The Determination of the Value for Molar Volume

Transcription

1 Objective The Determination of the Value for Molar Volume Using a chemical reaction that produces a gas, measure the appropriate values to allow a determination of the value for molar volume. Brief Overview The chemical reaction to be used is this: Mg(s) + 2 HCl(aq) ---> MgCl 2 (aq) + H 2 (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 H 2 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. An 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 1) Wear goggles and an apron during the entire lab, including setting up and cleaning up. 2) Equipment made of glass is breakable. Treat each piece of equipment with proper care. 3) The hydrochloric acid (HCl) we will use can cause severe burns - handle cautiously. Materials List Equipment battery jar 50 ml gas measuring tube (eudiometer) 30 cm piece of copper wire #1 2-hole stopper 150 ml beaker 600 ml beaker centimeter ruler distilled water bottle Reagents small piece of magnesium ribbon 10 ml of 6-molar hydrochloric acid The teacher will use a barometer and thermometer to determine the room pressure and temperature. Procedure 1) If it has not been done for you already, fill a battery jar or bucket 3/4 full with tap water. 2) Measure the Mg ribbon's length to the nearest 0.01 cm and record this value in your data table. 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.

2 4) Fill the gas measuring tube with 10 ml of acid at the front desk, then return to your work area. 5) Use the 150 ml beaker to fill the gas measuring tube to the top with tap water. Pour water slowly to wash acid drops near the open mouth of the tube downward. Fill the tube so full with water that the water bulges upward from the lip of the gas measuring tube. 6) Place the copper wire cage and stopper in the open mouth of the gas measuring tube. The cage should about 3-5 cm into the tube. Be aware that, if there is some acid near the top, the Mg will start reacting immediately. If this happens, continue on to the next steps at a normal rate of work. 7) 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. 8) Place the inverted tube into the water in the bucket, then remove your finger. 9) The acid is denser than the water and will drop down to the Mg ribbon. The reaction will start slowly. KEEP WATCHING!! When the reaction ceases, let everything sit for 2-3 minutes before continuing. Try to dislodge any bubbles that might be caught by the copper wire cage. 10) As you work, the teacher will read the barometer and thermometer. When the values are announced, record them in your data table. 11) Make the gas pressure inside the tube equal to the room pressure BEFORE measuring the volume. You do this by making the water level INSIDE the tube equal to the water level in the bucket. Do not raise the gas tube so high that its open end comes above the water level. 12) Read the volume of gas to the closest 0.01 ml and record this value in your data table. Note that zero is at the top of the tube. 13) Pour all liquids down the drain, and rinse all glassware with tap water. Wash your hands. DATA and CALCULATIONS magnesiun ribbon length Length of long ribbon mass of long ribbon of Mg room temperature room temperature room pressure "wet" gas volume Determine the: cm cm g Celsius Kelvin mmhg ml 1) mass of magnesium used. 2) moles of magnesium used. 3) moles of hydrogen gas produced.

3 4) pressure of the "dry" hydrogen gas. You will need the table below. Vapor Pressure of Water at Various Temperatures in mmhg o C ) volume of "dry" hydrogen gas at STP. (use combined gas law) 6) value (in liters/mole at STP) for molar volume. Use the data from your experiment. 7) percent error. The true value for molar volume at STP is L/mol.

4 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: g (b) Length of ribbon used: 5.30 cm. (c) room pressure: 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 g. 2) Moles of Mg used - you must divide by the atomic weight of Mg and the answer is moles of Mg. 3) Moles of H 2 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 H 2 gas - this value is determined by using Dalton's Law of Partial Pressures: P total = P H2 + P water vapor The total pressure is mmhg and the P water vapor value must be looked up in a table. The value used MUST be in mmhg. The P H2 is the "corrected pressure" of H 2 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 P 1 V 1 / T 1 = P 2 V 2 / T 2. The P 2 and T 2 values are the standard values, P 1 is the corrected pressure (the one from #4), T 1 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 H 2 - the molar volume of any gas at STP is known to be 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 ) 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!!

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