LABORATORY EXERCISE Date Submitted: __________________________ Team members: ______________________ ______________________ ______________________ ______________________ The Determination of the Value for Molar Volume Objective/Goal: Using a chemical reaction that produces a gas, measure the appropriate values to allow a determination of the value for molar volume. (How many liters of dry hydrogen gas at room temperature and pressure can be produced per mole of magnesium metal. Brief Overview: The chemical reaction to be used is this: Mg(s) + 2 HCl(aq) ---> MgCl2(aq) + H2(g) This reaction has a 1:1 mole ratio between magnesium used and hydrogen produced. This will allow an easy determination of the moles of H2 by determining the mass of Mg consumed in the reaction. The mass of Mg will be determined by comparing the length of ribbon used to the mass of one meter of ribbon. A hidden assumption of this lab is that two different lengths of the ribbon are uniform in width & thickness. The gas will be collected "over water." This means that the gas produced is in contact with liquid water. The consequence of this is that the gas in the tube WILL NOT be pure H 2; it will have some water vapor mixed in. The mixture is usually called "wet gas" and its production is unavoidable. However, using Dalton's Law of Partial Pressures, we will be able to remove the water vapor's pressure, thereby recovering the "dry" hydrogen gas' pressure. Corrections to the temperature and pressure at the time of the experiment will also be required, since the molar volume value we are seeking is measured at Standard Temperature and Pressure (STP). Using the Combined Gas Law, we will be able to make the necessary corrections. Safety Reminder: Wear goggles during the entire lab, including setting up and cleaning up. You will be asked to leave the laboratory THE VERY FIRST TIME you do not comply with this safety procedure. The hydrochloric acid (3M HCl) we will use can HURT you. Use it with great care. Materials List: Equipment 50 mL gas measuring tube 30 cm piece of copper wire 1-hole stopper 150 mL beaker 600 mL beaker full of water 500 ml graduated cylinder centimeter ruler water bottle pieces of chalk Reagents small piece (≈5 cm) of magnesium ribbon 15 mL of 3 molar (3 mols/ml) HCl Procedure 1) Fill the 600 mL beaker 3/4 full with tap water. 2) Measure the Mg ribbon's length with the ruler. Measure it and record this value in your data table. Your teacher will give you the mass of 1 m of the Mg ribbon. 3) Fold the Mg ribbon over twice, then create the copper wire cage around it. Wind the wire in different directions, so there are many small openings rather than one or two big ones. Leave 8-10 cm or so of wire unrolled to act as a handle so that the cage can be put up to approximately the 50-ml mark in the gas measuring tube. 4) Set up a ring stand and utility clamp in position to hold the gas collecting tube. 5) Fill the gas measuring tube with approximately 15 mL of acid at the front desk, then return to your work area. (The HCl is in excess in this experiment, so don’t fuss about the exact volume of HCl) Those not carrying the tube with acid must yield the right of way to those who are. This is an extra precaution when using concentrated acid). Fig 1: Copper cage containing folded Mg ribbon 6) Tilt the gas collecting tube slightly, and fill it to the top with tap water. Use the 150 mL beaker for this. Pour water slowly so that it forms a layer over the acid, avoid stirring up the acid layer in the bottom of the test tube. The final liquid in the top of the gas collecting tube will contain very little acid. Fill the tube so full with water that the water actually bulges upward from the lip of the gas measuring tube. Use the water bottle for the last few drops. 7) Place the copper wire cage in the open mouth of the gas measuring tube as in figure 2a. The cage should be positioned at about the 50 mL mark on the tube. Hook the copper wire over the edge of the gas collecting tube and clamp it there by inserting the rubber stopper. When properly set up, the tube will contain no air bubbles, and the water will completely fill the hole in the stopper as well as the tube. 8) Place your finger over the rubber stopper holes and invert the gas measuring tube. DO NOT remove your finger until you are under the water level in the next step. Figure 2 9) Place the inverted tube into the 600 ml beaker filled with water, then remove your finger. Clamp the tube in place as in figure 2b. 10) The acid, being denser than water, will stream down through the water and eventually reach the Mg ribbon, and react. 11) As you work, the teacher will read the barometer and thermometer. When the values are announced, record the room temperature and pressure in your data table. 12) After the reaction ceases, let everything sit for 2-3 minutes before continuing. Try to dislodge any bubbles clinging to the sides of the gas collecting tube and caught by the copper wire cage by tapping the tube gently. 13) To measure the volume of gas produced, you must ake the gas pressure inside the tube equal to the room pressure BEFORE measuring the volume. To do this, cover the hole in the stopper with your finger and transfer the gas collecting tube to to a 500 ml graduated cylinder. Raise or lower the gas collecting tube until the water level INSIDE the gas collecting tube is equal to the water level in the graduated cylinder. Be careful: do not raise the gas tube so high that its open end comes above the battery jar's water level. 