A gas sample has a pressure of 380 torr. What is this pressure in atmospheres (1 atm = 760 torr), to three significant figures?
0.500 atm
✓
2.00 atm
0.380 atm
5.00 atm
Correct Answer
0.500 atm
Dividing by the conversion factor gives 380 torr / 760 torr per atm = 0.500 atm. 2.00 atm comes from dividing 760 by 380, inverting the conversion. 0.380 atm wrongly treats torr as thousandths of an atmosphere. 5.00 atm is not supported by any correct use of the conversion.
Category: Gas Pressure
Question 2
What is a pressure of exactly 1 atm expressed in kilopascals?
760 kPa
101.325 kPa
✓
14.7 kPa
1.013 kPa
Correct Answer
101.325 kPa
One standard atmosphere is defined as 101,325 Pa, which is 101.325 kPa. 760 kPa confuses the number of torr in 1 atm with kilopascals. 14.7 kPa uses the value of 1 atm in pounds per square inch. 1.013 kPa is off by a factor of 100; 1 atm is about 1.013 bar, not 1.013 kPa.
Category: Gas Pressure
Question 3
An open-end mercury manometer is attached to a gas container. Atmospheric pressure is 760 torr, and the mercury level in the arm open to the air is 40 mm higher than in the arm connected to the gas. What is the gas pressure?
720 torr
760 torr
800 torr
✓
40 torr
Correct Answer
800 torr
The gas pushes the mercury toward the open arm, so the gas pressure exceeds atmospheric pressure by the 40 mm height difference: 760 + 40 = 800 torr (1 mm Hg is about 1 torr). 720 torr subtracts the difference, which applies only when the mercury is higher on the gas side. 760 torr ignores the height difference. 40 torr is the difference alone, which would be the reading of a closed-end manometer, not an open-end one.
Category: Gas Pressure
Question 4
A 2.00 L sample of gas at 1.00 atm is compressed to 0.500 L at constant temperature and amount. What is the new pressure?
0.250 atm
1.00 atm
2.50 atm
4.00 atm
✓
Correct Answer
4.00 atm
Boyle's law gives P2 = P1V1/V2 = (1.00 atm)(2.00 L)/(0.500 L) = 4.00 atm; cutting the volume to one fourth raises the pressure fourfold. 0.250 atm treats pressure as directly proportional to volume. 1.00 atm assumes pressure does not change on compression. 2.50 atm adds the volumes instead of using a ratio.
Category: Simple Gas Laws
Question 5
A gas occupies 3.00 L at 300 K. At constant pressure and amount it is heated to 450 K. What is its new volume?
4.50 L
✓
2.00 L
3.00 L
1.50 L
Correct Answer
4.50 L
Charles's law gives V2 = V1(T2/T1) = 3.00 L x (450 K / 300 K) = 4.50 L, because volume is directly proportional to Kelvin temperature. 2.00 L inverts the temperature ratio. 3.00 L assumes volume does not depend on temperature. 1.50 L is half the original volume and has no basis in the law.
Category: Simple Gas Laws
Question 6
At constant volume and amount of gas, how does the pressure of a gas depend on its temperature?
It is inversely proportional to the Kelvin temperature
It is directly proportional to the Kelvin temperature
✓
It is directly proportional to the Celsius temperature
It does not change with temperature
Correct Answer
It is directly proportional to the Kelvin temperature
In a rigid container, pressure is directly proportional to absolute temperature (Amontons's law, also called Gay-Lussac's law), so doubling the Kelvin temperature doubles the pressure. An inverse proportion to the Kelvin temperature describes no simple gas law for pressure. A direct proportion to the Celsius temperature fails because 0 degrees Celsius is not zero pressure. Saying pressure does not change contradicts the observation that heated rigid containers build pressure.
Category: Simple Gas Laws
Question 7
A balloon holds 0.50 mol of gas and has a volume of 5.0 L. If 0.50 mol more gas is added at the same temperature and pressure, what is the new volume?
