If you like us, please share us on social media.
The latest UCD Hyperlibrary newsletter is now complete, check it out.

ChemWiki: The Dynamic Chemistry E-textbook > Physical Chemistry > Physical Properties of Matter > Phases of Matter > Gases > Virtual: Gas Laws > Dalton's Law

Copyright (c) 2006-2014 MindTouch Inc.

This file and accompanying files are licensed under the MindTouch Master Subscription Agreement (MSA).

At any time, you shall not, directly or indirectly: (i) sublicense, resell, rent, lease, distribute, market, commercialize or otherwise transfer rights or usage to: (a) the Software, (b) any modified version or derivative work of the Software created by you or for you, or (c) MindTouch Open Source (which includes all non-supported versions of MindTouch-developed software), for any purpose including timesharing or service bureau purposes; (ii) remove or alter any copyright, trademark or proprietary notice in the Software; (iii) transfer, use or export the Software in violation of any applicable laws or regulations of any government or governmental agency; (iv) use or run on any of your hardware, or have deployed for use, any production version of MindTouch Open Source; (v) use any of the Support Services, Error corrections, Updates or Upgrades, for the MindTouch Open Source software or for any Server for which Support Services are not then purchased as provided hereunder; or (vi) reverse engineer, decompile or modify any encrypted or encoded portion of the Software.

A complete copy of the MSA is available at http://www.mindtouch.com/msa

Dalton's Law

Table of Contents

One of the important predictions made by Avogadro is that the identity of a gas is unimportant in determining the P-V-T properties of the gas. This behavior means that a gas mixture behaves in exactly the same fashion as a pure gas. (Indeed, early scientists such as Robert Boyle studying the properties of gases performed their experiments using gas mixtures, most notably air, rather than pure gases).

The Ideal Gas Law,

P V = n R T ,

predicts how the pressure, volume, and temperature of a gas depend upon the number of moles of the gas. The number of moles, n, is the total moles of all the gas-phase species.

Air, for example, is composed primarily of nitrogen and oxygen. In a given sample of air, the total number of moles is can be approximated as

n = nnitrogen + noxygen

 This expression for n can be substituted into the ideal gas law to yield


P V = ( nnitrogen + noxygen ) R T

All molecules in the gas have access to the entire volume of the system, thus V is the same for both nitrogen and oxygen. Similarly, both compounds experience the same temperature. One can therefore split this expression of the ideal gas law into two terms, one for nitrogen and one for oxygen.


P = nnitrogen R T/V + noxygen R T/V


P = Pnitrogen + Poxygen

The above equation is called Dalton's Law of Partial Pressure, and it states that the pressure of a gas mixture is the sum of the partial pressures of the individual components of the gas mixture. Pnitrogen is the partial pressure of the nitrogen and Poxygen is the partial pressure of oxygen.


Pnitrogen = nnitrogen R T/V


Poxygen = noxygen R T/V

You'll notice that the equations for the partial pressures are really just the ideal gas law, but the moles of the individual component (nitrogen or oxygen) are used instead of the total moles. Conceptually Pnitrogen is the contribution nitrogen molecules make to the pressure and Poxygen is the contribution oxygen molecules make.


Employ Boyle's Law and Dalton's Law of Partial Pressures to predict the pressure of a gas mixture. This exercise involves the same two-bulb apparatus employed in a previous exercise. The left bulb is filled with nitrogen gas (artificially colored blue) and the right bulb is filled with oxygen gas (artificially colored red). The volumes of the two bulbs are indicated and manometers are provided to measure the pressure inside each bulb. Each time you press the "New Conditions" button, the experiment will be reset with new volumes and initial pressures for the two bulbs.

When the stopcock is open, the nitrogen will expand into the right bulb and the oxygen into the left bulb to produce a uniform mixture.

Use Boyle's Law to calculate the partial pressure of nitrogen and the partial pressure of oxygen in the final gas mixture. Use these partial pressures and Dalton's Law to calculate the pressure for the gas mixture.

Open the stopcock and measure the final pressure. Was your calculation accurate? Practice the calculations until you are able to solve the problem consistently.


Volume of Left Bulb

Volume of Right Bulb

Last modified
09:33, 2 Oct 2013



(not set)
(not set)

Creative Commons License Unless otherwise noted, content in the UC Davis ChemWiki is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. Permissions beyond the scope of this license may be available at copyright@ucdavis.edu. Questions and concerns can be directed toward Prof. Delmar Larsen (dlarsen@ucdavis.edu), Founder and Director. Terms of Use