample is moved to another flask (B), with a pressure of 94.3 mm
Hg at 24.5 C. What is the volume of flask B? (Ans: 386 mL)
7. A bicycle tire is inflated to a pressure of 55 lb/in2 at 15 C. If the tire is heated to 35
C, what is the pressure in the tire? Assume that the tire volume is unchanged. 1 atm
= 14.7 lb/in2. (Ans = 59 lb/in2)
8. 2 NaN3(s)  2 Na(s)
+ 3 N2(g)
Calculate the volume of N2(g) produced at 21 C and 823 mm Hg when 60.0 g NaN3(s)
decomposes. (Ans: V=30.7L)
9.
Si(s) + 2 CH3Cl(g)  (CH3)2SiCl2(g)
If you placed 1.95 g of solid Si in a 6.56 L flask that contains CH3Cl(g) with a pressure
of 485 mm Hg at 25 C, how many grams of (CH3)2SiCl2(g) can be formed? What
pressure of (CH3)2SiCl2(g) would you expect in this flask at 95 C on completion of
this reaction? (Ans: 0.0694 mol (CH3)2SiCl2(g); 0.319 atm)
10. A 0.982g sample of an unknown gas exerts a pressure of 700 mm Hg in a 450. ML
container at 23 C. What is it’s molar mass? (Ans: 57.5 g/mol)
11. A gas has a density of 2.39 g/L at 23.0 C and 715 mm Hg. What is its molar mass?
(Ans: 61.7 g/mol)
12. Diethyl ether, (C2H5)2O vaporizes easily at room temperature. If the vapor has a
pressure of 233 mm Hg in a flask at 25. C, what is the density of the vapor? (Ans:
0.927 g/L)
13. 46 L of O2 at 25 C and 1.0 atm is pumped into a scuba tank along with 12 L of He at
25 C and 1 atm pressure. The volume of the tank is 5.0 L. What are the partial
pressures and mole fractions of oxygen and helium, and what is the total pressure in
the scuba tank?
14. The vapor pressure of water at 22 C is 19.8 mm Hg. If 456 ml of hydrogen gas were
collected over water and the atmospheric pressure that day was 742 mm Hg, how
many ml of dry hydrogen gas at STP were collected? (Ans: 401 mL)
15. Calculate the average speed of an oxygen molecule at 25 C. (Ans: (482 m/s)
16. What happens to the average speed of a helium atom if the temperature is raised from
25 C to 50 C? (Ans: Increases by 4%)
17. At 25 C, how much faster does the average He atom move than the average oxygen
molecule? (Ans: 2.8 times faster).
18. Tetrafluorethylene, C2F4, effuses through a barrier at a rate of 4.6 x 10-6 mol/hr. An
unknown gas, consisting of only boron and hydrogen, effuses at the rate of 5.8 x 10-6
mol/hr under the same conditions. What is the molar mass of the unknown gas?
(Ans: 63g/mol)