October 7: Determining the Molar Mass of Butane 

Determining the Molar Mass of Butane

Today, depending how far you previous got last class on friday, you either conducted the lab Determining the Molar Mass of Butane, or if you had already conducted this lab, you finished processing the calculations for the data/observations and Post Lab Questions. 

The objective of the lab: 

• To successfully collect and store butane gas from a cigarette lighter.
• To use the Ideal Gas Law (and Dalton’s Law of Partial Pressures) in calculating the number of moles of butane being stored.
• To use the mass and the number of moles of butane being stored in calculating the molar mass of butane.
• To compare the experimental molar mass of butane with the theoretical mass (based on its formula, which will be provided) and calculate a percent error.

For those who conducted the lab today, followed the procedure on there Butane Lab sheet which involved -

• Obtain a 2 or 3 dp balance and make sure it is properly set to zero.
• Place a clean, dry cigarette lighter on the triple beam balance and record its mass under Initial Mass of Cigarette Lighter in the Data and Observations section.  Remember to record the mass in agreement with the uncertainty of the device (if you can). 
• Add water to a trough (at least 2/3 full).  
• Fill a 500cm³ or 1dm³ measuring cylinder with water to the very brim.  Place a glass plate over the mouth of the bottle, then turn the cylinder upside down and place it in the trough.  When the mouth underwater, you may remove the glass plate.  Be sure that there are no bubbles in the bottle.  If the bottle has air bubbles trapped in it, you must try again to completely fill the bottle with water.
• Lift the bottle up slightly (but keep its mouth below the surface of the water) and hold the cigarette lighter beneath the mouth.  Press the button on the lighter so that bubbles of butane rise into the inverted bottle.  Be careful that no bubbles of butane miss the mouth of the bottle.
• As the cylinder fills with butane, the level of the water will fall.  Continue to collect butane gas until at least 200cm³ have been collected.
• When you have collected a measurable amount of butane, release the button on the lighter and remove it from the water.  Dry it with a paper towel and hair dryer and record its mass again under Final Mass of Cigarette Lighter in the Data and Observations section.  The second mass should be LESS than the first mass you recorded in step A.  If the second mass is higher than or the same as the first mass, be sure that it is thoroughly dry (including inside the metal casing around the gas outlet) and mass it again.  If the mass is STILL greater than or the same as the first mass, you should repeat the procedure from the beginning.
• Read the volume of the butane you collected from the side of the cylinder.  Record this volume under Volume of Butane Collected in the Data and Observations section.  When you have recorded the volume, you may set the bottle down inside the trough.
• Use a thermometer to record the temperature of the water in the trough.  You may assume that the gas temperature is the same as the temperature of the water.  Record the temperature to the correct number of decimal places under Temperature of Gas Collected in the Data and Observations section. 
• Use a weather website to identify the atmospheric pressure in the room.  This information will be recorded on the board in mmHg.  Record this pressure in your Data and Observations section exactly as it appears on the board under the heading Total Pressure of Gas in Cylinder.
• Refer to the graph on the next page to determine the vapor pressure of H₂O at the temperature you recorded in Step I.  Record the Vapor Pressure of Water in your Data and Observations section.
• Perform all calculations and conversions in the Calculations section.  Don’t forget to calculate a percent error when you have finished everything else.

Once everyone had finished this procedure, they continued to work on the calculations starting with the Data and observations worksheet for the lab which included the following:

Initial Mass of Cigarette Lighter:  _________________________ grams

Final Mass of Cigarette Lighter:  _________________________ grams

Volume of Gas Collected in mL:  _________________________ mL



Temperature of Gas Collected in ºC:  _____________________ºC



Atmospheric Pressure in mmHg*:  __________________ mmHg

Vapor Pressure of Water at Measured Temperature:  _________________ mmHg

This Process also involved the need of this graph below to find out some of the values. 

Once all the Observations and data was processed students moved on to the 5 Post Lab Questions which applied the same concepts as the above valuations.  The Questions are as follows: 

1.Identify at least one potential source of error in the experimental procedure that may lead to a loss of accuracy.

2.A gas sample is collected over water at 20.0ºC.  The volume of the gas collected is 45.0 mL and the atmospheric pressure is 771 mmHg.  How many moles of gas were collected?  (Remember to take into account the vapor pressure of water.  Consult the graph on Page 3 of this lab.)

3.The mass of the gas sample described in question 2 is found to be 0.0371 grams.  What is the molar mass of this gas?

4.It is known that the gas described in questions 2 and 3 is an element from the periodic table.  What is the probable identity of this mystery gas?

5.What volume will 0.010 grams of n-pentane, C₅H₁₂, occupy when stored under 12.0 atm of pressure and 21.0ºC?

In both worksheets students used the concept of the Ideal Gas Law:  PV=nRt. 
Also students applied the relations between the Molar Volume of a Gas, converting Temperatures and Pressures into the correct units. 

•  Which can be found on your (Gas Law Practice Problems 35 mins) handout. 

Homework: Finish Butane lab work, Watch/take notes on video 1.33 and prepare for the quiz on section 1.32 on this thursday. Continue with all worksheets.