Name: ______

Molar Volume of a Gas – the Ideal Gas Law

Introduction

The Ideal Gas Law is represented by the formula PV = nRT, where R is the ideal gas constant. In this laboratory investigation you will experimentally determine the value of R. To do this, you must first determine the values of the other variables in the ideal gas equation. You will generate a collect a sample of hydrogen gas and determine its volume, temperature, pressure, and the number of moles produced under laboratory conditions.

The hydrogen gas is generated in a graduated cylinder from the reaction between magnesium and hydrochloric acid. By wrapping the magnesium ribbon in a copper wire cage, you can ensure that the magnesium will remain in the acid environment. Hydrochloric acid is in excess in the reaction so that the moles of hydrogen gas produced may be determined from the moles of magnesium that react.

Hypothesis

Pre-Lab Discussion

  1. Describe how the values for P, V, n, and T are obtained in this investigation. Answer:
  1. Why do you think copper wire is used to make the cage for the magnesium ribbon in this reaction?
  1. When the graduated cylinder is inverted, why does the acid flow downward? Answer:
  1. How can you protect yourself from the hazards of working with 3M HCL? Answer:

Materials

Chemical splash goggles tap water

Laboratory apronlatex gloves

Metric ruler micropipette

1.0cm or less of magnesium ribbon 3.0M hydrochloric acid (HCL)

25cm thin-gauge copper wire one-hole rubberwash bottle

Stopper to fit graduated cylinderThermometer

Graduated cylinder, 10mL beaker, 400mLtable of vapor pressures of water

Safety

Wear goggles, lab aprons, and gloves at all time. HCl is corrosive, if HCl come in contact with skin wash with large quantities of water. Neutralize any acid spills on the work surface with baking soda. When inserting the stopper into the graduated cylinder, tap it down gently to avoid breaking the top of the cylinder.

Procedure

  1. Put on goggles and lab apron.
  2. Using a metric ruler, measure a record the exact length of the piece of magnesium ribbon provided by your teacher. The ribbon should be no longer than 1.0 cm.
  3. Record the mass of 100.0 cm of Mg ribbon. It 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. Leave a handle of copper wire approximately 6 cm long.
  5. Insert the handle end of the copper wire into the one-hole rubber stopper. When the stopper is inserted into the graduated cylinder, the copper wire cage and Mg ribbon will be inside the cylinder.
  6. Fill the 400-mL beaker or other container approximately half full with water.
  7. Use a dropper or micropipette to add approximately 3 mL of 3.0 M hydrochloric acid (HCL) to the graduated cylinder.
  8. Using the wash bottle, gently fill the graduated cylinder by drizzling water down the cylinders inner side to avoid mixing. Because HCL has a greater density than water, the acid will remain at the bottom of the cylinder.
  9. Insert the stopper into the graduated cylinder by tapping gently or so as to avoid cracking the cylinder. The copper wire cage should be suspended at the top of the cylinder. Holding your finger over the hole in the rubber stopper, quickly invert the cylinder into the beaker of water. When the top of the cylinder is under water you may remove your finger. Rest the cylinder in the beaker.
  10. Notice the appearance of the acid solution inside the cylinder. Record any indication of the chemical reaction.
  11. Then the magnesium ribbon is no longer reacting, tap the side of the cylinder to release any trapped bubbles.
  12. Let the cylinder sit for five minutes. Using the thermometer read and record the temperature in the beaker.
  13. Determined and record the atmospheric pressure in the lab. Determine the water vapor pressure from a reference table.
  14. Lift the graduated cylinder slightly until the levels of water inside and outside the cylinder are the same.
  15. Read and record the volume of gas in the cylinder. Remember that you are reading an inverted cylinder.
  16. After reading the volume of gas, remove the cylinder form the beaker and dispose of the contents of the beaker by pouring down the drain. Turn the cylinder right side up, remove the stopper holding the copper cage, and dispose of any remaining liquid down the sink.
  17. Clean workstation and wash hands before leaving.

Observations

Length of Mg ribbon_____

Mass of 100.0 cm Mg _____

Temperature of the reaction system _____

Atmospheric pressure_____

Water vapor pressure at system temp. _____

Volume of gas produced _____

Calculations-

  1. Calculate the number of moles of Mg that reacted, using the length of Mg ribbon you used, the mass of 100.0cm Mg ribbon provided by your teacher, and the molar mass of Mg.
  1. Write the balanced equation for the reaction between Mg and HCl.
  1. Determine the value of n. Use the balanced equation and the number of moles of Mg that reacted to calculate the moles of H2 produced.
  1. Determine the value of P. Calculate the pressure of the H2 gas collected by subtracting the water vapor pressure from the atmospheric pressure. Convert your pressure units from mmHg to atmospheres.
  1. Determine the value of V. Calculate the volume of gas collected in liters. Remember that you must read the bottom of the meniscus, but that the scale is inverted. Then convert the volume units from mL to L.
  1. Determine the value of T. Convert the temperature units of the gas collected from oC to kelvins.
  1. Using the pressure, volume, temperature, and moles of H2, calculate the value of the gas constant where R=PV/nT. Include all units in the answer.

Critical Thinking: Analysis and conclusions

  1. Why is it necessary to subtract the value for water vapor pressure from atmospheric pressure to determine the pressure of the H2 gas? Answer:
  1. What evidence of a chemical reaction did you observe? Answer:
  1. 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?
  1. Using the accepted value for the ideal gas constant, determine the percent deviation of the value you calculated. Then explain the possible sources of experimental error in this investigation.

Critical Thinking:

  1. What is the importance of your choice of units in expressing the value of the ideal gas constant?
  1. Convert the pressure of dry H2 gas to kilopascals and calculate the value of R in kPa-L/ mol-K. Answer:
  1. If all other conditions remained the same, how would the value of R change if your investigation made use of a gas other than hydrogen? Explain. Answer:
  1. How could you demonstrate that the copper wire did not participate in the chemical reaction? Answer:

Conclusion