Molar Volume of Gas

Background Information:

When magnesium reacts with hydrochloric acid (HCl), hydrogen gas is produced. The gas can be collected in a test tube 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 chemical 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 theoretical value.

Materials:

600 mL beaker, Thermometer

Flask, Polyethylene Pipet , 12M HCl

15 x 125mm (medium) test tube

1-hole rubber stopper to fit the test tube

Magnesium metal ribbon, 1 to 1.5 cm in length

Procedure:Caution: Put on your goggles and apron now!!

Warning: Concentrated hydrochloric acid is caustic and corrosive. Avoid contact and immediately rinse all spills with copious amounts of water.

  1. Fill your 600mL beaker two-thirds full of water of room temperature tap water. Take the temperature of the water and record. #5 on data table.
  2. Obtain a piece of magnesium ribbon from your instructor. Make sure that it has a length between 1.0cm and 1.5cm. Measure the length of the magnesium as precisely as possible and record. #1 on data table.
  3. In the fume hood, add 25drops of 12M concentrated HCl to your test tube.
  4. While holding the test tube in a tipped position, very slowly pour water from another beaker into the test tube, being careful to layer the water over the acid so that they do not mix. Add enough water to completely fill the test tube.
  5. Over the sink, place the 1-hole stopper in the test tube without creating air bubbles.
  6. Place the magnesium ribbon into the acid-water mixture through the hole in the stopper.
  7. With your finger covering the hole in the stopper, invert the test tube and place it in the 600mL beaker, being careful not to allow any air bubbles in the tube. Be sure that the magnesium ribbon does not fall out of the stopper and into the beaker.
  8. Hold the test tube against the bottom of the beaker. The acid will flow down the cylinder until it reaches the magnesium and the reaction will start. The magnesium might float into the test tube to meet the acid.
  9. Make sure all the magnesium has reacted with the acid before measuring the volume of the hydrogen gas created. If the magnesium should float to the surface, swirl the test tube until all the magnesium has reacted.
  10. Before you measure the volume of the hydrogen, make sure that the water level inside the tube is at the same height as the water in the beaker.
  11. Mark a line on the test tube with a lab marker separating the hydrogen gas created and the water still in the test tube.
  12. Remove the test tube from the water and empty its contents in the sink.
  13. Fill the test tube to the line with tap water and then measure this volume in a 25ml graduated cylinder. Record this as V1 in your data table, #4.
  14. Be sure to record the air pressure in the room using a barometer, #6, and the vapor pressure of water, #7, based on the temperature of your water.
  15. Repeat steps #1 through #14 one more time with another piece of magnesium.

Data Table: SHOW ALL WORK in the analysis section of your lab report!!!

Trial #1 / Trial #2 (UNITS)
1. Length of Mg (measured) / cm
2. Mass of Mg (conversion) / g
3. Moles of Mg (conversion) / mol
4. Volume of H2 (measure) = V1 / ml
5. Temp. of H2 (same as H2O) = T1 / K
6. Atmospheric Pressure (measure) / mmHg
7. Vapor Pressure of H2O (chart) / mmHg
8. Volume of H2 (calculation) = V2 / ml
9. Exp. Molar Volume of H2 (conversion) / L/mol
10. Theoretical Molar Volume of H2 / 22.4L/mol / 22.4L/mol
11. Experimental Error (calculation) / L/mol
12. Percent Error (calculation) / %

2. Use 1m = 0.840g and 1m = 100cm as conversions.

3. Use 1mol Mg = 24g Mg (from the periodic table) as conversions.

5. Use K = ˚C + 273.

8. Use the Combined Gas Law P1V1/T1 = P2V2/T2.Assume P2 and T2 are at STP (standard temperature = 0˚C = 273K and standard pressure = 760mmHg = 101.3kPa = 1atm)

And P1= PH2 = Patm – PH2O(Dalton’s Law of Partial Pressures).

9.Divide V2 by the molesof hydrogen gas that you should have created in the reaction, knowing that the mole ratio of Mg to H2 from the balanced chemical equationis 1:1 and then use 1L = 1000ml. (i.e. The moles of hydrogen produced should be the same as the moles of magnesium used in the reaction.)

11. Theoretical Molar Volume of H2- Experimental Molar Volume of H2(subtract).

12. The absolute value of the Experimental Error / Theoretical Molar Volume of H2x 100

Analysis: SHOW ALL WORK for the calculations done for the data table.

Conclusion:

  1. Why does the acid flow down the graduated cylinder when it is inverted?
  2. Write the balanced chemical equation for the reaction between magnesium and hydrochloric acid. What type of chemical reaction is this?
  3. How does your experimental value of the molar volume compare with the theoretical value? Why would there be a difference?
  4. If a student’s error was greater than 10%, what are some possible reasons?
  5. What are three commercial uses of hydrogen gas? Use your text or Google.
  6. How does this lab relate to the concepts that we are currently discussing in class? Specifically, which concept is being reinforced? Be detailed and clear in your response.