Dalton's law
Total pressure
psi
Mole fraction
Partial pressure
psi
Ideal gas law
Amount of moles
mol
Temperature
°F
Volume
Partial pressure
psi
Henry's law - method 1
Gas
Oxygen
Concentration
M
Partial pressure
psi
Henry's law - method 2
Gas
Oxygen
Mole fraction
Partial pressure
psi

Welcome to the partial pressure calculator - a device that can help you determine and understand partial pressure. Interested in some of the laws of chemistry, such as Dalton's law of partial pressure? Want to know how to calculate partial pressure? Worry no more. We present you with four partial pressure formulas, so keep reading!

Are you're also interested in thermodynamics? If so, check out our combined gas law calculator.

Dalton's law of partial pressures

Pressure is the force applied orthogonally over a surface. If a mixture of ideal gases (i.e., where the molecules don't interact with each other) is sealed within a container, the gases will diffuse and fill up all of the available space. The partial pressure of one component of this mixture is the pressure that this individual gas exerts.

Dalton's law states that:

the total pressure exerted on a container's walls by a gas mixture is equal to the sum of the partial pressures of each separate gas.

It can also be illustrated with an equation:

total pressure = p1 + p2 + ... + pn,

where p1, p2, and so on, up to pn, represent the partial pressure of each gaseous component.

It can also be presented as follows:

partial pressure = total pressure * mole fraction

where mole fraction is the ratio of moles of the selected gas to the moles of the entire gas mixture.

It shows that the partial pressure of one component is proportional to its mole fraction.

The above formula is one of our calculator's four partial pressure formulas. Which one you choose depends on the data you've collected beforehand.

How to calculate partial pressure? - Ideal gas law

The ideal gas law states that:

p * V = n * R * T

where:

  • p is the pressure of the gas
  • V is the volume of the gas
  • n is the number of moles of the gas
  • R is the gas constant, 8.3145 J/mol*K
  • T is the temperature of the gas

If you want to calculate the partial pressure of one component of a gas mixture, use the following formula (derived from the one above):

pi = (ni * R * T) / v

where:

  • pi is the partial pressure of the individual gas
  • ni is the amount of moles of the individual gas
  • T is the temperature of the mixture
  • V is the volume of the mixture

Now you know how to calculate partial pressure using the ideal gas law. Let's move on to the last two forms of the partial pressure equation - both using Henry's law.

Another partial pressure equations - Henry's law

Henry's law states that:

the partial pressure of a gas above a liquid is proportional to the amount of gas dissolved in that liquid.

The coefficient of this proportionality is the Henry's law constant. In the table below, you can find its value for some of the most common gases in water at 298 K:

Element Henry's law constant [litre*atm/mol] Henry's law constant [atm]
O2 769.23 4.259*104
H2 1282.05 7.099*104
Co2 29.41 0.163*104
N2 1639.34 9.077*104
He 2702.7 14.97*104
Ne 2222.22 12.3*104
Ar 714.28 3.955*104
CO 1052.63 5.828*104

Henry's law is only accurate at low gas pressures (pressures < 1000 hPa), constant temperatures (usually 293.15 K) and when the molecules are at equilibrium.

How to find partial pressure with Henry's law constant? There are two methods:

  1. Where the concentration of the solute is given:

pressure = KH1 * concentration

where

KH1 is Henry's law constant in [litre*atm/mol].

  1. Where the mole fraction of the solute is given:

pressure = KH2 * mole fraction

where

KH2 is Henry's law constant in [atm].

Let's use the Henry's law equation in an example. Let's say that you want to calculate the partial pressure of dinitrogen (N2) in a container. Its concentration is 1.5 moles / L. All you need to do is check the Henry's law constant in the table above, and input the numbers into the partial pressure formula:

pressure = 1.5 mol/litre * 1639.34 litre*atm/mol = 2459 atm = 249159 kPa

Simple, right?

Fun facts about pressure

  • It is essential to consider pressure if you are an underwater diver. Divers usually breathe a mixture of oxygen and nitrogen. When diving down to about 35 meters, the standard mixture is safe. As the pressure increases during deeper dives, oxygen becomes toxic, potentially causing narcosis. That's why technical divers (those who dive very deep) use different breathing mixtures to casual divers.

  • In medicine, while performing an arterial-blood gas test, physicians measure the partial pressure of carbon dioxide and oxygen. With these measurements, they can calculate the pH of their patient's blood.

  • You might find it surprising that air pressure changes with altitude and temperature. It's important for people trekking up high mountains - the lower the pressure, the harder it is to breathe.

Julia Żuławińska

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