The law discovered by John
Dalton in 1803,John Dalton
(1766-1844) chemist and
meteorologist,was born in
September 06, 1766 in
Dalton’s Law of partial pressure states that the
total pressure of mixture of nonreacting
gases is the sum of the partial pressures
exerted by each of the gases in the mixture.
The partial pressure of a gas
• is the pressure of each gas in a mixture.
• is the pressure that gas would exert if it were
by itself in the container.
Formula of Dalton’s Law:
P total = partial pressure A + partial pressure B
PT = PA + PB
= nART + nBRT
Therefore, PT = nTRT
Composition of a gas mixture and partial pressure
For a gas mixture the partial pressures of each of the
gases present are proportional to the number of moles of
eg. For gases A and B where the total pressure is PT
` partial pressure of A = number of moles A x total pressure
total number of moles (both gases)
PA = nA x PT
nA + nB
PB = nB x PT
nA + nB
Dalton’s Law Question 3: (tutorial book pg 29)
A gas mixture containing 2.45 g of N2 and 3.10 g of Ne occupies a
volume of 2.5 L. What is the pressure of the gas at 25oC?
Answer : 2.4 atm
Dalton’s Law Challenging Question 1: (tutorial book pg 29)
4.0 L of nitrogen at a pressure of 400 kN m-2 and 1.0 L of argon at a
pressure of 200 kN m-2 are tranferred into a 2.0 L container. Calculate:
(i) the partial pressure of nitrogen (Answer: 8.0 x 102 kN m-2 )
(ii) the partial pressure of argon (Answer: 1.0 x 102 kN m-2 )
(iii) the total pressure of the mixture (Answer: 9.0 x 102 kN m-2 )
TIPS: Recall Boyle’s Law
Container A (with volume 1.23 dm3) contains a gas under
3.24 atm of pressure. Container B (with volume 0.93
dm3) contains a gas under 2.82 atm of pressure.
Container C (with volume 1.42 dm3) contains a gas
under 1.21 atm of pressure. If all of these gases are put
into Container D (with volume1.51 dm3), what is the
pressure in Container D? (answer:5.51 atm)
Dalton’s Law Challenging Question 2:
The gases of three identical containers A, B, and C are under
pressures of 1.44 atm, 3.16 atm, and 2.52 atm, respectively.
These gases are then combined into Container D (with a
volume of 3.92 L) so that the pressure in Container D is 4.38
atm. Containers A, B, and C have the same volume. Find
that volume. (Answer :2.41L)
Dalton’s Law Challenging Question 3:
Dalton’s Law Challenging Question 4:
The gases carbon dioxide, oxygen and Argon are mixed
in a tank. All gases have the same partial pressure, and
the total pressure of the tank is 48,420 Pascals.
What is the partial pressure of O2?
• When a scuba diver dives, the
increased pressure causes
N2(g)to dissolve in the blood.
• If a diver rises too fast, the
dissolved N2 will form
bubbles in the blood, a
dangerous and painful
condition called "the bends".
• Helium, which does not
dissolve in the blood, is mixed
with O2 to prepare breathing
mixtures for deep descents.
Example of Dalton’s Law in Real Life
The gas laws are not just a series of abstract statements. Certainly, they do concern the
behavior of ideal as opposed to real gases. Like all scientific models, they remove from the
equation all outside factors, and treat specific properties in isolation. Yet, the behaviors of
the ideal gases described in the gas laws provide a key to understanding the activities of real
gases in the real world. For instance, the concept of partial pressure helps scuba divers avoid
a possibly fatal sickness.
Imagine what would happen if a substance were to bubble out of one's blood like carbon
dioxide bubbling out of a soda can, as described below. This is exactly what can happen to an
undersea diver who returns to the surface too quickly: nitrogen rises up within the
body, producing decompression sickness—known colloquially as "the bends." This condition
may manifest as itching and other skin problems, joint
pain, choking, blindness, seizures, unconsciousness, permanent neurological defects such as
paraplegia, and possibly even death.
If a scuba diver descending to a depth of 150 ft (45.72 m) or more were to use ordinary air in
his or her tanks, the results would be disastrous. The high pressure exerted by the water at
such depths creates a high pressure on the air in the tank, meaning a high partial pressure
on the nitrogen component in the air. The result would be a high concentration of nitrogen
in the blood, and hence the bends.
Instead, divers use a mixture of helium and oxygen. Helium gas does not dissolve well in
blood, and thus it is safer for a diver to inhale this oxygen-helium mixture. At the same
time, the oxygen exerts the same pressure that it would normally—in other words, it
operates in accordance with Dalton's observations concerning partial pressure.
Dalton’s Law Challenging Question 5:
A scuba tank contains O2 with a pressure of
0.450 atm and He at 855 mm Hg. What is
the total pressure in mm Hg in the tank?
Answer: 1197 mm Hg