Molar.volume.gas.Novp.student

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Molar volume lab vnovp2 Chemistry, Dr. Breinan p. 1

The Molar Volume of a Gas

Equal volumes of all gases, measured at the same temperature and pressure, contain equal numbers of

particles. This assumption was proposed by Amadeo Avogadro, an Italian chemist, in 1811. This

statement also means that if you have the same number of particles of any gas at the same temperature

and pressure, the volume should always be the same.

What is the volume occupied by one mole of gas particles? Because the volume occupied by a mole of

gas depends on the temperature and pressure of the gas, we must choose a standard temperature andpressure (STP): 273 K and 1 atm. The volume one mole of a gas occupies at STP is the standard molar

volume. It should be the same for any ideal gas.

In this laboratory exercise, you will determine the standard molar volume of a gas. You will react a

known mass of magnesium metal with an excess of hydrochloric acid and collect the generated hydrogen

gas in a gas-collection tube. The evolved gas will rise to the top of the water-filled tube, displacing an

equal volume of water. A series of calculations will be used to find the standard molar volume... read the

calculation section and sample calculation (if given) before doing the lab.

An important application of this idea is credited to Stanislao Cannizzaro, another Italian chemist who

came upon Avogadro’s hypothesis nearly 50 years after it had been proposed. He saw that this hypothesis

pointed a way to finding the molar masses of gaseous elements and compounds. If equal volumes of gases contain equal numbers of particles, then the masses of those gas volumes should be in the same

ratio as the masses of their constituent particles. These findings were instrumental in Mendeleev’s work

in organizing the periodic table in the middle 1800’s.

Objective

1. To calculate the standard molar volume of hydrogen gas (without using the ideal gas constant!)

SAFETY:

- Wear goggles gloves, and aprons

- The 6M HCl is very corrosive and cause severe burns or eye damage. Be extremely careful in pouring

the acid into the gas collection tube...pour over the sink and use only a small container. Bring the gascollection tube to the lowest level you can so you do not pour above someone’s head, face, or

exposed skin.

- Be careful with the gas collection tubes. They are expensive and can cut you if broken. Carry them

securely and do not allow them to roll off the lab bench.

Materials

gas-collection tube 1-hole rubber stopper beaker (400-mL)

magnesium ribbon copper wire ruler

Shared materials:

thermometer barometer large graduated cylinder 6M HCl

Prelab Complete on a separate sheet.

1. Prepare a data table for this lab.

2. Write a balanced equation for the reaction between magnesium and hydrochloric acid.

3. What are standard temperature and pressure (STP)?

4. List the special safety considerations in pouring the acid into the tube.

5. List the safety precautions to be used with the gas collection tubes.

6. What should you be careful to do/not do when filling the gas-collection tube with water?

7. What is the function of the fine copper wire?

8. Describe how the pressure of the gas in the tube is made equal to the atmospheric pressure.




9. Will you use 22.4 or “R” in your calculations?

Procedure

1. Put on your goggles, gloves and apron .

2. Record the barometric pressure and the mass of 1 meter

of magnesium ribbon provided by your teacher.

3. Record the room temperature

4. Obtain a piece of magnesium ribbon.

Measure and record the length.

5. Obtain a piece of fine copper wire approximately 15 cm in length. Roll the magnesium ribbon into a

small ball and wind the copper wire around or through the magnesium so it will not fall out (it is OK to

do this without gloves on). Be sure to leave about 5 centimeters of the copper wire extended from the

“ball”. This “handle” will allow the ball of magnesium to be anchored at the stoppered end of a gas-

collection tube. (See Figure 1). The copper wire simply holds to magnesium in place and will not react

with the acid.

6. Add approximately 350 mL of room-temperature tap water to a 400 mL

beaker

7. Over the sink, carefully pour about 10 mL of 6M HCl into the collection tube.

8. Incline the tube slightly and completely fill the gas collection tube with tap

water from the 400-mL beaker. The tube should overflow a little. While

pouring the tap water, try to rinse down any acid that may have remained on

the tube sides. Let the water run slowly down the insides of the tube to avoid

agitating the bottom acid layer- this will keep the acid at the bottom of the

tube.

9. While holding the copper handle, insert the encased magnesium about 4 cm

into the tube. Hook the handle over the edge of the tube and secure the wire by

inserting a 1-hole rubber stopper into the tube end. (See Figure 1). The tube

should be filled to capacity, so that the stopper displaces several milliliters of

tap water.

10. Cover the stopper hole with your finger. Invert the tube and submerge the

stoppered end in the 400-mL beaker containing the remaining tap water (See

Figure 2). Hold the tube so that the stoppered mouth is just above the bottom

of the beaker. Since the acid is more dense than the water, it will settle

through the tube and soon start to react with the magnesium.

12. Wait until the reaction has completely stopped (there is no more

magnesium) and let the bubbles settle. Gently tap the tube with

your finger to remove bubbles clinging to the tube sides. Figure 2

13. With your gloves on, cover the stopper hole with your finger and transfer the tube into a large

graduated cylinder filled with water. Do not allow any extra air to enter the tube! Raise or lower the

tube until the level of liquid inside the tube is equal to the level outside the tube. This ensures that the

gas pressure inside the tube is the same as the atmospheric pressure outside the tube. Record the

volume of the gas in the tube when the level is adjusted.

