Partial Pressure Calculator

Created by Julia Żuławińska
Reviewed by Bogna Szyk and Jack Bowater
Last updated: Nov 11, 2022

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. (Learn more in the mole fraction calculator).

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

According to the ideal gas law equation:

pV=nRT\small pV = nRT

where:

  • pp – Pressure of the gas;
  • VV – Volume of the gas;
  • nn – Number of moles of the gas;
  • RR – Gas constant, 8.3145 J/mol·K; and
  • TT –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=niRTmVm\small p_i = \frac{n_i R T_\text{m}}{V_\text{m}}

where:

  • pip_i – Partial pressure of the individual gas;
  • nin_i – Amount of moles of the individual gas;
  • TmT_\text{m} – Temperature of the mixture; and
  • VmV_\text{m} – 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 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 [L·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 – Henry's law constant in L·atm/mol.
  1. Where the mole fraction of the solute is given:

    pressure = KH2 × mole fraction

    where:

    • KH2 – Henry's law constant in [atm].

Let's use 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 mol/L (moles per liter). All you need to do is check Henry's law constant in the table above and input the numbers into the partial pressure formula:

pressure = 1.5 mol/L × 1639.34 L·atm/mol = 2459 atm = 249,159 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. Oxygen becomes toxic as the pressure increases during deeper dives, potentially causing narcosis. That's why technical divers (those who dive very deep) use different breathing mixtures than 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. (Learn more in the air pressure at altitude calculator.

FAQ

How to calculate partial pressure?

To calculate the partial pressure of a gas:

  1. Divide the dissolved gas moles by the moles of the mixture to find the mole fraction.

  2. Multiply the total pressure by the mole fraction to find the partial pressure of the chosen gas.

Alternatively, you can use the ideal gas equation or Henry's law, depending on your data.

What is Dalton's law of partial pressures?

Dalton's law of partial pressures tells us that a mixture of gases has a total pressure equal to the sum of the partial pressures of its constituent gases.

It also informs us that the partial pressure of a component substance is proportional to its mole fraction.

How do I calculate the partial pressure of oxygen?

To calculate the partial pressure of oxygen:

  1. Determine the concentration or mole fraction of oxygen in the mixture.

  2. Find the appropriate Henry's law constant:

    • For known concentration, use Kʜ₁ = 769.23 L·atm/mol.

    • If you know the mole fraction, use Kʜ₂ = 4.259 × 10⁴ atm.

  3. Calculate the partial pressure of O₂ by multiplying the known variable by its corresponding Henry's law constant.

What does a high Henry's law constant mean?

A high value of Henry's law constant means that the gas has low solubility in a liquid, at least at a given pressure.

Notice that Henry's law constant varies for different gases under the same conditions; therefore, it depends on the gas' nature.

When to use Henry's law?

You can use Henry's law to calculate partial pressures if the gas of interest is at:

  • Pressure under 1000 hPa (0.987 atm); and
  • Both chemical and thermal equilibrium.

How do I use Henry's law to find concentration?

To find concentration using Henry's law:

  1. Determine the gas' partial pressure.

  2. Find Henry's law constant, , for the chosen substance.

  3. Divide the partial pressure by the constant; the result is the concentration.

Julia Żuławińska
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
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