MOLAR MASS 
OF A GAS
Pat Fisher
ptfisher@mhs.k12.la.us
 
        Credits:
        Algebra 2  p.84 Glencoe/McGraw Hill
        Chemistry 1996 D.C. Heath pp. 228-241
        Chemistry: Connections to Our Changing World
        Prentice-Hall pp. 439-442 
 
CHEMISTRY-GAS LAWS
 
Using prior knowledge concerning the gas laws, lab skills and techniques, the student will design an experiment to measure the molar mass of the gas found inside a disposable lighter. 
 
Benchmarks: Time Frame:

SI-H-A1-A7 2 - 3 class periods

PS-H-A1-A4, C3, C7
 
OBJECTIVES:
  • Determine constants, variables and conversion factors required for procedures and calculations
  • Determine and record methods to collect data
  • Design procedures required
  • List materials and equipment needed
  • Assemble materials and equipment
  • Collect sample of butane and calculate molar mass
  • Work with scientific notation and calculators to perform calculations with large numbers
  • Observe phase change of butane during discovery
  • Use periodic table to calculate theoretical molar mass of butane
BACKGROUND INFORMATION 

SKILLS: KNOWLEDGE:

-Water displacement lab - periodic table usage

procedures -mole concept

-Massing using 0.001 g -Gas Laws including Ideal Gas

digital or centigram balance -variables for gas laws

-CBL with barometer probe -determining molar mass

-Reading a thermometer - graphic calculator

-Reading a graduated cylinder

-Converting: gas factors 

-Derive equations

 MATERIALS: 
  • Calculators - paper towels 
  • CBL with barometer probe - water
  • Thermometers - pencils
  • 100 mL graduated cylinders - SHEETS:
  • water trough a) discovery development
  • butane lighters b) data
  • digital or centigram balances c) assessment
GROUPINGS:
 
    Groups of 4, groups of 2., individual work
NATIONAL SCIENCE EDUCATION STANDARDS 9-12 

1) Science as Inquiry Content Standard A: Understanding about Scientific Inquiry

2) Physical Science Content Standard B: Structure & Properties of Matter 2,4

3) Science & Technology Content Standard E: Abilities of Technology Design
ENGAGEMENT: DAY ONE

Hold up a lighter and ask the following questions…waiting for student responses.

1) We have a ____? lighter

2) What is in the lighter? Butane

3) Is it a gas? Yes

  1. What is the compound classification? Saturated hydrocarbon
  2. Why does it look like a liquid or is it really a liquid? Gas under pressure.
EXPLORATION:

(Have BUTANE written on the board)

1) What is the molar mass of this gas? (give students time to calculate) 58.14g/mol

2) What is the ideal gas law? PV=nRT

possible variables? P, V, n, T

constant ? R 

3) What is Dalton's Law of Partial Pressure?? 
INSTRUCTIONS:

DAY ONE:

Working in groups of 4, design an experiment to measure the molar mass of this gas. Utilize class time today to develop procedures, determine and procure materials and equipment, and research water vapor pressure (if needed).

Determine variables, constants and conversion factors needed.

Tomorrow, actual discovery with data collection. (Teacher monitor and assist as needed.

DAY TWO:

(Review, clarify and assist as needed) Monitor lab procedures. Data collection will not take long. Students will complete calculations of molar mass using formula:

PV

n = RT Have water vapor pressure tables and barometer readings from weather station and CBL barometer probe.
ANALYSIS AND EXTENSION:

Pose questions on Analysis and Extension sheets (attached).
ASSESSMENT:

One possible assessment tool is attached. Can be used for both days.

Teacher could also add bonus points, if desired. 
  MOLAR MASS OF A GAS Group Members: __________________ _____________________
 

___________________ _____________________

As a group, design an experiment to measure the molar mass of the gas in the lighter. The group will utilize prior knowledge concerning gas laws, lab skills and equipment. State a hypothesis, the problem, procedures, SAFETY, materials and equipment. Determine and locate the variables utilized in your calculations upon completion of the data collection.

The discovery will be completed during the next class meeting.

GOALS:

  1. Constants and variables:

 

2. HYPOTHESIS:

 

 

 

3. PROBLEM:

 

 a) PROCEDURES:

 b) SAFETY:

 c) MATERIALS AND EQUIPMENT:
 

 

DISCOVERY PROCEDURES AND DATA COLLECTED

 

ANALYSIS:

        a)  Compare and contrast actual to theoretical values.

        b)  Evaluate the pressure (was it at or below 1 atm?)

        c)  Do gases behave "ideally?" Support your answer.

        d)  Can this data be utilized to determine the Avogadro Constant?
 

molar volume = volume of gas collected at STP

number moles gas collected
 

EXTENSION:

    a)Find the volume of a hypothetical cylinder occupied by one      butane molecule. Assume that the molecule has a diameter of 4.5 x 10-8 cm and travels the mean free path distance of 1.0 x 10-4 cm/molecule.

     

    Volume of cylinder =

     
     

    b)  Find the Avogadro constant.

  Avogadro constant = molar volume (L/mol X 1000 cm3/L

Volume of cylinder (cm3/molecule)
 

 

 3. How does your result compare with the correct value of 6.02 x 1023 ?
 
  
ASSESSMENT OF GAS DISCOVERY
 

Points Categories

GROUP OF 4 :

        a)  Identified the problem

        b)  Stated a hypothesis

        c)  Identified constants and variables
 
        d)  Listed materials and equipment

        e)  Designed procedures

        f)   Executed a plan

        g)  Applied process and problem solving

        h)  Reached a reasonable conclusion

        i)  Demonstrated group effort

 

GROUP OF 2:

10 Implemented procedures

 3 Collected data 

 5 Observations

30 Calculations, Analysis, Extension