Expt 031 -- Molar Volume of a Gas

The molar volume of the gas resulting from the reaction of metallic magnesium in hydrochloric acid is determined by water displacement.

Background

• Magnesium reacts with an excess of hydrochloric acid (HCl) to produce hydrogen:

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

• One mole of Mg is expected to produce one mole of hydrogen. The volume occupied by the hydrogen gas is called the molar volume.
• The volume of a gas is a function of both pressure and temperature. By convention, the molar volume is expressed for conditions of 1 atmosphere pressure (760 mm Hg) and 0 ºC.
• Because the hydrogen was collected over water, it is said 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. To find the pressure of the hydrogen, the contribution of the water vapor must be subtracted from the total:

  P H₂= P total - P H₂O

• The pressure of water vapor varies as a function of temperature. The value of the water vapor pressure is determined from a table.
• The pressure and volume relationships are expressed by:

  (P₁V₁ / T₁) = (P₂V₂/T₂)

• If the 1-subscripts represent the room temperature data, and the 2-subscripts represent the 0 ºC calculated values, then:

  V₂ = V₁ x ( P₁/P₂) x (T₂/T₁)

• Finally, the temperature must be expressed on the Kelvin scale (absolute scale), or:

  K = ºC + 273.15

Safety

• HCl is corrosive and toxic. Wear goggles and apron. Avoid ingesting the chemicals. Wash spills with water. Wash hands after the experiment.
• If mercury thermometers are used and a thermometer breaks during the experiment, notify the instructor immediately.

Procedure

1. Fill a 600-mL beaker about 3/4 full with water.
2. Obtain a short piece of magnesium ribbon. Record its length to the nearest mm. Also, record the mass of 1 meter of magnesium ribbon. The mass/meter will be provided by the 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.

   !!!Click here to See Picture.

4. Clean a 10-mL graduated cylinder with soap and water. Wash the cylinder thoroughly. Place 3 mL of 3 M HCl in the cylinder. Wash any spills. Wash hands.
5. Using a plastic wash bottle or a dropper, carefully add water to fill the graduated cylinder to the top without mixing the acid and water; layer the water on top of the acid.
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 a 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.

   !!!Click here to See Movie.

8. Note any evidence of a chemical reaction.

   !!!Click here to See Movie.

   !!!Click here to See Movie.

9. Allow the apparatus to stand for 5 minutes after the magnesium has reacted completely. Tap the sides gently to dislodge any gas bubbles that may have become attached to the sides of the graduated cylinder.
10. 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 calibration marks are upside down.)

    !!!Click here to See Movie.

11. Record the temperature of the water and the barometric pressure.

Data Analysis

Find the volume of 1 mole of hydrogen gas at 760 mm Hg pressure and 0 ¼C.

Questions

1. Suggest ways in which the precision of this experiment may be improved.
2. The Mg ribbon is placed in water, not in acid. What makes the reaction start?
3. Suppose the Mg ribbon contains 3 % by mass of MgO. MgO does not lead to hydrogen formation in this experiment. Predict and explain the effect of this result.

Handout

Name ___________________________ Class _______

Teacher __________________________

SmallScale 031 Molar Volume of a Gas

length of Mg ribbon (mm)

mass of 1 meter Mg ribbon (g)

vol of hydrogen (mL)

water temperature (ºC)

barometric pressure (mm Hg)

water vapor pressure (mm Hg)

observations

Data Analysis

Find the volume of 1 mole of hydrogen gas at 760 mm Hg pressure and 0 ºC.

Answer the questions.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 031 Molar Volume of a Gas

Watch the movies, and note any evidence of a chemical reaction. Use the data below to answer the questions.

length of Mg ribbon (mm) = 10.4 mm

mass of 1 meter Mg ribbon (g) = 0.809 g/m

vol of hydrogen (mL) = 8.7 mL

water temperature (ºC) = 22 ºC

barometric pressure (mm Hg) = 751 mm Hg

water vapor pressure (mm Hg) = 19.8 mm Hg

Data Analysis

Find the volume of 1 mole of hydrogen gas at 760 mm Hg pressure and 0 ºC.

