R: The Ideal Gas Constant Pre-Lab Assignment Read the entire laboratory investigation and the relevant pages in your textbook, then answers the questions that follow in the space provided below. 1 Describe how you will obtain the values of P, V, n, and T in this investigation. 2 Why is copper wire used to make the cage for the magnesium ribbon in this reaction? 3 When the gas collection tube is inverted, why does the acid flow downward? 4 Why is it important to tap the side of the graduated cylinder before taking gas volume readings? 5 How can you protect yourself from the hazards of working with 6-M HCl? 6 Where is the proper place to dispose the liquid after the experiment is complete? dd Write a Purpose statement and a Hypothesis statement for this lab.
NAME: DATE: PERIOD: Calculation of the Ideal Gas Constant, R DATA COLLECTION Data for the Calculation of number of moles of Hydrogen Gas Length of Mg ribbon in centimeters Mass of Mg ribbon: 100-cm = 1.00-g m Mg, Mass of Magnesium Data for the Calculation of the Temperature of Hydrogen Gas T H2, Temperature of the reaction system Data for the Calculation of the Pressure of Hydrogen Gas P atm, Atmospheric pressure in cm Hg P atm, in mm Hg P H2O, Vapor Pressure of Water @ T H2 P ht, Water level ht. Difference: cm/13.55 Corrected Pressure = P mm P H2O P ht Data for the Calculation of Volume of Hydrogen Gas Final Volume of Gas Mixture Initial Volume of Gas Mixture Volume of Hydrogen Gas CALCULATIONS Calculation of n (for PV = nrt) Balanced Equation for the Reaction between Magnesium and HCl Find n: Moles of H 2(gas) from of Mg (solid) Convert Pressure to atm. Convert Volume to L. Convert Temperature to K Calculation of R (R = PV/nT)
CRITICAL THINKING 1 Why is it necessary to subtract the value for water vapor pressure from atmospheric pressure to determine the pressure of the H2 gas? (Interpreting Data) 2. List at least three evidences of a chemical reaction did you observe? (Making inferences) 3. At the end of the reaction, how did the appearance of the copper wire compare with that of the magnesium ribbon? What can you conclude about the effect of HCL on copper wire? (Making comparisons drawing conclusions) 4. Using the accepted value for the ideal gas constant, determine the percent error of the value you calculated. 5. Explain the possible sources of experimental error in this investigation.
CHS CHEMISTRY Calculation of the Ideal Gas Constant, R Problem Calculate an experimentally determined value of the ideal gas constant. Materials Chemical splash goggles barometer Metric ruler Jumbo Beral pipet 1.0 cm or less of magnesium ribbon 6.0 M hydrochloric acid (HCL) Beaker, 400-mL wash bottle w/ distilled water 25 cm thin-gauge copper wire thermometer Graduated cylinder, 10-mL table of vapor pressures of water Eudiometer, 50-mL (EXPENSIVE!!) Safety Wear your goggles at all times during the investigation. Hydrochloric acid (HCL) is corrosive. Avoid spills and contact with your skin and clothing. If HCL comes in contact with your skin or clothing (it will damage your clothing) inform your teacher and flush the acid with large quantities of water. Neutralize any acids spills on the work surface with baking soda. The Material Safety Data Sheet for HCl is posted in the laboratory area. Procedure 1. Put on your goggles and lab apron. 2. Using a metric ruler, measure and record the exact length of the piece of magnesium (Mg) ribbon provided by your teacher. The ribbon should be no longer than 1.0 cm. 3. Your teacher will give you the mass of 100.0 cm of Mg ribbon. Record this mass, which will be used as a conversion factor to determine the mass of your piece of Mg ribbon. 4. Wrap the copper wire around the magnesium ribbon; making a cage that surrounds the ribbon, as demonstrated by your instructor. Make a hook on the end of the copper wire approximately as demonstrated by your instructor. 5. Use a Jumbo pipet to add approximately 3 ml (see example) of 6.0 M hydrochloric acid (HCL) to the eudiometer. CAUTION: hydrochloric acid is corrosive. Avoid contact with skin or clothing. Flush any spills with water and notify your teacher. 6. Using the wash bottle, gently fill the eudiometer by drizzling water down its inner side to avoid mixing. See diagram in lab area. Because HCL has a greater density than water, the acid will remain at the bottom of the eudiometer until you start the reaction. 7. Hang the hook of the copper wire onto the edge of the eudiometer as shown by the sample set-up. Top off the liquid in the eudiometer with the wash bottle and cover the end securely with parafilm (this is really neat stuff). 8. With the tip of a pencil (Ticonderoga will do, or equivalent), poke a hole the diameter of the lead in the parafilm. 9. Hold your finger over the hole and quickly invert the eudiometer into the beaker of water. DO NOT ALLOW AIR TO LEAK INTO THE APPARATUS!! 10. When the end of the eudiometer with the parafilm and Mg sample is underwater, you may remove your finger. Rest the cylinder in the beaker, but steady it by holding on to it.
11. Notice the appearance of the acid solution inside the cylinder. Record any indication of a chemical reaction. 12. When the Mg ribbon is no longer reacting, tap the side of the cylinder to release any trapped bubbles. 13. Let the cylinder set for 5 minutes. Look for additional bubbles; release them, as above, if any are present. 14. Read and record the temperature in the beaker to the nearest 10 th degree (ESTIMATE). 15. Determine and record the atmospheric pressure in the lab to the nearest 100 th cm (ESTIMATE!!) 16. Determine the water vapor pressure from a reference table posted in the laboratory area. 17. Record the difference in the height of the water inside the eudiometer with the level of water in the beaker. Measure this in centimeters. 18. Read and record the volume of gas in the eudiometer to the nearest 10 th ml. Don t estimate this. 19. CLEAN UP: After recording the volume of gas, remove the eudiometer from the beaker and dispose of its contents by pouring them into the bucket labeled Eudiometer Waste. Pour the contents of the beaker down the drain. Please remove the parafilm and copper wire before draining the contents of the eudiometer. Note the appearance of the copper wire and return it to your lab station for the next group. Discard the parafilm in the trash. Place any remaining magnesium ribbon in the Metal Waste beaker. 20. Wipe your work area with a damp paper towel and wash your hands before leaving the laboratory.