Consider a mixture of two gases, A and B, confined in a closed vessel. A quantity of a third gas, C, is added to the same vessel at the same temperature. How does the addition of gas C affect the following: (a) the partial pressure of gas A?

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Understand the concept of partial pressure: The partial pressure of a gas in a mixture is the pressure that the gas would exert if it occupied the entire volume of the mixture alone.

Recall Dalton's Law of Partial Pressures: The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture.

Consider the initial state: Initially, gases A and B are in the vessel, each contributing to the total pressure with their respective partial pressures.

Analyze the effect of adding gas C: When gas C is added to the vessel, it increases the total pressure of the system because it contributes its own partial pressure.

Determine the effect on the partial pressure of gas A: The partial pressure of gas A remains unchanged because it depends only on the amount of gas A and the volume and temperature of the vessel, which are constant.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Dalton's Law of Partial Pressures

Dalton's Law states that in a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of each individual gas. Each gas in the mixture behaves independently, and its partial pressure is determined by its mole fraction and the total pressure. This principle is crucial for understanding how the addition of gas C influences the partial pressures of gases A and B.

Ideal Gas Law

The Ideal Gas Law, expressed as PV = nRT, relates the pressure (P), volume (V), temperature (T), and number of moles (n) of a gas. This law helps predict how changes in the amount of gas, such as adding gas C, will affect the overall pressure and the partial pressures of the other gases in the system. It assumes ideal behavior, which is a good approximation under many conditions.

Mole Fraction

Mole fraction is the ratio of the number of moles of a particular gas to the total number of moles of all gases in the mixture. It is a key factor in calculating partial pressures, as the partial pressure of a gas is directly proportional to its mole fraction in the mixture. Understanding mole fractions is essential for determining how the addition of gas C alters the distribution of pressures among gases A and B.

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Acetylene gas, C2H2(𝑔), can be prepared by the reaction of calcium carbide with water:

CaC2(𝑠)+2 H2O(𝑙)⟶Ca(OH)2(𝑎𝑞)+C2H2(𝑔)

Calculate the volume of C2H2 that is collected over water at 23°C by reaction of 1.524 g of CaC2 if the total pressure of the gas is 753 torr. (The vapor pressure of water is tabulated in Appendix B.)