Question

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`?

Answer 1

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) = ("number of moles of gas" O_2)/("total number of moles of gas")`

`chi_(O_2) = (2cancel("moles"))/((3+2+1)cancel("moles")) = 1/3`

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

`P_(O_2) = "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) = 1/3 * "768 kPa" = "256 kPa"`

Note:

The total pressure can be calculated using this;

`P_"total" = P_"gas A" + P_"gas B" + P_"gas C" +...`