CHEM 103 | CHEM103 Exam 3: General Chemistry
Updated and Latest Questions and Correct
Answers with Rationale - Portage Learning
1. According to Boyle’s Law, what happens to the volume of a gas when the pressure is
doubled while maintaining a constant temperature?
A. The volume doubles
B. The volume is reduced by half
C. The volume stays the same
D. The volume quadruples
Correct Answer: B
Expert Explanation: Boyle’s Law establishes an inverse relationship between the pressure
and volume of a gas at constant temperature. When the external pressure exerted on a gas
is increased, the gas molecules are forced closer together, occupying less space.
Mathematically, the product of pressure and volume remains constant (P1V1 = P2V2).
Doubling the pressure (2P) requires the volume to become one-half (1/2V) to keep the
equation balanced. This principle explains why divers must manage air pressure in their
lungs as they ascend or descend in water.
2. A weather balloon is filled with 10.0 L of helium at 25 degrees Celsius. If the temperature
drops to -10 degrees Celsius at constant pressure, what is the new volume?
A. 4.00 L
B. 11.3 L
C. 8.83 L
D. 9.50 L
Correct Answer: C
Expert Explanation: Charles’s Law states that the volume of a gas is directly proportional
to its absolute temperature in Kelvin. To solve this, temperatures must be converted from
Celsius to Kelvin by adding 273.15, resulting in 298.15 K and 263.15 K respectively. Using
the formula V1/T1 = V2/T2, we find that the decrease in temperature leads to a decrease in
volume. The calculation (10.0 L * 263.15 K) / 298.15 K yields approximately 8.83 L. This
thermal contraction is why balloons appear deflated when moved from a warm room to a
cold outdoor environment.
3. How many moles of gas are contained in a 5.0 L tank at 2.0 atm of pressure and 300 K?
A. 0.25 moles
B. 0.41 moles
,C. 0.81 moles
D. 1.2 moles
Correct Answer: B
Expert Explanation: The Ideal Gas Law, PV = nRT, allows for the calculation of moles when
pressure, volume, and temperature are known. In this problem, we substitute the known
values: P = 2.0 atm, V = 5.0 L, R = 0.0821 L·atm/mol·K, and T = 300 K. Solving for n gives
(2.0 * 5.0) / (0.0821 * 300), which equals approximately 0.406 moles. Rounding to the
appropriate significant figures results in 0.41 moles. This equation is fundamental for
predicting the behavior of gases under various physical conditions in a laboratory setting.
4. What is the molarity of a solution prepared by dissolving 58.44 g of NaCl in enough water
to make 500.0 mL of solution?
A. 1.00 M
B. 2.00 M
C. 0.50 M
D. 5.84 M
Correct Answer: B
Expert Explanation: Molarity is defined as the number of moles of solute per liter of
solution. First, convert the mass of NaCl to moles using its molar mass (58.44 g/mol), which
equals 1.00 mole. Next, convert the volume of the solution from milliliters to liters,
resulting in 0.500 L. Dividing 1.00 mole by 0.500 L gives a concentration of 2.00 M.
Understanding molarity is crucial for chemical reactions that take place in aqueous
environments.
5. A chemist needs to prepare 250 mL of a 0.10 M HCl solution from a 12.0 M stock solution.
How much stock solution is required?
A. 1.20 mL
B. 30.0 mL
C. 2.08 mL
D. 25.0 mL
Correct Answer: C
Expert Explanation: The dilution equation M1V1 = M2V2 is used to calculate the volume
of a concentrated stock solution needed to achieve a desired concentration. Plugging in the
values: (12.0 M) * V1 = (0.10 M) * (250 mL). Rearranging the equation to solve for V1 gives
(0.10 * 250) / 12.0, which equals 2.08 mL. It is essential to measure the stock solution
accurately to ensure the final concentration is correct. This procedure is standard practice
for preparing reagent solutions in general chemistry labs.
, 6. Which of the following describes the relationship in Gay-Lussac’s Law?
A. Pressure and temperature are directly proportional
B. Volume and moles are directly proportional
C. Pressure and volume are inversely proportional
D. Volume and temperature are inversely proportional
Correct Answer: A
Expert Explanation: Gay-Lussac’s Law states that the pressure of a fixed mass of gas is
directly proportional to its absolute temperature at constant volume. As temperature
increases, the kinetic energy of the gas molecules increases, causing more frequent and
forceful collisions with the container walls. This result in an increase in pressure that can
be mathematically expressed as P/T = k. If the temperature of a sealed container is
doubled, the pressure inside will also double. This law explains why pressurized cans carry
warnings against disposal in fires.
7. A mixture of nitrogen, oxygen, and argon gases has partial pressures of 0.70 atm, 0.20 atm,
and 0.05 atm respectively. What is the total pressure?
A. 1.00 atm
B. 0.75 atm
C. 0.95 atm
D. 1.10 atm
Correct Answer: C
Expert Explanation: Dalton’s Law of Partial Pressures states that the total pressure of a
gas mixture is the sum of the pressures of each individual gas. In this scenario, you simply
add the partial pressures: 0.70 atm + 0.20 atm + 0.05 atm. The resulting total pressure is
0.95 atm, assuming no chemical reaction occurs between the gases. This law is critical for
understanding atmospheric chemistry and the composition of breathing gases for scuba
diving. Each gas behaves independently within the mixture as if the other gases were not
present.
8. What volume does 1.00 mole of an ideal gas occupy at Standard Temperature and Pressure
(STP)?
