1. Tricky Gas Law Problems

2. Ex #1
The first reservoir has a vol of 500L. It’s filled with gas at a press of 510 kPa & a temp of 20°C. The second reservoir has a vol of 250L & is initially empty.
When the valve in the tube connecting the two reservoirs is opened, the gas enters the second reservoir and the temp of the gas in the two reservoirs drops to 10°C.

3. P1V1 = PV T T (510kPa)(500L) = P2(750L) (293K) (283K) P2 = 328.40 kPa
What will be the new pressure of the gas in the two reservoirs?
P1V1 = PV
T T
Vol of BOTH reservoirs b/c valve is open
(510kPa)(500L) = P2(750L)
(293K) (283K)
P2 = kPa
P2 = 328 kPa

4. Ex #2
2.00 moles of magnesium metal reacts with HCl to produce hydrogen gas & magnesium chloride at a pressure of 200kPa & a temp of 25oC.
What is the volume of H2 gas?
Mg + 2HCl  H2 + MgCl2
1 : 2  1 : 1
2.00mol : ?  ? : ?
2.00 mol
PV = nRT
(200kPa)(V) = (2.00mol)(8.31)(298K)
V = 24.8L

5. Ex #3
The combustion of acetylene, C2H2(g), produces carbon dioxide gas & water vapour.
If 15 g of acetylene is burned, what volume of CO2(g) will be obtained at STP?
2 C2H2(g) + 5 O2(g)  4 CO2(g) + 2 H2O(g) + Energy
Moles of C2H2 = (15 g) ÷ (26.04 g/mol)
Moles of C2H2 = mol
Moles of C02 = mol

6. Recall STP: PV = nRT (101.3kPa)(V) = (1.1521mol)(8.31)(273K)
P = 101.3kPa
T = 0oC or 273K
1 mol in 22.4L
PV = nRT
(101.3kPa)(V) = (1.1521mol)(8.31)(273K)
V = L
V = 26 L

7. Ex #4
During a lab experiment, you put an unknown compound into some water.
You observe a bubbling reaction. With the help of a syringe, you remove the gas produced.
Consider the following data:
Mass of the 140 mL syringe: g
Mass of the syringe with 140 mL of the gas: g
Temperature in the lab: K
Pressure in the room: kPa

8. Which of the following is most likely the identity of the gas produced and collected?
O2 N2 CO2 H2
PV = nRT
(101.0kPa)(0.140L) = (?)(8.31)(298K)
n = mol
Mass = g – g
Mass = 0.25 g

9. Molar mass = 0.25g ÷ mol
Molar mass = g/mol
So gas is CO2

10. Ex #5
Two gas bottles with identical volumes contain different gases at the same temp and pressure.
One contains 16 g of SO2. What mass of He is contained in the other bottle?
nSO2 = 16g ÷ 64g/mol
= 0.25 mol
nHe = 0.25 moles
massHe = 0.25mol x 4 g/mol
massHe = 1g

11. Ex #6
An empty 1L container weighs 480g. When this container is filled with nitrogen gas, its total mass is 620g. When it is filled with an unknown gas at the same temp and press, its total mass is 770 g.
Which of the following is the unknown gas?
A) Acetylene, C2H2
B) Butane, C4H10
C) Ethane, C2H6
D) Methane, CH4

12. nN2 = 140g ÷ g/mol
nN2 = mol
nunknown = 5.00 moles
massunknown = 290g
Molar massunknown = 290g ÷ 5.00 mol
Molar massunknown = 58 g/mol
Unknown gas = BUTANE

13. WOW!
Gas Ques can be TRICKY!
Ahhh!