Dalton's Partial Pressure Calculator

Frequently Asked Questions

What is the law of partial pressures?

The law of partial pressures states that the total pressure of a mixture of non-reacting gases equals the sum of the partial pressures each gas would exert if it occupied the container alone.

How is a partial pressure calculated?

The partial pressure of a gas equals its mole fraction, its share of the total moles present, multiplied by the total pressure of the mixture.

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How This Calculator Works

Dalton's law of partial pressures states that in a mixture of non-reacting gases, the total pressure is the sum of the pressures each gas would exert alone. Each gas contributes a partial pressure equal to its mole fraction times the total pressure. The mole fraction is simply that gas's share of all the moles present.

This lets you split a measured total pressure into the individual contributions of each component once you know how many moles of each gas are present.

How To Use It

Enter the total pressure in atmospheres and the moles of each gas. Gas A is required, while gases B and C are optional. The result lists the partial pressure and mole fraction for each gas you entered.

Where It Applies

Analyzing breathing gas mixtures in diving and medicine.

Correcting for water vapor when collecting a gas over water.

Understanding atmospheric composition by pressure.

Practical Applications

Dalton's Law of Partial Pressures is foundational to respiratory physiology, industrial gas handling, and diving safety. In the lungs, the partial pressure of oxygen in alveolar air (pO₂ ≈ 104 mmHg at sea level) drives oxygen diffusion into the blood. At altitude, total atmospheric pressure drops but the mole fraction of oxygen stays the same (21%), so pO₂ falls proportionally - at 5,500 m (18,000 ft) it is roughly half the sea-level value, explaining why supplemental oxygen is needed above that altitude.

Scuba divers face the opposite problem: at depth, increased total pressure raises the partial pressure of nitrogen in compressed air. At 30 m (4 bar absolute), pN₂ reaches ~3.1 bar - high enough that nitrogen narcosis becomes a risk. Deep technical divers switch to trimix (helium, oxygen, nitrogen) to keep each component's partial pressure within safe limits at target depth. In chemical engineering, partial pressures determine vapor-liquid equilibrium and are used to design distillation columns and absorption towers.

Dalton's Partial Pressure Calculator

Frequently Asked Questions

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