DALTON’S LAW
By Raymond Greenlaw
Learning Objectives
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State Dalton’s Law
Understand Dalton’s Law
Apply Dalton’s Law
Explain relevance of Dalton’s Law to scuba
John Dalton
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English chemist
Father of atomic theory
Discovered colored blindness
Discovered gas law in early 1800s
State Dalton’s Law
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The total pressure exerted by a mixture of gases is
equal to the sum of the pressures that would be
exerted by each of the gases if it alone were
present and occupied the volume.
State Dalton’s Law
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Mathematically,
Ptotal = P1 + P2 +
…
+ P n,
where Pi is the partial pressure of gas i. In this
case we have n total gases.
Visual of Dalton’s Law
Understand Dalton’s Law
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At sea level air is roughly 20% Oxygen and 80%
Nitrogren.
Pressure at sea level is one atmosphere.
According, to Dalton’s Law
PO2 = .20 × 1 ata = .2 ata
PN2 = .80 × 1 ata = .8 ata
That is partial pressure of oxygen is .2 ata and of
nitrogen .8 ata.
Popcorn and Marbles
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Thought experiment with popcorn and marbles
Apply Dalton’s Law
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What is the partial pressure of oxygen in air at sea
level in pounds per square inch?
PO2 = .21 × 14.7 psi = 3.1 psi
Note, the .21 represents the fraction of air that is
oxygen at sea level.
Apply Dalton’s Law
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Suppose the total pressure of three mixed gases
above sea level is 8 psi. Suppose two of the gases
have the same partial pressure and a third gas has
a partial pressure of 4 psi. What are the partial
pressures of the other two gases?
Solution
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From Dalton’s Law we know that
Ptotal = P1 + P2 + P3
We are told Ptotal = 8 psi.
We are told for gas three the psi is 4, so
P3 = 4.
 Since we are told P1 = P2, we get
8 = (2 × P1) + 4, so P1 = P2 = 2 psi.
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Explain Relevance of Dalton’s Law to
Scuba
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The air delivered to a diver at depth is delivered
by the regulator at ambient pressure.
At sufficient pressures, gases such as nitrogen,
oxygen, and contaminants, can have adverse or
dangerous effects on a diver.
Gases breathed at high pressure will pass into
solution in the body’s tissues.
Gas toxicity, a dangerous condition, could result.
References
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NAUI Master Scuba Diver Manual, 2010.
En.wikipedia.org/wiki/John_Dalton
www.thescubaguide.com
Figures borrowed from around the web, please let
me know if any of the figures are not in the public
domain and I will replace them.
Questions
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Thanks for coming!
www.raymondgreenlaw.com