The Molar Volume of a Gas

Introduction

The molar volume is the volume occupied by one mole of gas. This volume is a function of temperature and pressure.

In this experiment you will react a known mass of magnesium with an excess of hydrochloric acid (HCl) to produce hydrogen gas as shown:

Mg(s) + 2HCl(aq) --> MgCl₂(aq) + H₂(g)

The hydrogen gas produced will be collected by the displacement of water. You will then compute the volume at standard conditions using the gas model and equations. The STP volume will be used to calculate the molar volume of hydrogen gas.

Since you will be working with small quantities of materials, it is essential that you work with great care.

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1. Fill a 600-mL beaker about 3/4 full with water.
2. Obtain a short piece of magnesium ribbon from your instructor. Record its length to the nearest mm in your data table. Also record the mass of 1 meter of magnesium ribbon. This data will be provided by your instructor.
3. Obtain a piece of cotton thread about 10 cm long. Tie one end around the piece of magnesium ribbon, leaving about 7-8 cm of thread free. Gently bend the piece of magnesium ribbon so it will fit into a 10-mL graduated cylinder.
4. Obtain 3 mL of 3 M hydrochloric acid (HCl) in your 10mL graduated cylinder, taking care to keep it off your skin.
5. Using a plastic squeeze bottle or a dropper, and using care to mix the acid and water gently, fill the graduated cylinder to the top with distilled water.
6. Lower the piece of magnesium ribbon into the graduated cylinder, coiling 1-2 cm extra thread into the top. Drape the remaining thread over the edge of the graduated cylinder and firmly insert a one-hole cork stopper. If there is an air space, add water through the hole in the cork.
7. Place your finger over the hole in the cork and invert the graduated cylinder. Lower the stoppered end of the graduated cylinder into the beaker of water. Note any evidence of a chemical reaction.
8. Allow the apparatus to stand for 5 minutes after the magnesium has completely reacted. Then, tap the sides gently to dislodge any gas bubbles that may have become attached to the sides of the graduated cylinder.
9. Move the graduated cylinder vertically (keeping the open end submerged) until the water level inside the cylinder is the same as the water level in the beaker. This is done to equalize the pressure of the gas trapped inside the cylinder with atmospheric pressure. Read the volume of gas in the cylinder. (Remember, the scale is upside down.)
10. Record the temperature of the water and barometric pressure.
11. Repeat the experiment.
    

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1. The accepted value for molar volume of any gas at STP is 22.4 L. Compute the absolute difference between the accepted value and the average value obtained from these experimental data. Calculate the percent error.
2. Identify possible sources of error in this experiment.
3. Identify the other product of the reaction. What happens to that product?

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1. Calculate the mass of the piece of magnesium strip using the known mass per meter of strip and the length of the strip used. You may assume the magnesium ribbon was of uniform thickness and width.
2. Calculate the moles of Mg reacted (the atomic mass of magnesium is 24.3). The number of moles of H₂ produced is equal to the number of moles of magnesium consumed.
3. Since you collected the gas over water, it is considered to be wet. The total pressure of a gas mixture is equal to the sum of the partial pressures of each gas, in this case, hydrogen and water vapor. Find the pressure exerted by the water vapor in the graduated cylinder by using a table for the pressure of water vapor as a function of temperature:

   Phydrogen = Pbarometric - Pwater

4. Convert water temperature from Celsius to Kelvin.
5. Find the volume of gas generated by finding the difference in water volumes.
6. Find the volume of hydrogen gas at STP using the gas laws.
7. Calculate the molar volume of hydrogen gas using the information from your experimental results. If X mL (your corrected volume) of hydrogen gas are produced by Y moles of hydrogen (the number of moles that were formed), the molar volume is X/Y.

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Safety

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1. Calculate percentage error

   % Error= 100% X (theoretical value - experimental value)/ (theoretical value)

2. Main sources of error are in measurement of length (and mass) of the magnesium and the volume of hydrogen.
3. The magnesium chloride is present in solution as Mg²⁺ and Cl^- ions.
   

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1. Find the mass of the Mg ribbon (assuming a length of 1 cm)

   0.01 meter/1.00 meter = x grams/mass of a 1-meter length

2. Calculate the number of moles of Mg reacted (which is equal to the number moles of hydrogen produced)

   #moles magnesium=mass magnesium reacted/(24.3 g/mol Mg)

3. Find the pressure exerted by the dry hydrogen gas

   Phydrogen = Pbarometric - Pwater

   (The vapor pressure of water at various temperatures can be found in a chemistry handbook)

4. Convert water temperature from Celsius to Kelvin

   K = °C + 273

5. Correct the volume of dry hydrogen from laboratory conditions to STP.
6. Calculate the molar volume.

   molar volume = mL of hydrogen gas at STP/moles of hydrogen produced
   

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• 6 mL 3M hydrochloric acid (90 mL/15 lab pairs) (add 25 mL concentrated HCl to 50 mL of distilled water slowly with stirring. Add enough distilled water to bring the final volume to 100 mL.)
• 2 1-cm lengths magnesium ribbon (The instructor must carefully weigh one meter sections of magnesium ribbon and give this information to the students. It is important the the length of the magnesium strips be about 1 cm. Longer lengths will lead to production of too large a volume of gas; lengths of 0.9 cm or less are preferred.)
• 10-mL graduated cylinder
• #2 cork stopper with one hole which can be bored with a #1 cork borer
• 600-mL beaker
• 20 cm cotton thread (one spool)
• plastic squeeze bottle or dropper
• ring stand
• clamp
• thermometer

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The Woodrow Wilson microscale version of this lesson was developed by:

Ginger Tannenbaum
Fairfield Senior High
1941 S. Staunton Drive
Fairfield, OH 45014

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