A chemist conducts an experiment in which 2.00 L of hydrogen gas is collected over water at 1.00 atm and 298.15 K.

Question

A chemist conducts an experiment in which 2.00 L of hydrogen gas is collected over water at 1.00 atm and 298.15 K. The phrase 'over water” means that the gas was collected by bubbling it into an inverted bottle filled with water, which is sitting in a water bath. The gas is trapped in the bottle, displacing the water into the water bath. However, the gas collected is now saturated with water vapor. The partial pressure of water vapor at 298.15 K is 0.0300 atm. Using Dalton’s Law, calculate the pressure of the hydrogen gas in atm.

Answer 1

According to Dalton's law of partial pressures, the total pressure is equal to the sum of the partial pressures of each gas. In equation form, this looks like this ...

P_t = P₁ + P₂ + ...

P_t = total pressure

P₁ = partial pressure of gas 1

P₂ = partial pressure of gas 2

etc...

In this case, we have the following...

P_t = 1.00 atm

P₁ = 0.0300 atm as partial pressure of water vapor

P₂ = ? = partial pressure of hydrogen gas

P_t = P₁ + P₂

1.00 atm = 0.0300 atm + P₂

P₂ = 1.00 atm - 0.0300 atm

P₂ = 0.97 atm = pressure of hydrogen gas