Molar Mass and the Ideal Gas Law Prelab

Transcription

1 Molar Mass and the Ideal Gas Law Prelab Name Total /10 SHOW ALL WORK NO WORK = NO CREDIT 1. What is the purpose of this experiment? 2. Determine the mass (in grams) of magnesium metal required to produce 40.0 ml of H 2 with a temperature of 26.9 o C and an atmospheric pressure of 745 mmhg. Be sure to take into account water vapour using equation (3). 3. A sample of a halogen gas has a mass of g and exerts a pressure of torr at 14.0 o C in a ml flask. Calculate the molar mass of the halogen gas. Is the halogen gas F 2, Cl 2, Br 2 or I 2? R = L atm/mole K 1 atm = 760 torr 1

2 Molar Mass and the Ideal Gas Law This experiment will enable you to collect a gas (H 2 ) evolved in a given reaction and measure its temperature, volume and pressure. Assuming this is an ideal gas, the number of moles of hydrogen gas formed in this reaction can be calculated using the ideal gas equation. The molar mass of an unknown metal can be determined by finding the amount of hydrogen gas produced when reacting it with hydrochloric acid. Special mention goes to Brian Stahl, Jon Witkowski, Leigh Bukowski and Nadia Szymanski for developing this laboratory experiment to be used here in the Behrend chemistry curriculum. Introduction Hydrogen gas (H 2 ) is colorless, odourless and the most abundant element in the universe. Hydrogen combines with oxygen to make water which is essential to life. Combined with carbon, it makes up a large class of organic compounds called hydrocarbons. Hydrocarbons include the gas you use for your car to the propane you use to light your barbecues. Hydrogen gas is also used to produce ammonia, which is used for fertilizer production. Hydrogen is also a very flammable and explosive gas. Hydrogen gas, when mixed with oxygen gas, will explode upon ignition and burn violently. Great care must be taken when working with hydrogen gas. In the laboratory, hydrogen gas can be generated by reacting acids with metals such as magnesium. The magnesium oxidizes to the magnesium ion and the hydrogen is reduced to the diatomic gas. The known reaction being performed in today s experiment is: Mg (s) + 2HCl (aq) MgCl 2 (aq) + H 2 (g) (1) In this experiment, HCl is present in excess, so each mole of magnesium you start with should lead to one mole of H 2 gas. The reaction you will be working with in the second part of this experiment is: M (s) + 2HCl (aq) MCl 2 (aq) + H 2 (g) (2) where M represents an unknown metal. Again, HCl is in excess so each mole of hydrogen produced came from one mole of unknown metal. The unknown metal will be identified by determining its molar mass. In Part I of this experiment, you will test this stoichiometric relationship from equation (1) with a known amount of magnesium. In Part II, you will use this relationship from equation (2) to determine the molar mass of the unknown metal you are given. In order to calculate this molar mass for Part II, you will first have to determine how many moles of hydrogen gas are produced in the reaction. 2

3 We are going to assume ideality and use the following equation: n H 2 (Patm PH O )V 2 b (3) RT where the value of the constant R is ml atm/mole K and V b, is the volume change in the buret. P atm is the atmospheric pressure and PH 2O is the vapor pressure of water at a certain temperature. The temperature used in the equation is the temperature measured after the reaction has occurred since the reaction between metals and acids is exothermic. A table of these vapor pressures at different temperatures is at the end of this laboratory experiment. Be sure to estimate between temperatures when necessary. Remember, when converting from mmhg to atm, the conversion is 1 atm = 760 mmhg. Procedure Part I Data that you should have written down for EACH trial for Part I are: Chemical reaction. Atmospheric pressure (P atm ) Mass of Magnesium Moles of Magnesium Theoretical yield of moles of H 2 (eq (1)) Final Buret reading Initial Buret reading Volume change (V b ) Temperature of solution after reaction PH 2O (based on temperature listed on pg 6) P atm - PH 2O Moles of H 2 gas produced experimentally (eq (3)) % yield of H 2 Average % yield of H 2 Show a sample calculation for everything in your notebook. 1. Obtain a small test tube and rinse with distilled water. Weigh approximately g g of magnesium. Be accurate to 0.1 mg ( g). Do not weigh out more than you need, if this happens, cut some of the magnesium and reweigh. 2. Place the magnesium in the small test tube. 3. Your apparatus for this experiment has already been set up for you and your instructor will inform you of the names of the different parts of the apparatus. Check that all of the connections on the apparatus are snug so there is no leakage and that the rubber tubing isn t pinched or folded shut. 3

