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Replacement of Hydrogen by a Metal


									Lab-05: Replacement of Hydrogen by a Metal
Introduction: The Law of Definite Composition holds true for all compounds. Therefore, it is possible to find out how many grams of one element are required to unite with one gram of another element, hydrogen for example. As a rule, metals do not readily combine with hydrogen, but they do replace hydrogen from compounds, such as hydrochloric acid. Magnesium will react with hydrochloric acid to liberate hydrogen gas. Gases are usually collected by bubbling the gas into the bottom of an inverted tube(eudiometer) filled with water. The gases entering the bottom of the tube displace the water in the tube. The total volume of gas collected in the eudiometer can easily be calculated by measuring the amount of water displaced. When the open end of a eudiometer is placed under the water in a large open container, the pressure against the water in the open container is equal to the atmospheric pressure. If the water level in the eudiometer is equal to the level of the water in the container outside the eudiometer, then the total pressure inside the eudiometer is equal to the atmospheric pressure. If the water levels inside and outside the eudiometer are not equal, then the total pressure inside the tube is not equal to the atmospheric pressure and must be adjusted accordingly. The height difference between the two levels of water must be measured in centimeters. Each centimeter of water in a column corresponds to 0.0978 kilopascals (kPa) of pressure. The total pressure inside the eudiometer can be calculated by the water level difference in centimeters times 0.0978 kPa/cm, then adding or subtracting that term to the atmospheric pressure. (PT = PAtm + or - cmwater x 0.0978 kPa/cm) When a gas like hydrogen is collected in a eudiometer over water, the tube contains both the hydrogen gas and water vapor. Using Dalton's Law, the total pressure in the tube is equal to the pressure of the hydrogen gas plus the pressure of the water vapor: (PT = PHydrogen + Pwater vapor). Thus, the pressure of the H2 gas can be calculated. Once the partial pressure of the hydrogen gas has been calculated and the volume has been measured, the number of moles of hydrogen gas can be calculated using the Ideal Gas Law Formula of PV = nRT: where: P = pressure; V = volume; n = number of moles; T = absolute temperature; R = the Ideal Gas Constant = 8.31 (liter kPa)/(mole K). When the number of moles of hydrogen has been determined, the mass of hydrogen can be determined through the use of gram molecular conversions, and the mass of magnesium can be calculated through stoichiometry. Once these masses have been calculated, their ratios can be easily determined. In this experiment, we shall attempt to find out just how many grams of magnesium are required to replace one gram of hydrogen from hydrochloric acid. Objective: The objective of this experiment is to determine the mass of magnesium required to replace one gram of hydrogen from a compound. Material: Laboratory balance; large battery jar; ring stand; buret clamp; 50ml buret or eudiometer; graduated cylinder; thermometer, Magnesium ribbon; thread; concentrated hydrochloric acid(HCl) Procedure: 1) Fill a battery jar to half full with water , and allow it to sit for a while to equilibrate to room temperature. 2) Measure the length and roll your piece of magnesium in a loose coil. 3) Tie one end of a 25 cm piece of thread to the loose coil of magnesium. 4) Carefully pour about 5 ml concentrated HCl into the eudiometer. (Have the instructor help if needed) 5) Slowly, add enough water to completely fill the eudiometer, being very careful not to mix water with HCl. 6) Lower the tied magnesium coil to a depth of 5 cm into the eudiometer. 7) Place a one-holed rubber stopper in the end of the tube holding the thread in place. 8) Place your thumb firmly over the end of the rubber stopper, and invert the open end of the eudiometer into the battery jar and under the surface of the water. (Make sure no air gets into the eudiometer) 9) Clamp the eudiometer in the upside down position, & allow the reaction to take place. 10) When no magnesium ribbon remains, measure the difference in the water levels to the nearest 0.1 cm. 11) Record the volume of hydrogen gas collected in the eudiometer. 12) Measure the temperature of the solution in the battery jar, and record today's barometric pressure.

Results, Lab Data: Make a data table to record all observations and measurements. Lab 05: Hydrogen Replacement By a metal Experimental Data: Length of Mg strip __________ Volume of gas __________ Names _____________________________________ _____________________________________ Temperature __________  water level __________ Atm Pressure __________ VP in tube __________

Calculations: Determine each of the following 1) _____________ PTotal in the eudiometer. 2) _____________ PH2 in the eudiometer. 3) _____________ Moles of H2 in the eudiometer. 4) _____________ Balanced reaction between HCl & Mg. 5) _____________ Moles of Mg required to produce the H2 in the rxn. 6) _____________ Mass of Mg in your strip.

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