# Partial Pressure and Mole Fraction

# Dalton's Law of Partial Pressures

_{total}= P

_{A}+ P

_{B}+ P

_{C}+ P

_{D}+ . . . (at constant volume & temperature)

_{total}

_{_ }â€“Total pressure

_{A }, P

_{B }, P

_{C }, P

_{D }, . . . represent the partial pressures of gases A, B, C, D, and so on.

**mole fraction**of a gas is the fraction or ratio of moles of that particular gas against the total number of moles of gases present in the mixture. Mole fraction is defined as follows:

_{
}

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