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

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

John Dalton

Dalton’s Law of Partial Pressures For a mixture of gases in a container, P Total = P 1 + P 2 + P This is particularly useful in calculating the pressure of gases collected over water.

What is the total pressure in the flask? P total in gas mixture = P A + P B +... Therefore, P total = P H 2 O + P O 2 = 0.48 atm Dalton’s Law: total P is sum of PARTIAL pressures. 2 H 2 O 2 (l) ---> 2 H 2 O (g) + O 2 (g) 0.32 atm 0.16 atm 0.32 atm 0.16 atm

When a scuba diver is several hundred feet under water, the high pressures cause N 2 from the tank air to dissolve in the blood. If the diver rises too fast, the dissolved N 2 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 O 2 in scuba tanks used for deep descents.

The % of gases in air Partial pressure (STP) 78.08% N mm Hg 20.95% O mm Hg 0.94% Ar 7.1 mm Hg 0.03% CO mm Hg P AIR = P N + P O + P Ar + P CO = 760 mm Hg Total Pressure = 760 mm Hg

 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.

A student collects some hydrogen gas over water at 20 degrees C and 768 torr. What is the pressure of the H 2 gas? 768 torr – 17.5 torr = 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: n Total = n 1 + n 2 + n V Total = V 1 + V 2 + V 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!!!

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

(295.0 torr)(3.00 L)(1.00 L) 4.00 L (P) (530.0 torr) 354 torr

* g/mol ((1.00 atm – (17.5 torr* 1 atm/760 torr))(250.0 L) ( atm*L / mol*K )(293 K) 10.2 mol N 2 (x mol) 285 g N 2