Understanding gas collection over water: practical examples and solutions

📚 Quick Study Guide

• 🌊 Purpose: Gas collection over water is a common lab technique used to collect gases produced in a reaction. The gas displaces water in a container, allowing for its collection and measurement.
• 🌡️ Key Concept - Vapor Pressure: Since the gas is collected over water, it becomes saturated with water vapor. The total pressure of the collected gas ($P_{total}$) is the sum of the pressure of the dry gas ($P_{gas}$) and the vapor pressure of water ($P_{H_2O}$) at that temperature: $P_{total} = P_{gas} + P_{H_2O}$.
• 📐 Leveling the Water: To ensure accurate measurements, the water level inside the collection container should be equal to the water level outside. This makes the total pressure inside the container equal to atmospheric pressure.
• 📝 Calculating Dry Gas Pressure: To find the pressure of the dry gas, you need to subtract the vapor pressure of water at the experimental temperature from the total pressure: $P_{gas} = P_{total} - P_{H_2O}$. Vapor pressure values are typically provided in tables.
• 🔢 Ideal Gas Law: You can use the ideal gas law, $PV = nRT$, to determine the number of moles ($n$) of gas collected, where $P$ is the pressure of the dry gas, $V$ is the volume of the gas, $R$ is the ideal gas constant, and $T$ is the temperature in Kelvin.

Practice Quiz

1. What is the primary reason for collecting gas over water?
   1. To keep the gas dry.
   2. To easily measure the gas volume.
   3. To increase the gas pressure.
   4. To remove impurities from the gas.
2. Which of the following equations correctly relates the total pressure ($P_{total}$), the pressure of the dry gas ($P_{gas}$), and the vapor pressure of water ($P_{H_2O}$)?
   1. $P_{total} = P_{gas} - P_{H_2O}$
   2. $P_{gas} = P_{total} + P_{H_2O}$
   3. $P_{total} = P_{gas} + P_{H_2O}$
   4. $P_{H_2O} = P_{total} + P_{gas}$
3. Why is it important to level the water inside and outside the collection container?
   1. To minimize the amount of gas collected.
   2. To ensure the pressure inside equals atmospheric pressure.
   3. To prevent water from entering the container.
   4. To make the calculation of vapor pressure easier.
4. What information do you need to look up in a table to calculate the dry gas pressure?
   1. The density of water.
   2. The atmospheric pressure.
   3. The vapor pressure of water at the experimental temperature.
   4. The molar mass of the gas.
5. If the total pressure is 760 torr and the vapor pressure of water at the experimental temperature is 25 torr, what is the pressure of the dry gas?
   1. 735 torr
   2. 785 torr
   3. 760 torr
   4. 25 torr
6. Which law is most directly used to calculate the number of moles of gas collected?
   1. Dalton's Law of Partial Pressures
   2. Charles's Law
   3. Ideal Gas Law
   4. Graham's Law
7. If you collect a gas over water at 298 K and the vapor pressure of water at this temperature is 24 torr, what must you do with the temperature value before using it in the Ideal Gas Law?
   1. Convert torr to atm
   2. Nothing, 298 K is ready to use
   3. Convert to Celsius
   4. Use the value as a constant