Question

A compound is heated to produce a gas whose molecular weight is to be determined. The gas is collected by displacing water in a water-filled flask inverted in water. Which of the following is needed to calculate the molecular weight of the gas, but does NOT need to be measured during the experiment?

(A) Mass of the compound used in the experiment
(B) Temperature of the water in the trough
(C) Vapor pressure of the water
(D) Barometric pressure
(E) Volume of water displaced from the flask

Answer 1

The vapor pressure of water is required to calculate the molecular weight of a gas in a water displacement experiment but does not need to be measured as it is a known value and can be found in tables. The correct option is (C) Vapor pressure of the water.

Step-by-step explanation:

To calculate the molecular weight of a gas collected by water displacement, the vapor pressure of water is needed but does not need to be measured during the experiment. This is because the vapor pressure of water at a given temperature is a known value that can be found in published tables.

To determine the molecular weight of the gas, you need to subtract the water vapor pressure from the total barometric pressure to find the partial pressure of the gas, using Dalton's Law. Therefore, the measured experimental values such as the mass of the compound, the temperature, the barometric pressure, and the volume of water displaced are used along with the known vapor pressure of water to calculate the molar mass of the gas.