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DALTON’S LAW By Raymond Greenlaw Learning Objectives State Dalton’s Law Understand Dalton’s Law Apply Dalton’s Law Explain relevance of Dalton’s Law to scuba John Dalton English chemist Father of atomic theory Discovered colored blindness Discovered gas law in early 1800s State Dalton’s Law The total pressure exerted by a mixture of gases is equal to the sum of the pressures that would be exerted by each of the gases if it alone were present and occupied the volume. State Dalton’s Law Mathematically, Ptotal = P1 + P2 + … + P n, where Pi is the partial pressure of gas i. In this case we have n total gases. Visual of Dalton’s Law Understand Dalton’s Law At sea level air is roughly 20% Oxygen and 80% Nitrogren. Pressure at sea level is one atmosphere. According, to Dalton’s Law PO2 = .20 × 1 ata = .2 ata PN2 = .80 × 1 ata = .8 ata That is partial pressure of oxygen is .2 ata and of nitrogen .8 ata. Popcorn and Marbles Thought experiment with popcorn and marbles Apply Dalton’s Law What is the partial pressure of oxygen in air at sea level in pounds per square inch? PO2 = .21 × 14.7 psi = 3.1 psi Note, the .21 represents the fraction of air that is oxygen at sea level. Apply Dalton’s Law Suppose the total pressure of three mixed gases above sea level is 8 psi. Suppose two of the gases have the same partial pressure and a third gas has a partial pressure of 4 psi. What are the partial pressures of the other two gases? Solution From Dalton’s Law we know that Ptotal = P1 + P2 + P3 We are told Ptotal = 8 psi. We are told for gas three the psi is 4, so P3 = 4. Since we are told P1 = P2, we get 8 = (2 × P1) + 4, so P1 = P2 = 2 psi. Explain Relevance of Dalton’s Law to Scuba The air delivered to a diver at depth is delivered by the regulator at ambient pressure. At sufficient pressures, gases such as nitrogen, oxygen, and contaminants, can have adverse or dangerous effects on a diver. Gases breathed at high pressure will pass into solution in the body’s tissues. Gas toxicity, a dangerous condition, could result. References NAUI Master Scuba Diver Manual, 2010. En.wikipedia.org/wiki/John_Dalton www.thescubaguide.com Figures borrowed from around the web, please let me know if any of the figures are not in the public domain and I will replace them. Questions Thanks for coming! www.raymondgreenlaw.com