Question

We often collect small samples of gases in the laboratory by bubbling the gas into a bottle or flask containing water. Explain why the gas becomes saturated with water vapor and how we must take the presence of water vapor into account when calculating the properties of the gas sample.

Short answer

When a gas is collected by bubbling it through water in a bottle or flask, it becomes saturated with water vapor due to contact between the gas and water molecules. The amount of water vapor depends on factors like temperature, pressure, and water solubility. To account for water vapor when calculating gas properties, we can use the concept of partial pressure and determine the partial pressure of the water vapor. The total pressure is the sum of the dry gas pressure and the partial pressure of water vapor, which can be substituted into equations like the ideal gas law to calculate the desired properties while considering the water vapor's presence.

Step-by-step solution

Understand the process of gas collection by bubbling into water

When a gas is collected in a bottle or flask containing water, it is passed through a tube that is submerged below the water level. As the gas bubbles up through the water, it becomes mixed with water molecules. As the gas molecules pass through the water column, they come into contact with the water molecules, which can dissolve to some extent in the gas. Therefore, the collected gas in the bottle or flask will have some amount of water vapor mixed with it, making the gas saturated with water vapor.

Explain saturation with water vapor

When the gas bubble gets in contact with the water, water molecules are trapped inside the bubble and are carried up with the gas. Furthermore, gas molecules can also dissolve in the water. This exchange process continues until the gas reaches an equilibrium state, called saturation. At this point, the gas cannot absorb any more water molecules, and it becomes saturated with water vapor. The amount of water vapor in the gas depends on factors like temperature, pressure, and the solubility of water in the gas.

Importance of accounting for water vapor

When we collect a gas sample using the bubbling technique, it is crucial to consider the presence of water vapor because it affects the calculations of gas properties. For instance, when determining the pressure, volume, or moles of the gas using the ideal gas law, the presence of water vapor will have an impact on these calculations as it contributes to the total pressure and moles inside the flask or bottle.

Calculate the properties of the gas sample

To account for the water vapor in the gas sample, we can use the concept of partial pressure. We first determine the partial pressure of the water vapor in the gas sample, which can be obtained from a table of vapor pressures for water at various temperatures or by using the Antoine equation. Then, we can calculate the total pressure exerted by the gas sample using the following relation: Total pressure = Pressure of dry gas + Partial pressure of water vapor From here, we can substitute this corrected total pressure into the ideal gas law or any other required equations to calculate the desired properties of the gas sample. By doing this, we ensure that the presence of water vapor is taken into account in our calculations, giving us accurate results for the gas sample's properties.

Key concepts

Water Vapor Saturation

When collecting gas samples by bubbling them through water, the gas inevitably becomes saturated with water vapor. This occurs as gas bubbles rise through the water and water molecules attach themselves to the gas. As the gas travels, **water molecules blend into the gas stream**, until the gas reaches what's called a saturated state.

In this state, known as **equilibrium**, the gas cannot absorb more water vapor because the rate of evaporation equals the rate of condensation. Several factors affect this saturation process:

• **Temperature:** Higher temperatures generally increase the amount of water vapor that can exist within a gas.
• **Pressure:** Greater pressure can increase water retention in a gas.
• **Solubility:** The inherent ability of gas to dissolve water varies between types of gases.

Understanding saturation is important for accurate calculations concerning gas samples.

Partial Pressure

In the context of gas collection, partial pressure is a vital concept that helps in determining the impact of water vapor on a gas sample. Partial pressure refers to the pressure a component of a mixture of gases would exert if it occupied the entire volume on its own.

When gas is collected and contains water vapor, the total pressure consists of both the gas' pressure and the water vapor's pressure. This is calculated using:\[P_{\text{total}} = P_{\text{dry gas}} + P_{\text{water vapor}}\]This equation allows us to separate the contribution of water vapor pressure from the total pressure. Partial pressure of the water vapor can be obtained from reference tables or equations like the Antoine equation, which provides water's vapor pressure at various temperatures.

Knowing both the partial pressures lets us accurately apply the ideal gas law or other calculations when working out the specific properties of the dry gas in the mixture.

Ideal Gas Law

The Ideal Gas Law is an essential formula used to relate the pressure, volume, and temperature of gas samples. The law is succinctly expressed as:\[PV = nRT\]where **P** stands for pressure, **V** for volume, **n** for moles of gas, **R** for the ideal gas constant, and **T** for temperature.

However, when a gas contains water vapor, as often is the case in gas collection with bubbling methods, adjustments need to be made before the Ideal Gas Law can be applied. **The total pressure used in calculations must exclude the water vapor’s partial pressure** to accurately solve for the properties of the collected dry gas.

This adjusted pressure ensures that the calculation reflects true volumes, moles, and other variables without the influence of water. Hence, incorporating the understanding of water vapor saturation and partial pressures, permits precise gas property assessments. Without these considerations, measurements would often be mysterious or inaccurate. The Ideal Gas Law becomes a reliable tool only with applicable corrections for water vapor presence.