CHEMISTRY LAB



LAB: Molar Volume of a Gas

determined via a reaction between magnesium and hydrochloric acid

Introduction

This lab will investigate the relationship between the volume of hydrogen gas generated compared to the moles of hydrogen gas generated – in other words, you will be finding the liters per mole of hydrogen gas. You will be mixing magnesium metal (Mg) and hydrochloric acid solution (HCl). The reaction will generate hydrogen gas, H2 (g). The unbalanced reaction can be written as:

Mg (s) + HCl (aq) ( MgCl2 (aq) + H2 (g)

For all trials, the hydrochloric acid will be in excess (assuming you follow the directions).

Materials

|Magnesium metal |50 mL beaker |Vernier gas pressure sensor and tubing |

|1.0 M HCl |600 mL beaker |Vernier interface, cables, and PC |

|Electronic Balance |100 mL graduated cylinder |Vernier temperature probe |

|125 mL Erlenmeyer Flask |plastic syringe |2-hole rubber stopper |

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Figure 1

Pre-lab

1. Determine the volume of the flask when it has the stopper in place. It is NOT the number given on the flask. Hint: figure out how much water the flask will hold with the stopper on.

Record the volume of the flask here: ______________________________

Safety: Hydrochloric acid is a caustic acid and should be handled with caution. Safety goggles must be worn at all times during this lab.

Procedure

1. Set up the lab apparatus as directed by your teacher. See Figure 1 on the previous page.

2. Turn on the computer. Double-click the Logger Pro icon. Go to File > Open > Advanced Chemistry w Vernier > 05 Molar Volume. Click Open.

3. Connect the temperature probe and gas pressure sensors to the Vernier Lab Pro (green box). Be sure the cable is connected properly from the Lab Pro to the PC.

4. Connect the gas pressure sensor to the stopper via the plastic tubing and be sure all connections are snug.

5. In the Logger Pro software, go to Experiment > Data Collection. Set Length to 700 seconds and Sampling Rate to 1 samples/second.

6. For trial 1, add 0.01 g of Mg to the flask. Record the exact mass given on the balance in Data Table 1.

7. Measure 5 mL of 1.0 M hydrochloric acid into the syringe by placing a small amount of HCl in a 50-mL beaker and then drawing it up into the syringe.

8. Connect the syringe to the stopper with the valve closed by twisting it.

9. Prepare a room temperature water bath in a large beaker and submerge the flask. The bath should be deep enough to completely cover the gas level in the flask. Place the temperature probe in the water bath.

10. Press “Collect” to begin data collection.

11. After about 20 seconds, open the valve on the syringe, press the plunger to add the 5 mL of hydrochloric acid to the flask, pull the plunger back to its original position, and close the two-way valve.

12. Keep the flask immersed in the water bath as the reaction proceeds. Once the maximum pressure is reached you may stop collecting data. Note: this is NOT the initial peak you may see at the start of the reaction.

13. Carefully remove the stopper from the flask to relieve the pressure in the flask. DO NOT open the two-way valve to release pressure in the flask.

14. Record the temperature, initial pressure, and maximum pressure in Data Table 1.

15. Empty and rinse the flask with deionized water.

16. Repeat the procedure for trials 2 and 3. Note: the mass of Mg changes for each trial!

Data Table 1

| |Trial 1 |Trial 2 |Trial 3 |

|Approximate Mass of Mg (g) |0.01 |0.02 |0.03 |

|Actual Mass Mg Used (g) | | | |

|Volume H2 Generated (L) | | | |

|Temperature (K) | | | |

|Initial Pressure (kPa) | | | |

|Maximum Pressure (kPa) | | | |

|Pressure change (kPa) | | | |

Calculations

1. Calculate the moles of Mg used for each trial and enter it in Data Table 2 below.

2. Using the balanced equation and stoichiometry, calculate the moles of H2 gas generated for each trial. This is a theoretical amount, but for purposes of this lab we’ll assume it’s the same as the actual amount in your flask.

Show your work for at least one of your trials here:

Data Table 2

| |Trial 1 |Trial 2 |Trial 3 |

|Moles Mg Used | | | |

|Moles H2 Generated | | | |

Analysis

Part One: Determining the Molar Volume of a Gas

Gases under what we call “ideal” conditions behave the same – they will take up the same volume and exert the same pressure assuming all other things are equal. It does not matter what type of gas atom or molecule it is. Carbon dioxide (CO2) will behave the same as hydrogen (H2), which will behave the same as xenon (Xe).

Because gas volumes/pressures change if temperature or pressure changes, chemists use conditions called Standard Temperature and Pressure (STP) as a way of comparing gases. STP is 273.15K (0.00°C) and 101.325 kPa (1.000 atm) of pressure. If we always correct to STP then we will always be comparing gases under the same conditions.

3. Before you can determine the molar volume of a gas, we must correct the conditions in our lab to STP using the combined gas law. You should use the pressure change as P1, volume H2 generated as V1, and the temperature as T1 (all three values are in Data Table 1). P2 and T2 are STP, which means you will be solving for V2.

Show your work for at least one of your trials here:

Data Table 3

| |Trial 1 |Trial 2 |Trial 3 |

|Volume of H2 gas at STP (L) | | | |

4. Now that you have your volumes corrected to STP you can calculate the Molar Volume of a gas (in other words, the volume of one mole of your gas). To do this, divide the volume of H2 gas by the moles H2 generated for each trial.

Show your work for at least one of your trials here:

Data Table 4

| |Trial 1 |Trial 2 |Trial 3 |Average |

|Molar Volume (L/mol) | | | | |

5. The accepted value for the Molar Volume of a gas at STP is 22.4 L/mol. Compare your average Molar Volume from Data Table 3 to the accepted value by calculating a percent error.

Show your work for percent error here:

Part Two: Determining the Ideal Gas Constant

Recall that the ideal gas law relates pressure, volume, moles, and temperature. Another way we can check our lab results is by calculating R, the gas constant, from our lab data and comparing it to the accepted value of R.

Ideal Gas Law: PV = nRT

Data Table 5

Note: Transfer the appropriate values from Data Table 1 and 2 to the table below. Then solve for R.

| |Trial 1 |Trial 2 |Trial 3 | |

|Temperature (K) | | | | |

|Volume H2 (L) | | | | |

|Pressure of H2 (kPa) | | | | |

|Moles H2 | | | | |

|R (L•kPa/mol•K) | | | |Average = |

6. The accepted value for R is 8.314 L•kPa/mol•K. Compare your average value for the gas constant to the accepted value by calculating a percent error.

Show your work for percent error here:

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