DALTON’S LAW

• The law discovered by John Dalton in 1803,John Dalton (1766-1844) chemist and meteorologist,was born in September 06, 1766 in Cumberland, England

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.

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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.

Partial Pressure

Ptotal = P1 + P2 + P3 + . . .

Dalton’s Law Question 1:

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Formula of Dalton’s Law:

P total = partial pressure A + partial pressure BPT = PA + PB

= nART + nBRT V V Therefore, PT = nTRT

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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 each gas.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 2 :

mol gas

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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?Answer:16,140 Pascal

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Scuba Diving

• 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