2.5 L
5.0 L
10.0 L
✓
7.5 L
Correct Answer
10.0 L
Avogadro's law says volume is directly proportional to moles at constant temperature and pressure, so doubling the moles from 0.50 to 1.00 mol doubles the volume to 10.0 L. 2.5 L treats volume as inversely proportional to moles. 5.0 L assumes the volume does not change. 7.5 L adds only half of the original volume.
Category: Simple Gas Laws
Question 8
How many moles of an ideal gas occupy 24.62 L at 1.00 atm and 300.0 K? Use R = 0.08206 L atm/(mol K) and give three significant figures.
0.100 mol
10.0 mol
0.0821 mol
1.00 mol
✓
Correct Answer
1.00 mol
Rearranging PV = nRT gives n = PV/RT = (1.00 atm)(24.62 L) / (0.08206 L atm/(mol K) x 300.0 K) = 24.62 / 24.618 = 1.00 mol to three significant figures. 0.100 mol and 10.0 mol are off by a factor of ten, a typical slip in the arithmetic. 0.0821 mol is simply the value of R, not the result of the calculation.
Category: Ideal Gas Law
Question 9
At 0 degrees Celsius and 1 atm, what volume does one mole of an ideal gas occupy?
22.4 L
✓
24.5 L
1.00 L
8.31 L
Correct Answer
22.4 L
At 273.15 K and 1 atm (STP), V = nRT/P = (1.00)(0.08206)(273.15)/1 = 22.4 L, the standard molar volume. 24.5 L is the molar volume near room temperature, about 25 degrees Celsius, not 0 degrees Celsius. 1.00 L confuses moles with liters. 8.31 L uses the numerical value of R in joules, which does not give a volume.
Category: Ideal Gas Law
Question 10
At the same temperature and pressure, which gas has the greatest density?
N2
CO2
✓
CH4
He
Correct Answer
CO2
Gas density equals PM/RT, so at equal temperature and pressure density rises with molar mass, and CO2 (44 g/mol) is the heaviest of the four. N2 (28 g/mol) is lighter than CO2. CH4 (16 g/mol) is lighter still. He (4 g/mol) is the least dense of the group.
Category: Ideal Gas Law
Question 11
A gas has a density of 1.25 g/L at 273 K and 1.00 atm. What is its molar mass to three significant figures? Use R = 0.08206 L atm/(mol K).
22.4 g/mol
17.9 g/mol
28.0 g/mol
✓
44.0 g/mol
Correct Answer
28.0 g/mol
Molar mass equals dRT/P = (1.25 g/L)(0.08206)(273 K)/(1.00 atm) = 28.0 g/mol, consistent with N2. 22.4 g/mol is the molar volume in liters mistaken for a mass. 17.9 g/mol comes from dividing 22.4 by the density instead of multiplying. 44.0 g/mol is the molar mass of CO2 and does not match the data.
Category: Ideal Gas Law
Question 12
A container holds 0.20 mol of He and 0.80 mol of Ar at a total pressure of 5.0 atm. What is the partial pressure of He?
4.0 atm
0.20 atm
5.0 atm
1.0 atm
✓
Correct Answer
1.0 atm
The mole fraction of He is 0.20 / 1.00 = 0.20, and its partial pressure is 0.20 x 5.0 atm = 1.0 atm. 4.0 atm is the partial pressure of Ar. 0.20 atm is the mole fraction itself, not a pressure. 5.0 atm is the total pressure of the whole mixture.
Category: Mixtures and Stoichiometry
Question 13
Hydrogen gas is collected over water at a total pressure of 755 torr. The vapor pressure of water at that temperature is 24 torr. What is the pressure of the dry hydrogen?
731 torr
✓
779 torr
755 torr
24 torr
Correct Answer
731 torr
Gas collected over water is mixed with water vapor, so by Dalton's law the hydrogen's pressure is 755 - 24 = 731 torr. 779 torr adds the water vapor pressure instead of subtracting it. 755 torr ignores the water vapor in the sample. 24 torr is the pressure of the water vapor alone.
Category: Mixtures and Stoichiometry
Question 14
For the reaction N2 + 3 H2 -> 2 NH3, what volume of NH3 forms from 6.0 L of H2 with excess N2, with all gases at the same temperature and pressure?
