Dalton's Law Calculator
Calculate total pressure, partial pressures, or correct for vapor pressure ($P_total = P_1 + P_2 + \dots$).
Total Pressure
The Science of Gas Mixtures
John Dalton discovered a simple but profound truth: In a mixture, every gas acts like it owns the place. The total pressure is just the sum of these independent contributions.
Calculating Pressure
There are two main ways to approach partial pressures: adding up individual components or using mole fractions to split the total.
The Concept
Imagine a container with 10 red particles (1 atm) and 10 blue particles (1 atm).
- IndieThe red particles hit the walls just as often as if the blue ones weren't there.
- SumThe wall feels both sets of impacts. So, Total Pressure = 1 atm + 1 atm = 2 atm.
Collecting Gas Over Water
In chemistry labs, we often capture gases by bubbling them into an upside-down flask filled with water. But there's a catch: the collected gas isn't pure.
Water evaporates! So the "gas" in the flask is actually a mixture of your target gas AND water vapor. To find the true pressure of your gas, you must subtract the pressure of the water vapor.
| Temp (°C) | Pwater (mmHg) | Pwater (kPa) |
|---|---|---|
| 20°C | 17.5 | 2.34 |
| 25°C | 23.8 | 3.17 |
| 30°C | 31.8 | 4.24 |
| 40°C | 55.3 | 7.38 |
The Atmosphere
Atmospheric pressure (101.3 kPa) is just the weight of all air gases combined. Nitrogen contributes ~78 kPa, and Oxygen contributes ~21 kPa.
Scuba Diving
At 40m deep, total pressure is 5 atm. This means PN2 is 3.9 atm (5x normal). This high partial pressure forces nitrogen into the blood, causing narcosis.
Respiratory Physiology
Oxygen moves from lungs to blood because PO2 in alveoli (104 mmHg) is higher than PO2 in deoxygenated blood (40 mmHg). Gases always flow from high P to low P.
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Frequently Asked Questions
What is Dalton's Law?
Dalton's Law of Partial Pressures states that in a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases. Mathematically: Ptotal = P1 + P2 + ... + Pn.
What is 'Partial Pressure'?
It is the pressure that a specific gas would exert if it were the only gas in the container. In an ideal gas mixture, gases act independently, as if the others weren't there.
What is 'Collecting Gas Over Water'?
In labs, gases are often collected by displacing water in an inverted tube. This gas is 'wet' because it mixes with water vapor. To find the pressure of the 'dry' gas, you must subtract the water's vapor pressure: Pgas = Ptotal - PH2O.
What is Mole Fraction?
Mole fraction (X) is the ratio of moles of one gas to the total moles in the mixture. Dalton's Law can also be written as: Ppartial = Xgas × Ptotal.
Does the mass of the gas matter?
Not directly. Pressure depends on the number of particles (moles), temperature, and volume. Heavier gas particles move slower but hit walls with more momentum; lighter ones move faster but hit with less momentum—it averages out so that 1 mole of any ideal gas exerts the same pressure.
Why do my units need to match?
Physics requires consistency. You cannot add atmospheres (atm) to kilopascals (kPa). All partial pressures must be converted to the same unit before summing.
What is the composition of air?
Dry air is roughly 78% Nitrogen, 21% Oxygen, and 1% Argon/Other. The total atmospheric pressure (1 atm) is simply the sum: 0.78 atm (N2) + 0.21 atm (O2) + 0.01 atm (Ar).
How does temperature affect vapor pressure?
As water gets warmer, more molecules escape liquid phase into gas phase. At 25°C, PH2O is roughly 23.8 mmHg. At 100°C (boiling), it equals the atmospheric pressure (760 mmHg).
Can I use Dalton's Law for liquids?
No, Dalton's Law applies specifically to mixtures of gases. However, Raoult's Law is a similar concept used for vapor pressure above liquid mixtures.
What happens if the gases react?
Dalton's Law fails if gases chemically react (e.g., Ammonia + Hydrogen Chloride forms solid Ammonium Chloride). The law assumes gases are inert towards each other.
How does scuba diving relate to Dalton's Law?
As divers descend, Ptotal increases. This increases the partial pressure of Nitrogen in the lungs. High PN2 dissolves into blood and can cause Nitrogen Narcosis (a drunk-like state).
Does Dalton's Law work at high pressures?
At extremely high pressures, gas molecules are forced close together and intermolecular forces interfere. The gas stops behaving 'ideally', and Dalton's Law becomes less accurate.
How do I calculate Ptotal from moles?
First, use the Ideal Gas Law (PV=nRT) to calculate the pressure each gas would exert if alone. Then, simply add those calculated pressures together.
What is the partial pressure of O2 in the lungs?
In the atmosphere, PO2 is ~160 mmHg. In the alveoli, it drops to ~104 mmHg because fresh air mixes with CO2 and water vapor remaining in the lungs.
Who was John Dalton?
He was an English chemist (1766-1844) best known for introducing the Atomic Theory into chemistry and for his law of partial pressures.