Pnuematic Trough(Lourdes Ramos Quevedo)
Author
Lourdes Ramos Quevedo, Academy of Scientific Exploration
Principles
Water displacement, gas collection, gas laws, molar mass
Standards
Students know how to apply the gas laws to relations between the pressure, temperature, and volume of any amount of an ideal gas or any mixture of ideal gases. (CHEM 4c)
Students know how to solve problems by using the ideal gas law in the form PV=nRT. (CHEM 4h)
Select and use appropriate tools and technology (such as computer-linked probes,
spreadsheets, and graphing calculators) to perform tests, collect data, analyze relationships, and display data. (I&E a)
Identify and communicate sources of unavoidable experimental error. (I&E b)
Identify possible reasons for inconsistent results, such as sources of error or uncontrolled conditions. (I&E c)
Materials needed
graduated cylinder
delivery tube
butane lighter
thermometer
Procedure
1. Determine the initial mass for the butane lighter provided by your instructor ( +/- .01 gram ).
2. Set up the water trough for collecting a gas in an inverted graduated cylinder by water displacement. Use a 100 mL graduated cylinder.
3. Release butane from the pressurized container and collect a sample of gas with a volume of approximately 70.0 mL to 90.0 mL. Collect every bubble that leaves the lighter. If you miss a bubble, you will have to repeat the entire procedure beginning with re-weighing the clean and dry lighter.
4. Adjust the cylinder up or down so that the water level inside the graduated cylinder is the same as the level outside the cylinder. THIS IS VERY IMPORTANT! If you miss this simple step, the pressures inside and outside the cylinder will not be equal, and you will have an inaccurate estimation of the pressure inside the cylinder where the gas is collected.
5. Record the volume of butane that was collected.
6. Measure and record the temperature of the water in the basin ( +/- 0.1 °C ). This will be used to determine the vapor pressure of water H2O (g) in the graduated cylinder. (A Table of water vapor pressure may be referenced.)
7. Read the barometric pressure in the lab (inches of Hg ) and convert this to mm Hg (using the conversion factor 1 inch Hg = 25.4 mm Hg) and then to kPa.
8. Make sure the lighter is dry then, measure and record the mass of it ( +/- .01 gram).
9. Clean and return all materials to the starting position for the next class.
Explanation
In this lab, the molar mass of butane, C4H10, is determined experimentally. A simple calculation using a periodic table would give us the correct answer for the molar mass of butane, however, here we will see how close you can come to the accepted value. The ideal gas law equation, PV = nRT, can be rearranged to solve for n, the number of moles:
n = PV
RT
We are trying to find the molar mass, which is expressed in units of grams per mole (g/mol). The mass of the butane released from the pressurized container can be used in conjunction with the volume of the gas that is collected. You can use the method of water displacement at room conditions and then substitute the measurements of volume, temperature, and pressure into the ideal gas law equation in order to find n, the number of moles of butane. Once we know the mass, and the number of moles, we will divide the mass by the number of moles to calculate the molar mass (g/mol).
Questions
What is the experimental molar mass calculated based on your gas collected?
How many grams of butane did you collect?
What volume would the same number of grams of ethane (C2H6) have occupied at the same conditions of temperature and pressure?
Everyday examples of the principles illustrated
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Photos
