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Question

1.) Oxygen is collected by water displacement at an atmospheric pressure of 750.0 torr and a temperature of 25 degrees celsius. What is the pressure of the oxygen in torr?

2.) A chemical reaction produced 980,0 mL of nitrogen gas (N₂) at STP. What is the mass of the gas produced?

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Answer 1

Whenever a gas is collected over water, or by water displacement, one must consider the vapor pressure of the water at the particular temperature of the experiment. In this case, we need to look up the vapor pressure of water at 25ºC, and then subtract that from the total pressure of 750.0 to obtain the pressure of the oxygen gas. The vapor pressure of water at 25ºC is 23.8 torr.

Pressure of O₂ gas = 750.0 torr - 23.8 torr = 726.2 torr

At STP (Standard Temperature and Pressure) 1 mole of any ideal gas = 22.4 L. Since we have 980.0 ml of N₂ gas, we can find the moles and then the mass.

980.0 ml N₂ x 1 L/1000 ml x 1 mol N₂ / 22.4 L x 28 g N₂ / mol N₂ = 1.225 g N₂