# When a container is filled with 3.00 moles of H_2, 2.00 moles of O_2, and 1.00 mole of N_2, the pressure in thecontainer is 768 kPa. What is the partial pressure of O_2?

May 31, 2015

Partial Pressure of ${O}_{2}$ = 256 kPa.

Dalton's law of partial pressures covers two concepts;

• Mole fraction
• Partial pressure

Let's talk about Mole Fraction first.

You have a mixture of ${H}_{2}$, ${O}_{2}$ and ${N}_{2}$ in the container. The mole fraction of gas ${O}_{2}$ is worked out by dividing the number of moles of gas ${O}_{2}$ by the total number of moles of gas.

${\chi}_{{O}_{2}} = \left(\text{number of moles of gas" O_2)/("total number of moles of gas}\right)$

${\chi}_{{O}_{2}} = \left(2 \cancel{\text{moles"))/((3+2+1)cancel("moles}}\right) = \frac{1}{3}$

Next, the equation of the partial pressure of a gas can be written as;

${P}_{{O}_{2}} = \text{mole fraction of " O_2 xx "Total Pressure}$

Since now we know the mole fraction of ${O}_{2}$, and the total pressure, we can just substitute the values into the partial pressure equation.

${P}_{{O}_{2}} = \frac{1}{3} \cdot \text{768 kPa" = "256 kPa}$

Note:

The total pressure can be calculated using this;

${P}_{\text{total" = P_"gas A" + P_"gas B" + P_"gas C}} + \ldots$