14) When the gas pressures are equalized, read the volume of gas from the scale on the tube and record this value in your data table. Please note that zero is at the top of the tube. Read the value from the top down, not from the 50 mL mark going up. 15) Remove the gas collecting tube from the graduated cylinder. Hold the gas collecting tube over the 600ml beaker. Do not block the stopper tube, firstly because there is now acid at the bottom of the gas collecting test tube, and secondly, because you are trying to empty the contents of the gas collecting test tube into the beaker. Add a few pieces of chalk to the solution Figure 3: Measuring the volume of gas. In b, room in the beaker and wait until the reaction stops. pressure is equal to the combined pressure of Then you may pour all liquids down the drain, and H2 (g) and H2O (g). rinse all glassware three times with tap water. When done rinsing, let the water run for 2 more minutes. 16) Throw the wire cage in the trash. Do not keep it. Data Table magnesium ribbon length (± mass of 1 meter of Mg (± room temperature (± Room pressure (± cm) g) C) kPa) "wet" gas volume (± mL) Results Sample calculation mass of magnesium used moles of magnesium used. moles of hydrogen gas produced pressure of the "dry" hydrogen gas ж Please see below for some remarks on how to calculate this volume of "dry" hydrogen gas at STP molar volume (in liters/mole at STP) (according to the data from your experiment) percent error result Ж Since hydrogen has collected over water, the gas in the tube consists of a mixture of hydrogen gas and water vapour. The total pressure caused by these two gases is equal to the room pressure. (see figure 4) Mathematically, this can be expressed as: PH2 + PH2O = Proom. The pressure of the room can be determined by reading a barometer. The pressure of the water vapour PH2O, can be determined from table 1. The values in table 1 were obtained by measuring the pressure of water vapour above liquid water at various temperatures. Discussion: Answer these questions, then write out your own discussion. Q1: How did you know the “Mg + HCl” reaction had begun? Q2: After the “Mg + HCl” reaction ceased, why did you let everything sit for 2-3 minutes before continuing? To allow the tube and its contents to come to room temperature, because T affects the behaviour of gases. Q3: You used Boyle’s law to calculate the volume of hydrogen at STP, what 2 parameters must be kept constant in order for Boyle’s law to apply? Q4: Can a liquid have a molar volume? What information would you have to know in order to calculate it? Is the molar volume of a liquid useful? Explain. Conclusion: Write out your own conclusions. Figure 4: Correcting for the vapour pressure of water. Comparing the pressure in the gas collecting of combined pressures of H2(g) and H2O(g) to the pressure of H2(g) alone. T 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Table 1 Vapour Pressures of Water at Various Temperatures kPa T kPa 0.61129 16 1.8185 0.65716 17 1.938 0.70605 18 2.0644 0.75813 19 2.1978 0.81359 20 2.3388 0.8726 21 2.4877 0.93537 22 2.6447 1.0021 23 2.8104 1.073 24 2.985 1.1482 25 3.169 1.2281 26 3.3629 1.3129 27 3.567 1.4027 28 3.7818 1.4979 29 4.0078 1.5988 30 4.2455 1.7056 For teacher version Explanation of Discussion Answer Techniques Some of the sample answers contain instructions for what to write in your work to be turned in. In general, your work should be neat and showing all units. The values you write down MUST support the answer you arrive at for each discussion problem. The explanations for the discussion are based on the following sample data: (a) Mass of 1.00 m of Mg ribbon: 0.809 g (b) Length of ribbon used: 5.30 cm. (c) room pressure: 752.3 mmHg (d) room temperature: 23.0 °C (e) volume of wet gas collected 42.5 mL 1) Mass of Mg used - you must use a proportion (two ratios set equal to each other). The data in (a) and (b) are used. Using the sample data, the value arrived at is 0.0429 g. 2) Moles of Mg used - you must divide by the atomic weight of Mg and the answer is 0.00176 moles of Mg. 3) Moles of H2 produced - this depends on the molar ratio between Mg and H 2. Show a balanced equation in your answer to justify the ratio you arrive at. Then, indicate in writing how many moles of H 2 were produced. 4) Corrected Pressure of H2 gas - this value is determined by using Dalton's Law of Partial Pressures: Ptotal = PH2 + Pwater vapor The total pressure is 752.3 mmHg and the Pwater vapor value must be looked up in a table. The value used MUST be in mmHg. The PH2 is the "corrected pressure" of H2 or the "pressure of the dry gas." It is the value that should be used in the following calculation. 5) Volume of Collected Gas at STP - this calculation will use the Combined Gas Law where P1V1 / T1 = P2V2 / T2. The P2 and T2 values are the standard values, P1 is the corrected pressure (the one from #4), T1 is the room temperature (remember: all temps in Kelvins!!) and V 1 is the volume of gas you measured. Make sure you show how the problem is set up with all units included. The answer is 37.7 mL. 6) Molar volume of H2 - the molar volume of any gas at STP is known to be 22.414 L/mol. To calculate this value, you should take your volume calculated in the step just above and convert it to liters. You do not need to show this step. Then divide that value by the moles of H 2 that was determined in calculation #3. I have deliberately left that value out, BUT I have told you how to get it. I will not tell you the answer to #6, but when rounded to the nearest 0.1 place, it is within 1.0 of the correct answer of 22.4. 7) Percent error - numerator: the absolute difference between the correct (usually called the true) value and the value you measured (the experimental value). Denominator: the true value. Multiply by 100 to get a percentage, round to the nearest 0.1 place. You're done!! Q1: Effervescence Q2: After the “Mg + HCl” reaction ceased, why did you let everything sit for 2-3 minutes before continuing? To allow the tube and its contents to come to room temperature, because T affects the behaviour of gases.