2.0 L
4.0 L
✓
6.0 L
9.0 L
Correct Answer
4.0 L
At the same temperature and pressure, gas volumes react in the same ratio as moles, so 6.0 L H2 x (2 NH3 / 3 H2) = 4.0 L NH3. 2.0 L uses the 1-to-3 ratio for N2 to H2 instead of the NH3 ratio. 6.0 L assumes a 1-to-1 ratio. 9.0 L inverts the 2-to-3 ratio.
Category: Mixtures and Stoichiometry
Question 15
According to Graham's law, how does the rate at which H2 (2.0 g/mol) escapes through a pinhole compare with that of O2 (32.0 g/mol) at the same temperature?
16 times as fast
4 times as slow
4 times as fast
✓
2 times as fast
Correct Answer
4 times as fast
Rates are inversely proportional to the square root of molar mass, so the ratio is the square root of 32.0/2.0, which is the square root of 16, or 4; the lighter H2 is 4 times as fast. 16 times as fast forgets to take the square root. 4 times as slow reverses the direction, since lighter gases move faster. 2 times as fast takes the square root twice.
Category: Effusion and Diffusion
Question 16
Why does a rubber balloon filled with helium shrink faster than an identical balloon filled with air at the same temperature?
Helium atoms are larger than the molecules in air
Helium reacts slowly with the rubber of the balloon
Helium atoms have more kinetic energy on average
Helium atoms are lighter and move faster on average
✓
Correct Answer
Helium atoms are lighter and move faster on average
At the same temperature, lighter particles have higher average speeds, so helium (4 g/mol) passes through tiny pores in the rubber faster than N2 and O2. The idea that helium atoms are larger is wrong; they are smaller, and larger particles would not escape faster. Helium is a noble gas and does not react with rubber. The average kinetic energy depends only on temperature, so helium atoms do not have more of it than air molecules at the same temperature.
Category: Effusion and Diffusion
Question 17
According to kinetic-molecular theory, the average kinetic energy of the molecules in a gas sample depends only on which property?
Absolute temperature
✓
Molar mass
Pressure
Volume
Correct Answer
Absolute temperature
Kinetic-molecular theory states that average kinetic energy is directly proportional to the Kelvin temperature, regardless of the identity of the gas. Molar mass affects how fast the molecules move, not their average kinetic energy at a given temperature. Pressure can change at constant temperature (by compression) without changing the average kinetic energy. Volume can also change at constant temperature without changing it.
Category: Kinetic-Molecular Theory
Question 18
At the same temperature, which gas has the highest root mean square speed?
Ne
He
✓
Ar
Kr
Correct Answer
He
All four gases have the same average kinetic energy at the same temperature, so the one with the smallest molar mass moves fastest, and He (4 g/mol) is the lightest. Ne (20 g/mol) is about five times heavier than He. Ar (40 g/mol) is heavier still. Kr (84 g/mol) is the heaviest and slowest of the four.
Category: Kinetic-Molecular Theory
Question 19
In the van der Waals equation, what does the constant a correct for?
The volume occupied by the molecules
The mass of each gas molecule
Attractions between gas molecules
✓
The speed of the gas molecules
Correct Answer
Attractions between gas molecules
The a term raises the measured pressure back toward the ideal value because attractions between molecules reduce how hard they strike the walls. The volume occupied by the molecules is handled by the b constant. The mass of each gas molecule is not corrected for by either constant. The speed of the molecules depends on temperature and molar mass and is not a van der Waals correction.
Category: Real Gases
Question 20
Under which conditions does a real gas deviate most from ideal gas behavior?
Low pressure and high temperature
Low pressure and low temperature
High pressure and high temperature
High pressure and low temperature
✓
Correct Answer
High pressure and low temperature
At high pressure the molecules' own volume is a large share of the container, and at low temperature intermolecular attractions matter more, so both effects push a real gas away from ideal behavior. Low pressure and high temperature are the conditions where gases behave most ideally. Low pressure and low temperature include only one of the two non-ideal factors. High pressure and high temperature also include only one, since fast molecules overcome attractions.
Category: Real Gases