Partial Pressure and Mole Fraction
Dalton's Law of Partial Pressures
A 1 liter flask contains 0.4 mol of helium and 1.2 moles of hydrogen gas. Find the mole fractions and partial pressures of both gases, if the total pressure of the mixture is 790 mmHg.
The total number of moles of gases present in the container is 0.4 + 1.2 = 1.6 moles
The mole fraction of helium = 0.4/1.6 = 0.25
The mole fraction of hydrogen = 1.2/1.6 = 0.75
Notice that the sum of the mole fractions is always one. If it is not, you probably made an error somewhere in your calculation. Here,
0.25 + 0.75 = 1.0
Next, we have to find the partial pressures of the gases. We know that the partial pressures of the gases should add up to get the total pressure of the gases. Now that we know the total pressure and the mole fractions, we can calculate the partial pressures of helium and hydrogen.
Partial pressure of gas A = mole fraction of gas A x total pressure
Partial pressure of helium = 0.25 x 790 mmHg = 197.5 mmHg
Partial pressure of hydrogen = 0.75 x 790 mmHg = 592.5 mmHg