High School Chemistry: Mastering Vapor Pressure Correction

📚 Understanding Vapor Pressure Correction

Vapor pressure correction is a crucial aspect of experimental chemistry, particularly when collecting gases over water. When a gas is collected over water, it becomes saturated with water vapor. The total pressure of the collected gas is the sum of the pressure of the desired gas and the vapor pressure of water. To determine the true pressure of the desired gas, we must subtract the vapor pressure of water at the given temperature from the total pressure.

📜 Historical Context

The study of vapor pressure dates back to the early investigations of gas behavior by scientists like Robert Boyle and John Dalton. Dalton's Law of Partial Pressures, formulated in the early 19th century, provides the theoretical foundation for vapor pressure correction. These early experiments highlighted the importance of accounting for the presence of water vapor when measuring gas volumes and pressures accurately.

⚗️ Key Principles of Vapor Pressure Correction

• 💧 Dalton's Law of Partial Pressures: The total pressure ($P_{total}$) of a mixture of gases is equal to the sum of the partial pressures of each individual gas: $P_{total} = P_{gas} + P_{H_2O}$
• 🌡️ Temperature Dependence: The vapor pressure of water increases with temperature. It's essential to use the correct vapor pressure value for the temperature at which the gas is collected.
• 📐 Correction Formula: The pressure of the dry gas ($P_{gas}$) is calculated by subtracting the vapor pressure of water ($P_{H_2O}$) from the total pressure ($P_{total}$): $P_{gas} = P_{total} - P_{H_2O}$

🧪 Real-world Examples

Example 1:

Hydrogen gas is collected over water at 25°C. The total pressure is 768 torr. The vapor pressure of water at 25°C is 24 torr. What is the pressure of the dry hydrogen gas?

Solution:

$P_{H_2} = P_{total} - P_{H_2O} = 768 \text{ torr} - 24 \text{ torr} = 744 \text{ torr}$

Example 2:

Oxygen gas is collected over water at 30°C. The total pressure inside the collection tube is 780 mmHg. The vapor pressure of water at 30°C is 32 mmHg. Calculate the partial pressure of the dry oxygen gas.

Solution:

$P_{O_2} = P_{total} - P_{H_2O} = 780 \text{ mmHg} - 32 \text{ mmHg} = 748 \text{ mmHg}$

📝 Practice Quiz

1. ❓ A gas is collected over water at 20°C. The total pressure is 760 torr. The vapor pressure of water at 20°C is 18 torr. What is the pressure of the dry gas?
2. ❓ Methane gas is collected over water at 28°C. The total pressure is 772 torr. The vapor pressure of water at 28°C is 28.3 torr. Calculate the pressure of the dry methane gas.
3. ❓ Nitrogen gas is collected over water at 23°C. The total pressure is 755 torr. The vapor pressure of water at 23°C is 21.1 torr. Find the pressure of the dry nitrogen gas.
4. ❓ Ammonia gas is collected over water at 26°C. The total pressure is 765 torr. The vapor pressure of water at 26°C is 25.2 torr. Determine the pressure of the dry ammonia gas.
5. ❓ Carbon dioxide gas is collected over water at 29°C. The total pressure is 775 torr. The vapor pressure of water at 29°C is 30.0 torr. What is the pressure of the dry carbon dioxide gas?

💡 Conclusion

Mastering vapor pressure correction is essential for accurate gas measurements in chemistry. By understanding Dalton's Law and the temperature dependence of vapor pressure, you can confidently correct for the presence of water vapor and obtain reliable experimental results. Keep practicing, and you'll become a pro at handling gas collection experiments! 🚀