Determination of the Molar Volume of H2 Gas

Lab 9

Equipment Setup• Today you will set up your equipment and run

your experiment prior to the pre-lab lecture.• After you are finished, return to your seats.• Here’s how to proceed:

Equipment• Weigh out a Mg strip• Roll into a ball• Insert into alligator clip

Equipment Continued

• Carefully add 10 mL of 6M HCl to the eudiometer tube.

• Fill the tube carefully with distilled water.

• Place the stopper in the tube and eliminate all air bubbles.

Equipment Continued

• With your finger over the hole in the stopper, invert the tube and place in 100 mL dH2O.

• Clamp the tube to a buret clamp.

• The stopper should be 4 mm from the bottom.

• Record the time.

Equipment Continued

• Record the volume of H2 gas from your eudiometer tube after 1 hour.

• Go ahead…

Purpose

The molar volume of all gases at STP is 22.414 L/mol. In this experiment, we will be collecting H2 gas by

displacement of water to determine the molar volume of H2 gas experimentally, i.e. confirm that it is 22.433 L/mol. Note the slight deviation from ideal behavior.

Discussion

Physical properties of gases can be related through mathematical equations, known as laws:• For Single Gases -

• Boyle’s Law: V = ; at constant T and n

• Charles’ Law: V = bT; at constant P & n• Avogadro’s Law: V = an; at constant T and P• Ideal Gas Law: PV = nRT

• For Mixed Gases - • Dalton’s Law:PT = P1 + P2 + P3 …

k

P

Reaction

The net ionic equation for this redox reaction is:

2H+(aq) + Mg(s) Mg2+

(aq) + H2(g)

reduced oxidized

Data• Accepted Molar Volume is 22.433 L/mol• Atmospheric Pressure can be obtained from the

barometer• Vapor Pressure of Water in mmHg is obtained from

Table 9.1• Water Bath Temperature in degrees Celsius is

obtained using the digital thermometer • The pressure difference in mm H2O is measured

with a meter stick.• Mass of Magnesium is obtained from the balance

Data Continued

• Partial Pressure of H2 gas in mmHg from Dalton’s law.

• Volume of H2 gas is obtained from your Eudiometer tube.

• Volume of H2 gas at STP is obtained from:

Example: = =

2 2

2

H H

H O

P V

T

STP 2

STP

P V

T

(730.0mmHg)(33.18mL)

293.6K2(760.0mmHg)(V )

273.15K

Data Continued

• Theoretical moles of H2 are obtained from:

moles H2 =

• Molar Volume of H2 gas is obtained from:

molar VolH2 =

2(mass Mg)(1mol H )

(fw Mg)(1mol Mg)

2

2

(vol of H at STP, L)

(theoretical moles H )

Safety Concerns• Reagents:

• Hydrochloric Acid• Magnesium Metal

• Eye Contact: • irreversible damage, severe burns, painful sensitization to

light• Skin Contact:

• Burns, rash, and ulceration• Ingestion:

• digestive tract burns, circulatory system failure, abdominal pain, diarrhea, vomiting, and death

• Inhalation: • sore throat, coughing, shortness of breath, lung edema,

burns, erosion of teeth, damaged mucosa, chest pain, fever, and leukocytosis

Waste

• Acidic solutions / waste need to be disposed in the acid waste container in the fume hood.

Lab 10 Reminder

• Lab 10 is next.