Answer the questions.

Curriculum-

Use when studying gas laws, vapor pressure, oxidation reduction, acids, or the mole.

Safety-

• HCl is corrosive and toxic. Wear goggles and apron. Avoid ingesting the chemicals. Wash spills with water. Wash hands after the experiment.
• If mercury thermometers are used and a thermometer breaks during the experiment, ask students to notify the instructor immediately.
• Magnesium is a flammable metal. Polishing the magnesium wire before cutting it into small pieces may lead to cuts, Wear gloves when polishing the magnesium.
• If mercury thermometers are used, have a commercial mercury spill clean-up kit available in case a thermometer breaks.

Time-

Teacher Preparation: 15 minutes

Class Time: 50 minutes

Materials-

• 4 mL of 3 M hydrochloric acid
• 1-cm length magnesium ribbon (The instructor must carefully weigh one meter sections of magnesium ribbon and give this information to the students. It is important 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
• thermometer (alcohol or metal preferred; if mercury thermometers are used, have a mercury spill kit available.)

Disposal-

• The solutions generated during the experiment may be disposed of safely at the sink.
• Save unused Mg.

Data Table-

length of Mg ribbon (mm) = 10.4 mm

mass of 1 meter Mg ribbon (g) = 0.809 g/m

vol of hydrogen (mL) = 8.7 mL

water temperature (ºC) = 22 ºC

barometric pressure (mm Hg) = 751 mm Hg

water vapor pressure (mm Hg) = 19.8 mm Hg

observations = When the cylinder is inverted, lines inside the liquid (called schlieren) show mixing of the acid and water. Bubbles form around the magnesium, The ribbon reacts and gets smaller. When the ribbon is almost gone, it comes loose from the string and floats to the top.

Data Analysis-

Analyze shared data from all of the groups working on the experiment.

mass Mg = 10.4 mm x(1 m/1000 mm ) x (0.806 g Mg/1 m Mg)

= 0.0084 g Mg

mol H₂ = 0.0084 g Mg x (1 mol Mg/24.3 g Mg) x (1 mol H₂/1 mol Mg)

= 0.00035 mol H₂

P H₂ = P total - P H₂O = 751 - 19.8 = 731 mm Hg

T = 22 + 273.15 = 295 K

V₂ = V₁ x ( P₁/P₂) x ( T₂/T₁)

V₂ = 8.7 mL x (731/760) x (273/295) x (1 L/1000 mL)

= 0.0077 L

molar volume = (0.0077 L /0.000347 mol) = 22 L/mol

Answers-

Q1. Suggest ways in which the precision of this experiment may be improved.

A1. The scale is too small. To measure a larger mass, a larger cylinder must be used. Eudiometer tubes, sealed, well-calibrated tubes, are used for this experiment on a large scale. [Also, using zinc instead of magnesium gives a larger mass to weigh and a substance that can be obtained in purer form.]

Q2. The Mg ribbon is placed in water, not in acid. What makes the reaction start?

A2. The HCl solution is more dense that water. When the cylinder is inverted, this avid spontaneously mixes downward.

Q3. Suppose the Mg ribbon contains 3 % by mass of MgO. MgO does not lead to hydrogen formation in this experiment. Predict and explain the effect of this result.

A3. The volume of hydrogen will be 3% less than it should have been, so the reported value with be too small by 3%. [If all of the magnesium had reacted to form MgO, then the volume would be 0 and the apparent molar volume would be 0.]

CoopLearn-

Analysis of shared data is especially appropriate for this experiment.

Literature Data-

The accepted value for the molar volume is 22.414 L/mol.

Vapor Pressure of Water at Various Temperatures:

• Temperature (ºC) Pressure (mm Hg) Pressure (kPa)

Reference-

An early version of this experiment was developed by Ginger Tannenbaum, Fairfield, OH, at the 1987 Dreyfus Woodrow Wilson summer program.

Key Words 1-

gases, molar volume, oxidation, reduction, redox, metal, corrosion, gas laws, water vapor, vapor pressure, acid

Elements-

Mg H