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Molar Volume of a Gas/Calculation of R Name______Lab Period ______Date______I

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Molar Volume of a Gas/Calculation of R Name______Lab Period ______Date______I Molar Volume of a Gas/Calculation of R Name_____________________ Lab Period ______Date__________ I. Introduction The molar volume of a gas is defined as the volume occupied by one mole of gas at standard temperature and pressure (STP). The theoretical value for the molar volume of an ideal gas at standard conditions is 22.4 L/mol. Since this is the volume occupied by one mole, it follows that the volume contains Avogadro’s number of molecules. The mass in the molar volume would be equal to the molecular mass of the gas in grams. When magnesium metal reacts with hydrochloric acid, hydrogen is produced. The gas can be collected in a graduated cylinder where its volume may be determined. Knowing the number of moles of magnesium used, we can calculate the volume of hydrogen produced per mole of magnesium consumed. The balanced equation for this reaction allows us to determine the volume of one mole of gas at standard temperature and pressure. After completing this experiment, you should be able to determine the molar volume of a gas. You will also collect this gas by water displacement and make a standard pressure and temperature comparison to the actual value. II. Pre-Lab Disscussion 1. Write a balanced equation for the reaction between hydrochloric acid and magnesium. III. Purpose The purpose is to experimentally determine the molar volume of hydrogen gas at standard temperature and pressure using Dalton’s Law of Partial Pressure and the Combined Gas Law. IV. Materials and Equipment safety goggles thermometer hydrochloric acid (6 M HCl) 25 mL graduated cylinder magnesium ribbon (Mg) 400 mL beaker V. Procedure 1. Put on your lab goggles. Fill a 400 mL beaker ¾ full of water. Fill 25 mL graduated cylinder to the very top. Place a small piece of paper towel covering the top of the graduated cylinder. Make sure it is just a tiny bit bigger than the mouth of the cylinder. Flip the graduated cylinder into the beaker, make sure you don’t have any bubbles. (Note: point the spout away from the direction you are flipping the cylinder) Practice flipping the graduated cylinder a few times to perfect your technique. 2. Once you can flip the graduated cylinder without air bubbles, get a strip of Mg ribbon. Measure your Mg ribbon with a metric ruler and record the length in cm on your data chart. 3. Pour 7 mL of HCl into the graduated cylinder. SLOWLY tilt the graduated cylinder, fill the graduated cylinder with COLD water to the very top. 4. Float the Mg on top of the water. Cover with a piece of paper towel and flip the graduated cylinder into the beaker, like you practiced. 5. MAKE SURE THE MAGNESIUM RIBBON DOESN’T FLOAT OUT FROM THE GRADUATED CYLINDER. 6. While the reaction is progressing, place the thermometer into the beaker and record the temperature. 7. When the reaction is complete, you do not see any Mg; hold the graduated cylinder vertically and read the bottom of the meniscus. Record the volume on your data chart. Molar Volume of a Gas/Calculation of R Name_____________________ Lab Period ______Date__________ 8. Record the atmosheric pressure from the whiteboard on your data chart. 9. After all of the data is recorded, take the graduated cylinder and paper towel out of the beaker and pour the water down the drain. Put equipment back into the bin. VI. Observations Lab group number Length of Mg ribbon cm Mass of 100.0 cm of Mg (on board) g Volume of H2 mL Temperature of Reaction ˙C Atmospheric pressure (on board) torr Water vapor pressure (on board) torr VII. Results and Discussion – Determining Molar Volume 1. Determine the mass of Mg used in the reaction using the length of Mg ribbon you used and the mass of 100.0 cm Mg ribbon provided by the teacher. _______________________ g Mg 2. Determine the value of V1. Calculate the volume of gas collected in liters. Then convert the volume units from mL to L. 1000 mL = 1 L V1=______________________L 3. Determine the value of T1. Convert the temperature into Kelvin. T1=______________________K 4. Determine the value of P1. Calculate the pressure of the dry H2 gas collected over water using Dalton’s Law of Partial Pressure. P1 = ____________________ atms 5. Determine the value of V2. Calculate the volume of H2 at STP (T2 and P2) using the Combined Gas Law. V2=______________________L Molar Volume of a Gas/Calculation of R Name_____________________ Lab Period ______Date__________ 6. Calculate the moles of H2 from mass of Mg. (mass – mol problem) n = __________________ mols H2 7. Calculate the molar volume of H2. (Molar Volume = L/mols) Molar volume H2 = _____________L/mol 8. Calculate the percent error. (calculated – actual)/ actual * 100 [actual = 22.4L/mol] Determining the Value of R 1. Determine the mass of Mg used in the reaction using the length of Mg ribbon you used and the mass of 100.0 cm Mg ribbon provided by the teacher. _______________________ g Mg 2. Determine the value of n. Calculate moles of H2 from mass of Mg. (mass – mol problem) Use the balanced equation from the Pre-Lab and the number of grams of Mg that reacted to calculate the moles of H2 produced. n = _______________________ mols H2 3. Determine the value of P. Calculate the pressure of the dry H2 gas collected over water using Dalton’s Law of Partial Pressure. Convert your pressure units from torr to atmospheres. P = ____________________ atms 4. Determine the value of V. Calculate the volume of gas collected in liters. Then convert the volume units from mL to L. V=______________________L Molar Volume of a Gas/Calculation of R Name_____________________ Lab Period ______Date__________ 5. Determine the value of T. Convert the temperature units of the gas collected from ˚C to kelvins. T=__________________________K 6. Using the pressure, volume, temperature, and moles of H2, calculate the value of the gas constant where R = PV/nT. Include all units in your answer. R=______________________L*atm/mol*K 7. Calculate the percent error. (calculated – actual)/ actual * 100 [actual = 0.0821 L*atm/mol*K] Conclusion: Why does the acid flow down the test tube when it is inverted? What was your percent error? Explain where your group may have made an error in this lab. .
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