General Chemistry Exam 3 Review: Step-by-Step Study Guidance

Q1. If 10 g of sodium bicarbonate (NaHCO3) is combined with an excess of hydrochloric acid (HCl), how many L of carbon dioxide (CO2) gas are produced, if the pressure is 1.0 atm and the temperature is 20 °C?

Background

Topic: Stoichiometry and Gas Laws

This question tests your ability to use stoichiometry to determine the amount of gas produced in a chemical reaction, and then apply the ideal gas law to find the volume of gas at given conditions.

Key Terms and Formulas

• Stoichiometry: Using balanced chemical equations to relate amounts of reactants and products.

• Ideal Gas Law: $PV = nRT$

• Molar mass: The mass of one mole of a substance (g/mol).

• Standard temperature conversion: $T(\text{K}) = T(\degree C) + 273.15$

• R (gas constant): $0.0821\ \text{L} \cdot \text{atm} / (\text{mol} \cdot \text{K})$

Step-by-Step Guidance

1. Write the balanced chemical equation for the reaction between NaHCO3 and HCl. Identify the mole ratio between NaHCO3 and CO2.

2. Calculate the number of moles of NaHCO3 used by dividing the given mass by its molar mass.

3. Use the mole ratio from the balanced equation to determine the moles of CO2 produced.

4. Convert the temperature from Celsius to Kelvin using $T(\text{K}) = T(\degree C) + 273.15$.

5. Set up the ideal gas law equation $PV = nRT$ and rearrange to solve for $V$ (volume of CO2).

Try solving on your own before revealing the answer!

Q2. What are the three principles of kinetic molecular theory?

Background

Topic: Kinetic Molecular Theory of Gases

This question tests your understanding of the basic postulates that describe the behavior of ideal gases at the molecular level.

Key Terms

• Kinetic Molecular Theory (KMT): A model that explains the physical properties of gases.

• Postulate: A fundamental assumption or principle.

Step-by-Step Guidance

1. Recall that KMT describes gas particles as being in constant, random motion.

2. Think about the assumptions regarding the size of gas particles relative to the space between them.

3. Consider how KMT explains the nature of collisions between gas particles and with the walls of the container.

Try summarizing the three main principles before checking the answer!

Q3. What is pressure?

Background

Topic: Properties of Gases

This question is about understanding the physical meaning of pressure in the context of gases.

Key Terms

• Pressure ($P$): The force exerted per unit area.

• SI unit: Pascal (Pa), but atmospheres (atm) and mmHg are also common.

Step-by-Step Guidance

1. Recall that pressure is related to the collisions of gas molecules with the walls of their container.

2. Think about how pressure can be mathematically defined as $P = \frac{F}{A}$, where $F$ is force and $A$ is area.

3. Consider how pressure is measured in different units and what those units represent.

Try defining pressure in your own words before checking the answer!

Q4. Rationalize Boyle’s Law, Charles’s Law, Avogadro’s Law, and Dalton’s Law according to kinetic molecular theory.

Background

Topic: Gas Laws and Kinetic Molecular Theory

This question asks you to connect the macroscopic gas laws to the microscopic behavior of gas particles as described by KMT.

Key Terms

• Boyle’s Law: $P \propto \frac{1}{V}$ at constant $T$ and $n$.

• Charles’s Law: $V \propto T$ at constant $P$ and $n$.

• Avogadro’s Law: $V \propto n$ at constant $P$ and $T$.

• Dalton’s Law: Total pressure is the sum of partial pressures.

Step-by-Step Guidance

1. For each law, recall the relationship it describes between variables (e.g., pressure and volume for Boyle’s Law).

2. Think about how changes in particle motion, collisions, or number of particles explain each law according to KMT.

3. For Dalton’s Law, consider how the independence of gas particles leads to additive pressures.

Try connecting each law to KMT before checking the answer!

Q5. If NH3 and HCl are each released into still air, how far will the NH3 have effused when the HCl has diffused 1.00 meter?

Background

Topic: Gas Diffusion and Effusion

This question tests your understanding of Graham’s Law, which relates the rates of diffusion or effusion of gases to their molar masses.

Key Formula

• Graham’s Law: $\frac{\text{Rate}_1}{\text{Rate}_2} = \sqrt{\frac{M_2}{M_1}}$

• Where $M$ is the molar mass of each gas.

Step-by-Step Guidance

1. Identify the molar masses of NH3 and HCl.

2. Set up Graham’s Law to compare the rates of NH3 and HCl.

3. Since the distance traveled is proportional to the rate, set up a proportion using the given distance for HCl.

4. Solve for the distance NH3 travels using the ratio from Graham’s Law.

Try setting up the proportion before checking the answer!

Q6. Explain the conditions where the assumptions of the gas laws fail. What formula describes the behavior of gases under these unique conditions?

Background

Topic: Real Gases and Deviations from Ideal Behavior

This question asks you to consider when the ideal gas law does not accurately describe gas behavior and what alternative equation is used.

Key Terms and Formula

• Ideal Gas Law: $PV = nRT$

• Van der Waals Equation: $\left(P + a\frac{n^2}{V^2}\right)(V - nb) = nRT$

• Conditions: High pressure, low temperature.

Step-by-Step Guidance

1. Recall the assumptions of the ideal gas law (e.g., negligible volume, no intermolecular forces).

2. Think about why these assumptions break down at high pressures and low temperatures.

3. Identify the Van der Waals equation as the formula that accounts for real gas behavior.

4. Understand what the $a$ and $b$ constants represent in the Van der Waals equation.

Try explaining the breakdown of assumptions and the new formula before checking the answer!