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Chemistry 108 Lab # 3 1 Name_ __________ __________________ Prelab #3 Gases: Percent Yield of Hydrogen Gas from Magnesium and Hydrochloric Acid Introduction For chemical reactions involving gases, gas volume measurements provide a convenient means of determining stoichiometric relationships. A gaseous product is collected in a long, thin graduated glass tube, called aeudiometer , by displacement of a liquid, u sually water. Magnesium reacts with hydrochloric acid, producing hydrogen gas: Mg ( s ) + 2HCl ( aq )  MgCl 2 ( aq ) + H 2 ( g ) Note: for every mole of Mg (s) that is reacted, one mole of H 2 (g) is produced. If we know the mass of Mg(s) we can convert to mole s of Mg(s). Then, since we get 1 mole of H 2 (g) for every mole of Mg(s), we can predict how many moles of H 2 (g) could be made (theoretical yield). We use an excess of HCl so that we would react all the Mg(s) before we used all of the HCl. Eudiometer Tube Set – up Reaction Set – up (after inverting eudiometer tube) Chemistry 108 Lab # 3 2 When the magnesium reacts with the acid, the evolved hydrogen gas is collected by water displacement and its volume is measured. The temperature of the gas is taken to be the same as the temperature of the water it is in contact with because, given a sufficient amount of time, the two will reach thermal equilibrium. The level of water in the eudiometeri s adjusted s o that it is equal to the level of water outside the eudiometer. This insures that the pressure in the eudiometer is equal to the prevailing atmospheric pressure. Because the hydrogen gas was collected above water, and water has a significant v apor pressure, to get the pressure of pure hydrogen (dry hydrogen), we must subtract the vapor pressure of water. The pressure of the dry hydrogen gas ( P H 2 ) is calculated from Dalton’s Law of Partial Pressures: P total = P H 2 + P H 2 O so P H 2 = P total – P H 2 O where P total (the pressure in the eudiometer) is atmospheric pressure, and P H 2 O (the water vapor pressure) is the pressure exerted by water vapor that has evaporated into the eudiometer. We will get the vapor pressure of water from the table below of vapor pressure vs. temperature. The number of moles of hydrogen gas collected can then be calculated from the ideal gas law: ( n = # moles H 2 ) n = PV (Use P H 2 here, not P total ) R T This will give you the experimental # moles of hydrogen gas collected . The theoretical # of moles of H 2 (g) can be calculated by converting the mass of Mg to moles Mg, and understanding that we get 1 mole of H 2 from every mole of Mg(s). From the theoretical yield and the experimental yield , one can calculate the percent yield : Percent Yield = experimental # moles H 2  100% theoretical #mole H 2 Chemistry 108 Lab # 3 3 PRELAB QUESTIONS: 1. Suppose you did this experiment and obtained the following data. The initial mass of magnesium metal was 0.0426 g. The volume of the gas produced in the eudiometer was 41.7 5 mL, the atmospheric pressure is 763 .2 torr , and the water temperature is 24 .1  C . As if you carried o ut this experiment and obtained the following data, fill in the data table and do the calculations below. DATA TABLE : Mass of Magnesium Metal .g Volume of Gas . mL Temperature of Gas ( a s s u m e d t o b e t h e s a m e t e m p . a s t h e w a t e r ) .  C Atmospheric Pressure .torr Water Vapor Pressure (at the above temperature , see table on last page ) .torr CALCULATIONS: 1. Theoretical (calculated) yield of H 2 gas (# moles H 2 ). a) convert mass Mg to #moles Mg b) Convert # moles Mg reacted to moles of H 2 that could be produced. (1 mole H 2 is produced for every 1 mole Mg reacted – this comes from the balanced chemical equation ): Mg ( s ) + 2HCl ( aq )  MgCl 2 ( aq ) + H 2 ( g ) # moles H 2 = _____________________________________ ( theoretical yield of H 2 ) 2. Experimental yield of H 2 gas (# moles H 2 ). a) Determine pressure of dry H 2 ( P H 2 ) by subtracting the vapor pressure o f water from the total pressure, P total = P H 2 + P H 2 O so P H 2 = P total – P H 2 O where P total (the pressure in the eudiometer) is atmospheric pressure, and P H 2 O (the water vapor pressure) is the pressure exerted by water vapor that has evaporated into the eudiometer. Use the table provided on the previous page to find the vapor pressure of water as a function of temperature. P H 2 = _____________________torr b) Convert this pressure from torr to atm (760.0 torr = 1atm) P H 2 = _____________________ atm Chemistry 108 Lab # 3 4 c) Use the Ideal Gas Equation to calculate the number of moles ( n ) of H 2 that you produced in your experiment (experimental yield). Make sure to use the correct units so that they match the units in the gas constant (R). 3. Calculate the Percent Yield. % Yield = experimental # moles H 2  100% theoretical #mole H 2 % Yield =______________________________ OTHER PRE – LAB QUESTIONS 1. When the volume of gas is measured in a eudiometer, the water levels are the same on the inside and outside of the eudiometer . Why do we need to do this ? __________________________________________________________________________ ________________________________________________________ __________________ __________________________________________________________________________ __________________________________________________________________________ 2. If a gas is collected by wate r displacement in a eudiometer , the atmospheric pressure is known, and the water vapor pressure is known, what equation will allow you to calculate the pressure of the dry gas? __________________________________________________________________________ _________________________________________________ _________________________ __________________________________________________________________________ _________________________________________________________________________