9.5 Gas Collected over Water

Gas  Collected Over Water

Figure 9.58 Gas collected over water

Ref: commons.wikimedia.org/

The law of partial pressure is frequently used to determine the yield of a water-insoluble gas formed in a reaction. The gaseous product bubble through the water and is collected into an inverted container. The water vapor that mixes with the gas contributes a portion of the total pressure, called the vapor pressure, which depends only on the water temperature.

Ptotal- P_H2O= Pgas

Apply PV/RT to find the moles

Knowing the molar mass, find the mass of the gas

Gas Stoichiometry – Explained

Figure 9.59 An apparatus for collecting Gases by the displacement of water

Ref: commons.wikimedia.org/

When KClO₃(s) is heated, O₂ is produced according to the equation . The oxygen gas travels through the tube, bubbles up through the water, and is collected in a bottle as shown.

The only gases that cannot be collected using this technique are those that readily dissolve in water (e.g., NH₃, H₂S, and CO₂) and those that react rapidly with water (such as F₂ and NO₂). Remember, however, when calculating the amount of gas formed in the reaction, the gas collected inside the bottle is not pure. Instead, it is a mixture of the product gas and water vapor. As we know all liquids (including water) have a measurable amount of vapor in equilibrium with the liquid because molecules of the liquid are continuously escaping from the liquid’s surface, while other molecules from the vapor phase collide with the surface and return to the liquid. The vapor thus exerts a pressure above the liquid, which is called the liquid’s vapor pressure. In the case shown above, the bottle is therefore actually filled with a mixture of O₂ and water vapor, and the total pressure is, by Dalton’s law of partial pressures, the sum of the pressures of the two components:

If we want to know the pressure of the gas generated in the reaction to calculate the amount of gas formed, we must first subtract the pressure due to water vapor from the total pressure. This is done by referring to tabulated values of the vapor pressure of water as a function of temperature. As shown in  figure below, the vapor pressure of water increases rapidly with increasing temperature, and at the normal boiling point (100°C), the vapor pressure is exactly 1 atm. The methodology is illustrated in Example 14.

Table 10.4 Vapor Pressure of Water at Various Temperatures

Figure 9.60 A Plot of the Vapor Pressure of Water versus Temperature

Ref: commons.wikimedia.org/

The vapor pressure is very low (but not zero) at 0°C and reaches 1 atm = 760 mmHg at the normal boiling point, 100°C.

Practice Problems

1. What is the density of laughing gas, dinitrogen monoxide, N₂O, at a temperature of 325 K and a pressure of 113.0 kPa?
2. Calculate the density of Freon 12, CF₂Cl₂, at 30.0°C and 0.954 atm.
3. Which is denser at the same temperature and pressure, dry air or air saturated with water vapor? Explain.
4. A cylinder of O₂(g) used in breathing by emphysema patients has a volume of 3.00 L at a pressure of 10.0 atm. If the temperature of the cylinder is 28.0 °C, what mass of oxygen is in the cylinder?
5. What is the molar mass of a gas if 0.0494 g of the gas occupies a volume of 0.100 L at a temperature 26 °C and a pressure of 307 torr?
6. What is the molar mass of a gas if 0.281 g of the gas occupies a volume of 125 mL at a temperature 126 °C and a pressure of 777 torr?
7. How could you show experimentally that the molecular formula of propene is C₃H₆, not CH₂?
8. The density of a certain gaseous fluoride of phosphorus is 3.93 g/L at STP. Calculate the molar mass of this fluoride and determine its molecular formula.
9. Consider this question: What is the molecular formula of a compound that contains 39% C, 45% N, and 16% H if 0.157 g of the compound occupies l25 mL with a pressure of 99.5 kPa at 22 °C?