How to Solve a Partial Pressure Problem

Skill Set: Ideal Gases

  1. Solve Ideal Gas Problem
  2. Solve Partial Pressure Problem

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In a mixture of gases, the pressure of each individual gas is known as the partial pressure. The partial pressure of each gas is related to the number of moles of the gas.

For an analogy see Ideal Gas Law Explained By Analogy.
Equations to know:
Px = PTotal ( nx / nTotal )

Px = partial pressure of gas x
PTotal = total pressure of all gases
nx = number of moles of gas x
nTotal = number of moles of all gases
Example Problem:

A balloon contains 0.2 moles of nitrogen and 0.5 moles of oxygen. If the total pressure in the balloon is 2.0 atm, what is the partial pressure of oxygen?


Step 1

Identify all of the known and unknowns in the partial pressure equation.


Step 1

Px = This is the partial pressure of oxygen, our unknown
PTotal = 2.0 atm
nx = 0.5 moles of oxygen
nTotal = 0.5 moles + 0.2 moles = 0.7 moles

Step 2

Plug in the numbers and solve for the unknown (in this case it is Px).

Step 2

We use the following equation:
Px = PTotal ( nx / nTotal )

Px = 2.0 atm (0.5 moles/0.7 moles) = 1.4 atm