In this experiment you will react a known mass of a metal with an excess of

# In this experiment you will react a known mass of a

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In this experiment you will react a known mass of a metal with an excess of hydrochloric acid and collect the generated hydrogen gas over water in a buret that is corked at one end. The evolved gas will rise to the top of the water-filled tube, displacing an equal volume of water. Since the collected hydrogen gas will be saturated with water vapor and at conditions other than at a pressure of 1.00 atm, adjustments must be made to the observed pressure. Dalton’s Law of Partial Pressures states that the total pressure of a gas mixture is equal to the sum of the partial pressures of each gas. In this experiment the pressure of the gas collected in the buret, P Total , will be: P Total = P H2 + P Water or P H2 = P Total P Water The quantity, P Total , is related to the atmospheric pressure, P Air (which varies daily and will be given to you by your instructor), and the difference in levels of water inside and outside the buret tube. This difference in levels, Δ h, can be measured using a ruler. We need to convert the difference in water levels to an equivalent difference in mercury levels. The difference is divided by 13.6, because mercury’s density is 13.6 times greater than water’s. Since the level of water inside the tube is higher than the level outside the tube, Δ h is divided by 13.6 and must also be subtracted from the atmospheric pressure: P Total = P Air 6 . 13 h Δ to give: P H2 = P Air 6 . 13 h Δ P Water The vapor pressure of water, P Water , varies with temperature as seen in this table: Temperature 0.0 0.2 0.4 0.6 0.8 ( o C) 15.0 12.8 12.9 13.1 13.3 13.5 16.0 13.6 13.8 14.0 14.2 14.3 17.0 14.5 14.7 14.9 15.1 15.3 18.0 15.5 15.7 15.9 16.0 16.3 19.0 16.5 16.7 16.9 17.1 17.3 20.0 17.7 17.8 18.0 18.2 18.4 21.0 18.6 18.9 19.1 19.3 19.6 22.0 19.8 20.1 20.3 20.6 20.8 23.0 21.0 21.3 21.6 21.8 22.1 24.0 22.4 22.6 22.9 23.2 23.5 25.0 23.8 24.0 24.3 24.6 24.9 26.0 25.2 25.5 25.8 26.1 26.4 27.0 26.7 27.1 27.4 27.7 28.0 Vapor Pressure for Water, mmHg If we collect a sample of hydrogen gas over water at temperature of 23.6 o C, and P Air = 769 mmHg, Δ h = 67 mm, then P H2 can be calculated as follows: P H2 = P Air 6 . 13 h Δ P Water P H2 = 769 6 . 13 67 21.8 = 742 mmHg = 0.976 atm
3 SAFETY/HYGIENE/WASTE DISPOSAL 1. Always wear your goggles! 2. Caution: many chemicals are corrosive. You should wear gloves and be sure your skin is not exposed. Discard the gloves after use, and wash your hands. Exposure may result in an itchy sensation. If you have any sensation of itching, burning, or tingling, thoroughly flush the area with water. Continue flushing until several minutes after the sensation has subsided. Inform your lab instructor. 3. Never raise containers of solution, especially corrosive solutions, to eye level or above. In particular, avoid this when filling a buret. 4. Never weigh chemicals directly on a balance pan. Use weighing paper, glassware, or other secondary container.

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