Partial Pressure
Definition and meaning of Partial Pressure in chemistry.
Partial pressure is the pressure one specific gas would create if it took up an entire container by itself. In any mixture of gases, the total pressure is just the sum of all these individual partial pressures. This simple addition rule is known as Dalton's Law of Partial Pressures.
In more detail
When different gases mix in a room, they do not push against each other. Instead, each gas behaves completely on its own, as long as no chemical reaction happens. Each gas bounces off the walls and creates its own separate pressure.
This concept lets chemists easily predict how gas mixtures will act in closed spaces. It is vital for understanding chemical equilibrium and even how human lungs breathe. Doctors use partial pressure to track oxygen levels in a patient's blood.
Chemists rely on it when they collect a new gas over a container of water. The water vapor adds its own partial pressure, which must be subtracted to find the true gas pressure. A common student misconception is thinking heavier gases create more partial pressure.
In reality, partial pressure depends only on the number of gas particles, not their weight. Ten moles of light helium exert the exact same partial pressure as ten moles of heavy argon at the same temperature. Another mistake is trying to apply Dalton's law to gases that undergo a violent chemical reaction together.
Key facts
| Field | Physical Chemistry |
|---|---|
| Governing rule | Dalton's Law of Partial Pressures |
| Formula | Ptotal = P1 + P2 + P3 |
| Common units | Atmospheres (atm), kilopascals (kPa), torr |
| Key factor | Depends on the number of moles of gas |
| Independence | Gases in a mixture act as if they are alone |
The air we breathe is a perfect example of partial pressures at work. Total normal atmospheric pressure at sea level is about 101.3 kilopascals (kPa). Nitrogen makes up most of the air, creating a partial pressure of roughly 79.0 kPa. Oxygen is the next biggest piece, adding its own partial pressure of about 21.2 kPa. Argon and carbon dioxide make up the remaining 1.1 kPa. When you add 79.0, 21.2, and 1.1 together, you get the total 101.3 kPa.
Frequently asked questions
How do you calculate the partial pressure of a specific gas?
You multiply the total pressure of the mixture by the mole fraction of that specific gas.
Does Dalton's Law apply to every single gas mixture?
It works perfectly for ideal gases at normal temperatures. It breaks down slightly at very high pressures where gas molecules crowd together.
Does the mass of the gas particles affect the partial pressure?
No, the mass does not matter. The partial pressure is determined by the total number of gas particles hitting the container walls.