14. Discard the tube contents and rinse all apparatus with tap water.

15. Clean up all spills... they may contain residual acid.

magnesium wound up

inside copper wire

Figure 1

Rubber stopper




16. Straighten out your wire and check in the wire and stopper to your teacher. You may leave the tube

to dry on your lab bench.




Assignment: Complete on a separate sheet

1. Present your data neatly. Your prelab data table will be graded unless you cross it out! In other

words, if you rewrite or neaten it up for the assignment, cross it out of the prelab!.

2. Perform the calculations on your data using the sample sheet on the back as a guide. Do NOT use the

value of R or 22.4 for this question! Practice explaining your work clearly (even though each step is

explained for you!) Each answer should be reported to significant figures. Also copy down and useextra figures on your calculator if you are plugging the answer from one question into another.. this

will reduce the rounding error from reporting intermediate calculations.

3. What is the accepted value for the standard molar volume of a gas? Calculate your percent error.

Extra credit. When a gas is collected over water, a correction should be made in the calculation using

Dalton’s law. Research this correction and explain.

Calculations guide:

How can we use the data to find the molar volume of gas at STP? The molar volume of a gas can befound by dividing the volume of any sample by the number of moles in the sample, so we simply need to

know how many moles of gas were produced and how many L there would be at STP. Refer to the

sample processing sheet as you read. NOTE: do not use R (the ideal gas constant) or 22.4 L/mol

anywhere in the processing except for the % error! Remember, the number 22.4 and the value of R came

from experiments like this one! You are doing the calculations just like the first scientists who

determined standard molar volume did!

The number of moles produced can be found by assuming that all of the magnesium reacted to produce

hydrogen. This is a problem in stoichiometry. The balanced equation for the single replacement reaction

carried out in this laboratory exercise is (copy from pre-lab):

Since the coefficients of magnesium and hydrogen are the same, the reaction involves an equal number of moles of each substance. To obtain the number of moles of magnesium, and the number of moles of

hydrogen, use the mass of magnesium ribbon that reacted and the molar mass of magnesium. Since it is

hard to measure the mass of your small piece of Mg directly, it is best to measure the mass of a larger

length and use that value as a conversion factor. (see sample processing, part 1)

The volume that you recorded in the experiment was NOT the volume of hydrogen at STP. Why? The

temperature (room temp) was certainly not 0°C. Your pressure was the barometric pressure in the

room… while probably close to one atmosphere, it is unlikely to be exactly one atmosphere. Since the

reaction did not take place at STP, the recorded volume must be adjusted to standard conditions. This is

easily done now that you know both the temperature and pressure of the hydrogen collected. The

combined gas law is used to find the volume the hydrogen alone would occupy at STP (sample

processing, part 2).

Finally, find the standard molar volume of hydrogen is found from the calculated moles produced and the

corrected volume (sample processing step 3).

See sample calculation, next page




Sample data and processing:

Room temperature: 28.0°C Mass of 1 m of Mg: 1.378 g

Barometric pressure: 747 mmHg Length of Mg strip: 4.6 cm

Mg + 2HCl --> MgCl2 + H2 Gas volume collected: 61.3 mL

Data processing:

Part 1: Find the number of moles of hydrogen produced by the reaction: Assume all

magnesium is turned into hydrogen. This is the theoretical yield of the reaction.

1a. Find the mass of Mg used. The mass of 1 m of Mg is used as a conversion factor:

Mgm1

g1.378

cm100

m1

1

Mgcm4.6×× = 0.063 g Mg

1b. Convert this quantity to moles (use molar mass):Mgg24.3

Mgmole1

1

Mgg0.063× = 0.0026 mol

Mg

1c. Use the balanced equation to “ratio it” to moles of H2:

Mgmol1

Hmol1

1

Mgmol0.0026 2× =0.0026 mol H2

Part 2: Convert the volume of gas produced to a volume of hydrogen at STP. Use

the combined gas law. Do not forget to use only Kelvin temperatures! Condition 1 is what you

collected. Condition 2 is STP.

Hg)mmK)(760(301.1

K)mL)(273.1Hg)(61.3mm(747

P

T

T

VPV

2

2

1

11

2 == = 54.6 mL

Part 3. Calculate the molar volume at STP:

standard molar volumemol0.0026

mL54.6

1c)(stepsampleof moles

2)(stepsampleof volumeSTP= = = 20900

mL

molor 21

L/mol

Extra Practice (not part of the write-up, but may be useful in studying for a quiz or test): Calculate the

molar volume of hydrogen using the following data (answer: 22.7 L/mole)

Room temperature: 50°C Mass of 1 m of Mg: 0.978 g

Barometric pressure: 753 mmHg Length of Mg strip: 5.13 cm

Gas volume collected: 55.9 mL

Molar.volume.gas.Novp.student

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