4 4. To test for leaks, be sure all the stoppers are in place and raise the leveling bulb to maximum height. This increases the pressure inside the apparatus. Watch the water level inside the buret for a minute or so. If there is no change, then your apparatus is airtight. If the level rises slowly and keeps rising, there is a leak and you must find it. If the level stops rising when you pinch the rubber tubing connecting the large test tube to the buret, you have found the source of your leak. If the level continues to rise, the leak is in a connection near the test tube. When your apparatus is working correctly, return the leveling bulb to a level near the top of the buret. 5. Measure 15 ml of 6M HCl and pour it into the big test tube. 6. Carefully, lower the small test tube containing the magnesium sample into the large test tube. Do not allow the magnesium and the acid to come in contact. 7. Recheck your apparatus for leaks. Before you begin the reaction, you have to stabilize the pressures both inside and outside the system to measure the initial volume in the buret. This means both pressures must be the same. First, open the system by removing the rubber stopper from the large test tube. Raise the leveling bulb until the water level in the buret is between 0.00 and 1.00 ml. At this point, insert the rubber stopper back into the large test tube. Place the leveling bulb next to the buret so the water levels in both are identical. The pressures are now identical both inside and outside the system. Read the volume in the buret while the pressures are identical. 8. Record the initial volume in the buret. Remember that the large markings on the buret correspond to millilitres and the smaller markings to 0.1 millilitres. You initial volume should be recorded to the nearest 0.01 ml 9. One student should be keeping the levels in the buret and leveling bulb fairly constant throughout step 10 while the other student performs step Tilt the large test tube so that a little of the hydrochloric acid solution flows into the small tube containing the magnesium metal. Bubbling should occur and the water in the buret will drop rapidly. Repeat this step until all of the magnesium metal has disappeared and there is no longer any bubbling. Keep tipping gently to insure good mixing. Avoid letting the solution from the test tube flow into the buret. 11. When the reaction is complete, place the leveling bulb next to the buret so the water levels are again identical. Record the final volume reading on the buret. 12. Once the final volume has been recorded, take the stopper off the big test tube and measure the temperature of the solution in the big test tube. 13. Calculate the moles of hydrogen produced experimentally and compare with the theoretical moles of hydrogen based on the mass of magnesium you measured in step Repeat steps 1-13 one more time and calculate the average percent yield of H 2. 4

5 Part II Data that you should have written down for EACH trial for Part II are: Unknown Metal (letter/number) Chemical reaction Atmospheric pressure Mass of Unknown Metal Final Buret reading Initial Buret reading Volume change (V b ) Temperature of solution after reaction PH 2O (based on temperature listed on pg 6) P atm - PH 2O Moles of H 2 gas produced (eq (3)) Moles of unknown metal (eq(2)) Molar Mass of unknown metal Average Molar Mass of unknown metal Identity of Unknown Metal Show a sample calculation for everything in your notebook. 1. You will be given an unknown metal to work with for Part II. Weigh out between and g of your unknown metal. 2. Repeat the same procedure that you performed in Part I. Do this twice. 3. If you find there is inconsistency in your results, you may want to perform a third run and disregard the most inconsistent run. Should you do this, explain your reasons for doing so. 4. Calculate the molar mass of your unknown metal and identify the unknown metal. 5

6 Vapor Pressure of Water at Different Temperatures Temperature ( o C) Vapor Pressure (mmhg) Temperature ( o C) Vapor Pressure (mmhg)