Student s Name: Date : Molar volume of butane

Transcription

1 Student s Name: Date : Background Molar volume of butane The purpose of this activity is to determine the molar mass of butane using gas from a butane lighter. The experiment depends on the fact that the molar volume of an ideal gas is 22.4 L at standard temperature and pressure, STP (0 o C and ). By measuring the volume occupied by a known mass of butane, and knowing that its molar volume is 22.4 L at STP, it is possible to calculate the molar mass of butane. Equipment and chemicals 250 ml conical flask gas jar lid or flat glass plate to fit diameter conical flask pneumatic trough or plastic barometer (or access to Internet site bucket ) butane lighter rubber bands or cable tie to fit lengthwise around lighter to hold gas button open 100mL, 50mL, 10 ml compressed air + hose (or hair dryer) measuring cylinders thermometer o C fume cupboard analytical balance Safety Handle the butane light with care International safety card information butane - STAWA14v2.DOC Rob Namestnik Page 1

2 Procedure 1. Fill the 250 ml erlenmeyer flask (conical flask) completely full with tap water. Cover the top with the glass plate without trapping any air in the top of the flask. 2. Fill the pneumatic trough or bucket approx. 2/3 full with tap water. Holding the glass plate in place with your finger, carefully turn the flask upside down and place it in the trough. BE SURE THAT NO AIR BUBBLES ENTER THE FLASK. Remove the glass plate carefully. 3. Pick up a butane lighter from the lecturer. Depending upon the lighter, you may, or may not, need to remove the safety device before proceeding. Weigh the dry lighter accurately on the analytical balance to 4 decimal places. 4. Carefully hold the lighter under the water, making sure that the gas opening is beneath the mouth of the inverted flask. Lift the mouth of the flask slightly while keeping it under the water at all times. Press the release lever of the lighter ensuring that all of the gas bubbles enter the flask. Continue to hold the lever down until you have collected approximately 200 ml of butane gas. You may use a rubber band or cable-tie to keep the gas release lever depressed for as long as required. conical flask water butane gas water pneumatic trough or plastic bucket lighter 5. Remove the lighter. Shake off excess water and then dry it using compressed air (found in the fume cupboard). Dab dry with a paper towel. Re-weigh the lighter and determine the mass of butane gas that was released. STAWA14v2.DOC Rob Namestnik Page 2

3 6. Do not remove the gas-filled flask from the water. Instead, carefully raise or lower the flask until the level of water inside the flask is the same as the level of water outside the flask. This ensures that the gas inside the flask is at atmospheric pressure (which you will measure a little later). You may want to mark this level on the flask as a precaution. water level equal inside the flask and in the trough Hold the flask at this level and carefully slip the glass plate under the flask. Holding the plate tightly in place with your finger, lift the flask out of the water. Set the flask upright on the bench, being careful not to spill any water. 7. Under a fume-hood remove the glass plate from the flask and allow the butane gas to safely escape. Using various measuring cylinders, fill the flask with water, keeping count of the exact volume of water required to fill the flask exactly to the top. The total volume of water added represents the volume of the butane gas collected. 8. Measure the temperature of the water in the plastic trough and record this as the temperature of the butane gas. 9. Record the barometric pressure and record this as the atmospheric pressure. (Use the Internet website ) STAWA14v2.DOC Rob Namestnik Page 3

4 Data Sheet Mass of butane light before collection Mass of butane lighter after collection Mass of butane collected g g g Volume of water required to fill flask to the top Volume of butane collected ml ml Water temperature Butane gas temperature Atmospheric pressure o C o C Water Vapour pressure Corrected butane gas pressure Vapour pressure of water at various temperatures Temperature C Pressure Temperature C Pressure STAWA14v2.DOC Rob Namestnik Page 4

5 Processing of Results 1. Calculate the mass of butane released from the lighter 2. Calculate the partial pressure of the butane by subtracting the vapour pressure of water from the measured atmospheric pressure. (Use the water vapour pressure table supplied on page 4) Pbutane = Patmos - Pwater = 3. Calculate the volume the butane would occupy at STP and hence the number of moles of butane released. 4. Using the measured mass of butane and moles calculated, determine the molar mass of butane. STAWA14v2.DOC Rob Namestnik Page 5

6 5. a) Determine the percentage error in your determination of the molar mass of butane by comparing the experimental value to the accepted value. b) Comment on the accuracy of your experimentally determined molar mass of butane quoting the percentage error. 6. Suggest how and where errors in the determination could have been made. 6. Use the safety link on page 1 to study then comment on the safety of using butane gas in the laboratory. STAWA14v2.DOC Rob Namestnik Page 6