Molar Volume of Hydrogen Gas

One mole of any gas occupies the same volume when measured under the same conditions of temperature and pressure. In this experiment, the volume of a sample of hydrogen gas is measured. The volume of one mole of hydrogen is calculated at room temperature and pressure using stoichiometry.
Mg + 2HCl → MgCl2 + H2
1. a standard 50 mL buret6. sufficient water to fill beaker and buret
2. clamp, buret clamp and stand7. 100 mL round-bottom flask
3. medium beaker
(≥250 mL)
8. rubber tube and 1-hole stopper
4. 0.02 to 0.04 g Mg ribbon9. 100 mL graduated cylinder
5. 50 mL HCl (1 M)10. steel wool
Experimental Setup for Mg/H2 Molar Volume Experiment
Pre-Lab Questions
Write your answers to these questions neatly on a separate piece of paper, showing all steps of all calculations. Be sure to do the calculations before the experiment and to re-write them if they aren’t neat and easy to read.
  1. What is the molar mass of magnesium metal?
  2. What is the molar mass of hydrochloric acid?
  3. What is the molar mass of hydrogen gas?
  4. What is the molar ratio of magnesium to hydrogen gas?
  5. Given 0.450 mol Mg, how many moles of H2 would be produced?
  6. Given 0.04 g Mg, how many moles of H2 would be produced?
  7. If you collect 0.224 L of hydrogen gas when you react 0.01 mol Mg, how many liters of gas would be collected if one mole of Mg reacted?

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  1. Clean a 3.5 cm piece of magnesium ribbon with steel wool and weigh using the lab balance. (This should weigh between 0.02 and 0.04 g; if not adjust the amount used.) Record the exact mass of Mg used.
  2. Fill the beaker about 3/4 full of water.
  3. Make sure the buret’s valve is closed. Fill it almost all the way with water and upend it in the beaker of water. Clamp securely in place using the ring stand and a buret clamp. Adjust the level of water to somewhere below zero by opening the valve. Record the level: this is V1. Take care with this and remember that the buret is upside-down.
  4. Push the tubing into the stopper and thread the other end up into the buret under the water.
  5. Clamp the round-bottom flask to the stand in a convenient location. Measure 50 mL of 1 M HCl solution into the flask using a graduated cylinder.
  6. In a carefully coordinated motion drop the clean, weighed piece of Mg ribbon into the HCl solution and quickly stopper it as tightly as possible. The H2 gas will travel through the tube and into the buret.
  7. Watch the reaction of Mg with HCl and write down your observations. You may carefully swirl the reaction vessel by briefly unclamping it. Clamp it back securely in place when done. Wait for the reaction to be complete and note the new volume on the buret (again take care: it is upside down). This is V2
  8. To find the volume of H2 produced simply subtract V1 from V2.
Write a lab report on a separate piece of paper and be sure to create and include a neat, readable data table. The table should show the starting volume reading, the ending reading, the volume of gas produced, and the mass and number of moles of Mg reacted. Keep this lab for future reference. The lab report should include the following sections:
Pre-Lab: Give the answers to the pre-lab questions as a part of the formal lab report.
Abstract: Briefly (50 words or fewer) state the purpose and findings of the lab. Free advice (isn’t it always?): do not write the abstract until you have written the rest of the report and done all of your calculations since you need to report your results in the abstract, anyway.
Materials: Write a list of the materials used in the lab.
Procedure: Write the procedure given in your own words in paragraph form.
Data: You will collect two kinds of data: qualitative and quantitative. The qualitative data include your own impressions and observations about the experiment and may include pictures. The quantitative data are the data you were told to collect in the procedure. Also include any other numerical data that is relevant.
Data Analysis: Calculate the volume of gas produced by subtracting the smaller volume reading from the larger. Calculate the number of moles of H2 produced based on the number of moles of magnesium reacted. Divide the volume (expressed in liters!) by the number of moles to find the molar volume. This is the volume of any type of gas under the conditions in the classroom at the time of the experiment. Show all work clearly.
Error Analysis: be sure to mention how possible errors may affect results. Do not simply say that ‘human error’ probably caused problems in the data. That is a given. Try to think of what could be changed in the experimental design to ensure better data. For instance, would repeating the procedure two, three or more times improve the results’ accuracy? Why? Is it possible that another gas contributed to the volume found? How much gas might have been lost because you couldn’t get the stopper on fast enough?
Conclusion: Briefly state your results. No new information should be introduced here.
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