Not so long ago, in a chemistry lab far far away…May the FORCE/area be with you

Gas Laws: Dalton’s LawAt the conclusion of our time together, you should be able

to:

1. Explain Dalton’s Law 2. Use Dalton’s Law to solve a problem

Dalton’s Law

John Dalton1766-1844

Dalton’s Law of Partial Pressures

For a mixture of gases in a container,

PTotal = P1 + P2 + P3 + . . .

This is particularly useful in calculating the pressure of gases collected over water.

Dalton’s Law of Partial Pressures

What is the total pressure in the flask?Ptotal in gas mixture = PA + PB + ...Therefore, Ptotal = PH2O + PO2 = 0.48 atmDalton’s Law: total P is sum of PARTIAL

pressures.

2 H2O2 (l) ---> 2 H2O (g) + O2 (g)

0.32 atm 0.16 atm

“Disorder in the American Courts”

These are statements people actually said in court, word for word, taken down and now

published by court reporters.

ATTORNEY:  Were you present when your picture was taken?

WITNESS:     Are you shitting me?

Health NoteWhen a scuba diver is

several hundred feet under water, the high pressures

cause N2 from the tank air to dissolve in the blood. If the

diver rises too fast, the dissolved N2 will form bubbles

in the blood, a dangerous and painful condition called "the bends". Helium, which is inert, less dense, and does not dissolve in the blood, is

mixed with O2 in scuba tanks used for deep descents.

Gases in the Air

The % of gases in air Partial pressure (STP)

78.08% N2 593.4 mm Hg20.95% O2 159.2 mm Hg

0.94% Ar 7.1 mm Hg

0.03% CO2 0.2 mm HgPAIR = PN + PO + PAr + PCO = 760 mm

Hg 2 2 2

Total Pressure = 760 mm Hg

Collecting a gas “over water” Gases, since they mix with other gases readily, must

be collected in an environment where mixing can not occur. The easiest way to do this is under water because water displaces the air. So when a gas is collected “over water”, that means the container is filled with water and the gas is bubbled through the water into the container. Thus, the pressure inside the container is from the gas AND the water vapor. This is where Dalton’s Law of Partial Pressures becomes useful.

Table of Vapor Pressures for Water

Solve This!

A student collects some hydrogen gas over water at 20 degrees C and 768 torr. What is the pressure of the H2 gas?

768 torr – 17.5 torr = 750.5 torr

Dalton’s Law of Partial Pressures

Also, for a mixture of gases in a container, because P, V and n are directly proportional if the other gas law variables are kept constant:

nTotal = n1 + n2 + n3 + . . .

VTotal = V1 + V2 + V3 + . . .

This is useful in solving problems with differing numbers of moles or volumes of the gases that are mixed together.

Did I ever tell you how much I hate computers!!!

Gas Laws: Dalton’s LawLet’s see if you can:

1. Explain Dalton’s Law 2. Use Dalton’s Law to solve a problem

Dalton’s Law Problems, Page 38:5

Using Dalton’s & Boyle’s Laws

(295.0 torr)(3.00 L) (1.00 L)

4.00 L (P)

(530.0 torr)

354 torr

Dalton’s Law Problems, Page 39:7

Using Dalton’s & Ideal Gas Laws

x 28.02 g/mol

((1.00 atm – (17.5 torr x 1 atm/760 torr)) (250.0 L)

(0.08206 atm*L/mol*K) (293 K)

10.2 mol N2

(x mol)

285 g N2

0.977 atm(250)

Surrounded by Engines!!!