Question

1. a sample of an unknown gas from a cylinder is collected over water in the apparatus shown above. after all the gas sample has been collected, the water levels inside and outside the gas collection tube are made the same. in addition to the volume of gas in the gas - collection tube, the measurements that must be made to calculate the molar mass of the gas include all of the following except (a) atmospheric pressure (b) temperature of the water (c) initial and final mass of the gas cylinder (d) mass of the water in the apparatus

Explanation:

Step1: Recall the ideal gas law formula

The ideal gas law is \(PV = nRT\), and molar mass \(M=\frac{m}{n}\). We can rewrite it as \(M = \frac{mRT}{PV}\).

Step2: Analyze each option

• Option A: Atmospheric pressure (\(P\)) is needed. Since the gas is collected over water, the pressure of the gas \(P_{gas}=P_{atm}-P_{H_2O}\) (vapor - pressure of water).
• Option B: Temperature (\(T\)) is needed. The value of \(R\) is fixed, and \(T\) (in Kelvin) is in the formula \(M = \frac{mRT}{PV}\).
• Option C: The initial and final mass of the gas cylinder gives the mass (\(m\)) of the gas.
• Option D: The mass of water in the apparatus is not relevant. The formula for molar mass \(M=\frac{mRT}{PV}\) does not involve the mass of water.

Answer:

D. mass of the water in the apparatus