Name:_________Answer Key___________________________________________________
AP Chemistry Review Gases
__D__1. A sealed container contains 0.20 moles of oxygen gas and 0.10 moles of hydrogen gas. If the
temperature is 25°C throughout the container, which of the following is true?
(A)
(B)
(C)
(D)
(E)
The total masses of the two gases are the same.
The average molecular speeds of the two gases are the same.
The partial pressures of the two gases are the same.
The average kinetic energy of the two gases are the same.
The molecular masses of the two gases are the same.
__C__2. A gaseous mixture at a constant temperature contains O2, CO2, and He. Which of the following
lists the three gases in order of increasing molecular speeds?
(A) He, CO2, O2
(B) He, O2, CO2
(C) CO2, O2, He
(D) O2, CO2, He
(E) O2, He, CO2
__E__3. Nitrogen gas was collected over water at 25°C. IF the vapor pressure of water at 25°C is 23
mmHg, and the total pressure in the container is measured at 781 mmHg, what is the partial
pressure of the nitrogen gas?
(A) 23 mmHg
(B) 46 mmHg
(C) 551 mmHg
(D) 735 mmHg
(E) 758 mmHg
__E__4. A balloon occupies a volume of 1.0 liter when it contains 0.16 grams of helium at 37°C and 1
atm pressure. If helium is added ot the balloon until it contains 0.80 grams while pressure and
temperature are kept constant, what will be the new volume of the balloon?
(A) 0.50 liters
(B) 1.0 liters
(C) 2.0 liters
(D) 4.0 liters
(E) 5.0 liters
__E__5. Which of the following assumptions is (are) valid based on kinetic molecular theory?
I.
II.
III.
(A)
(B)
(C)
(D)
(E)
Gas molecules have negligible volume.
Gas molecules exert no attractive forces on one another.
The temperature of a gas is directly proportional to its kinetic energy.
I only
III only
I and III only
II and III only
I, II, and III
___A__6. A 22.0 gram sample of an unknown gas occupies 11.2 liters at standard temperature and
pressure. Which of the following could be the identity of the gas?
(A)
(B)
(C)
(D)
(E)
CO2
SO3
N2
O2
He
7. A rigid 5.00 L cylinder contains 24.5 g of N2(g) and 28.0 g of O2(g).
(a) Calculate the total pressure, in atm, of the gas mixture in the cylinder at 298 K.
Convert to moles
24.5g N2 •
1𝑚𝑜𝑙 𝑁2
28 𝑔 𝑁2
= 0.875 mol N2
28.0g O2 •
1𝑚𝑜𝑙 𝑂2
32 𝑔 𝑂2
= 0.875 mol O2
Find pressure of each gas PV = nRT
PN2 (5.00L) = (0.875mol)(0.08206 (L •atm) /(mol • K))(298K)
PO2 (5.00L) = (0.875mol)(0.08206 (L •atm) /(mol • K))(298K)
PO2 = PN2 = 4.28 atm
Ptotal = PO2 + PN2 = 4.28 atm + 4.28 atm = 8.56 atm
(b) The temperature of the gas mixture in the cylinder is decreased to 280 K. Calculate each of the
following.
(i) The mole fraction of N2(g) in the cylinder
XN2 =
𝑚𝑜𝑙𝑠 𝑁2
𝑡𝑜𝑡𝑎𝑙 𝑚𝑜𝑙𝑠
= 0.875 mol / 1.75 mol = 0.5
(ii) The partial pressure, in atm, of N2(g) in the cylinder
PN2 V=nRT
PN2 (5.00L) = (0.875 mol)(0.08206 (L •atm) /(mol • K))(280K)
PN2 = 4.02 atm
Lower temperature means a lower pressure.
(c)
If the cylinder develops a pinhole-sized leak and some of the gaseous mixture escapes, would the ratio
moles of N 2 ( g )
in the cylinder increase, decrease, or remain the same? Justify your answer.
moles of O 2 ( g )
The ratio will decrease because N2 will effuse at a slightly faster rate due to its smaller
molar mass.
A different rigid 5.00 L cylinder contains 0.176 mol of NO(g) at298K. A 0.176 mol sample of
O2(g) is added to the cylinder, where a reaction occurs to produce NO2(g).
(d)
Write the balanced equation for the reaction.
2NO (g) + O2 (g) → 2NO2 (g)
(e)
Calculate the total pressure, in atm, in the cylinder at 298 K after the reaction is complete.
Limiting reactant problem!
0.176 mol NO •
0.176 mol O2 •
2 𝑚𝑜𝑙 𝑁𝑂2
2 𝑚𝑜𝑙 𝑁𝑂
2 𝑚𝑜𝑙 𝑁𝑂2
1 𝑚𝑜𝑙 𝑂2
= 0.176 mol NO2
= 0.352 mol NO2
NO is liming there is none left after the reaction is complete.
0.176 mol NO2 are formed.
1 𝑚𝑜𝑙 𝑂
2
0.176 mol NO • 2 𝑚𝑜𝑙 𝑁𝑂
= 0.088 mol O2 used
0.176 mol O2 original – 0.088 mol O2 used = 0.088 mol O2 left over
Find pressure of each gas
PNO2 (5.00L) = (0.176mol)(0.08206 (L •atm) /(mol • K))(298K)
PO2 (5.00L) = (0.088mol)(0.08206 (L •atm) /(mol • K))(298K)
PNO2 = 0.861 atm
PO2 = 0.430 atm
Ptotal = PO2 + PNO2 = 0.861 atm + 0.430 atm = 1.29 atm