A. 24.4 L
B. 1.00 L
C. 22.4 L
D. 11.2 L
Correct Answer: C
Updated and Latest Questions and Correct
Answers with Rationale - Portage Learning
1. According to Boyle’s Law, what happens to the volume of a gas when the pressure is
doubled while maintaining a constant temperature?
A. The volume doubles
B. The volume is reduced by half
C. The volume stays the same
D. The volume quadruples
Correct Answer: B
Expert Explanation: Boyle’s Law establishes an inverse relationship between the pressure
and volume of a gas at constant temperature. When the external pressure exerted on a gas
is increased, the gas molecules are forced closer together, occupying less space.
Mathematically, the product of pressure and volume remains constant (P1V1 = P2V2).
Doubling the pressure (2P) requires the volume to become one-half (1/2V) to keep the
equation balanced. This principle explains why divers must manage air pressure in their
lungs as they ascend or descend in water.
2. A weather balloon is filled with 10.0 L of helium at 25 degrees Celsius. If the temperature
drops to -10 degrees Celsius at constant pressure, what is the new volume?
A. 4.00 L
B. 11.3 L
C. 8.83 L
D. 9.50 L
Correct Answer: C
Expert Explanation: Charles’s Law states that the volume of a gas is directly proportional
to its absolute temperature in Kelvin. To solve this, temperatures must be converted from
Celsius to Kelvin by adding 273.15, resulting in 298.15 K and 263.15 K respectively. Using
the formula V1/T1 = V2/T2, we find that the decrease in temperature leads to a decrease in
volume. The calculation (10.0 L * 263.15 K) / 298.15 K yields approximately 8.83 L. This
thermal contraction is why balloons appear deflated when moved from a warm room to a
cold outdoor environment.
3. How many moles of gas are contained in a 5.0 L tank at 2.0 atm of pressure and 300 K?
A. 0.25 moles
B. 0.41 moles
,C. 0.81 moles
D. 1.2 moles
Correct Answer: B
Expert Explanation: The Ideal Gas Law, PV = nRT, allows for the calculation of moles when
pressure, volume, and temperature are known. In this problem, we substitute the known
values: P = 2.0 atm, V = 5.0 L, R = 0.0821 L·atm/mol·K, and T = 300 K. Solving for n gives
(2.0 * 5.0) / (0.0821 * 300), which equals approximately 0.406 moles. Rounding to the
appropriate significant figures results in 0.41 moles. This equation is fundamental for
predicting the behavior of gases under various physical conditions in a laboratory setting.
4. What is the molarity of a solution prepared by dissolving 58.44 g of NaCl in enough water
to make 500.0 mL of solution?
A. 1.00 M
B. 2.00 M
C. 0.50 M
D. 5.84 M
Correct Answer: B
Expert Explanation: Molarity is defined as the number of moles of solute per liter of
solution. First, convert the mass of NaCl to moles using its molar mass (58.44 g/mol), which
equals 1.00 mole. Next, convert the volume of the solution from milliliters to liters,
resulting in 0.500 L. Dividing 1.00 mole by 0.500 L gives a concentration of 2.00 M.
Understanding molarity is crucial for chemical reactions that take place in aqueous
environments.
5. A chemist needs to prepare 250 mL of a 0.10 M HCl solution from a 12.0 M stock solution.
How much stock solution is required?
A. 1.20 mL
B. 30.0 mL
C. 2.08 mL
D. 25.0 mL
Correct Answer: C
Expert Explanation: The dilution equation M1V1 = M2V2 is used to calculate the volume
of a concentrated stock solution needed to achieve a desired concentration. Plugging in the
values: (12.0 M) * V1 = (0.10 M) * (250 mL). Rearranging the equation to solve for V1 gives
(0.10 * 250) / 12.0, which equals 2.08 mL. It is essential to measure the stock solution
accurately to ensure the final concentration is correct. This procedure is standard practice
for preparing reagent solutions in general chemistry labs.
, 6. Which of the following describes the relationship in Gay-Lussac’s Law?
A. Pressure and temperature are directly proportional
B. Volume and moles are directly proportional
C. Pressure and volume are inversely proportional
D. Volume and temperature are inversely proportional
Correct Answer: A
Expert Explanation: Gay-Lussac’s Law states that the pressure of a fixed mass of gas is
directly proportional to its absolute temperature at constant volume. As temperature
increases, the kinetic energy of the gas molecules increases, causing more frequent and
forceful collisions with the container walls. This result in an increase in pressure that can
be mathematically expressed as P/T = k. If the temperature of a sealed container is
doubled, the pressure inside will also double. This law explains why pressurized cans carry
warnings against disposal in fires.
7. A mixture of nitrogen, oxygen, and argon gases has partial pressures of 0.70 atm, 0.20 atm,
and 0.05 atm respectively. What is the total pressure?
A. 1.00 atm
B. 0.75 atm
C. 0.95 atm
D. 1.10 atm
Correct Answer: C
Expert Explanation: Dalton’s Law of Partial Pressures states that the total pressure of a
gas mixture is the sum of the pressures of each individual gas. In this scenario, you simply
add the partial pressures: 0.70 atm + 0.20 atm + 0.05 atm. The resulting total pressure is
0.95 atm, assuming no chemical reaction occurs between the gases. This law is critical for
understanding atmospheric chemistry and the composition of breathing gases for scuba
diving. Each gas behaves independently within the mixture as if the other gases were not
present.
8. What volume does 1.00 mole of an ideal gas occupy at Standard Temperature and Pressure
(STP)?
A. 24.4 L
B. 1.00 L
C. 22.4 L
D. 11.2 L
Correct Answer: C
