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

Gas Laws: Dalton’s Law At the conclusion of our time together, you should be able to:. Explain Dalton’s Law Use Dalton’s Law to solve a problem. Dalton’s Law. John Dalton 1766-1844. Dalton’s Law of Partial Pressures. For a mixture of gases in a container,

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

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  1. Gas Laws: Dalton’s LawAt the conclusion of our time together, you should be able to: Explain Dalton’s Law Use Dalton’s Law to solve a problem

  2. Dalton’s Law John Dalton 1766-1844

  3. 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.

  4. Dalton’s Law of Partial Pressures • 2 H2O2 (l) ---> 2 H2O (g) + O2 (g) • 0.32 atm 0.16 atm What is the total pressure in the flask? Ptotal in gas mixture = PA + PB + ... Therefore, Ptotal = PH2O + PO2 = 0.48 atm Dalton’s Law: total P is sum of PARTIAL pressures.

  5. Health Note When 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.

  6. Gases in the Air The % of gases in air Partial pressure (STP) 78.08% N2 593.4 mm Hg 20.95% O2 159.2 mm Hg 0.94% Ar 7.1 mm Hg 0.03% CO2 0.2 mm Hg PAIR = PN + PO + PAr + PCO = 760 mm Hg 2 2 2 Total Pressure = 760 mm Hg

  7. 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.

  8. Table of Vapor Pressures for Water

  9. 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

  10. 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.

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