Colegio San Agustin - Makati
CHEMISTRY LABORATORY ACTIVITY SHEET
SY: 2011 – 2012 / CSA SCIENCE PROMOTES
THE CULTURE OF EXCELLENCE
ACTIVITY # / 2 / Boyle’s Law
REMARKS
INTRODUCTION
Have you ever used a simple hand pump to inflate a bicycle tire? You push on the pump, and air moves into the tire. The bicycle pump operates, in part, on the principle of Boyle’s Law, which describes the relationship between the pressure and volume of a gas.
In this investigation, you will observe the behavior of a gas, using a device called a Boyle’s law apparatus. These devices vary somewhat, but they all work in basically the same way. Observe the type of Boyle’s law apparatus provided for the class. The apparatus consists of a graduated syringe with a movable piston. Unless there is a leak, the number of gaseous particles trapped in the syringe remains constant.
Initially, the syringe is adjusted to trap a volume of gas at the same pressure as its surroundings. The piston then does not move because the pressure exerted by the gas in the syringe equals the pressure of the atmosphere pushing on the piston. If the piston is pushed downward, it compresses the gas trapped in the syringe. If the pressure on the piston is then decreased, the pressure of the trapped gas will push the piston up.
In order to read the volume of trapped gascorrectly, you must always read the measurement on the side of the piston that is in contact with the gas.
In this investigation, you will use a Boyle’s law apparatus to compress a sample of air. You should recognize that air actually is a mixture of gases – mostly nitrogen and oxygen – that behaves physically as a single gas. The data you obtain for the air sample may be treated as data for a single gas. As the pressure of the air sample changes, you will monitor and collect data on the resulting changes in volume. You can then use your data to find the atmospheric pressure and determine how closely your results agree with Boyle’s law.
Pre-Lab Questions
  1. State Boyle’s law in your own words. Then write the mathematical equation for Boyle’s law.
  2. When the piston in the Boyle’s law apparatus is at rest, what is the relationship between the pressure of the trapped gas and the pressure on the outside of the piston?
  3. What are the possible sources of external pressure on the piston during this investigation?
  4. What is the benefit of collecting three sets of data in the investigation?

PROBLEM
How does the volume of an enclosed sample of gas change as the pressure of the gas is changed?
MATERIALS
safety goggles
Boyle’s law apparatus / Weights
SAFETY
Wear your goggles and lab apron at all times during the investigation.
PROCEDURE
  1. Put on your safety goggles.
  2. Adjust the initial volume (about 30 mL) to atmospheric pressure as directed by your teacher.
  3. Remove the plunger of the syringe to allow air to enter.
  4. Close the other end of the syringe(the end where the needle is being placed) by placing clay. Ensure that there is no leak.
  5. Bring the plunger to the trapped air.
  6. Using the scale of volume written on the body of the syringe, record the initial volume of the trapped air.
  7. Apply pressure on the trapped air by placing 25 g – weight on the plunger. Record the new volume of the trapped air.
  8. Repeat procedure #5 using 50 g, 75 g, 100 g, and 125 g weights.
  9. Remove all the weights from the piston and reset the apparatus to the initial volume recorded on procedure #6
  10. Repeat the procedure two (2) more times.
  11. Clean up your work area and wash your hands before leaving the laboratory.

OBSERVATION
Data Table
Pweights / V1 (mL) / V2 (mL) / V3 (mL) / Vavg (mL) / 1/ Vavg (mL-1) / Ptotal / Ptotal x Vave
0
25
50
75
100
125
CALCULATIONS AND QUESTIONS
Calculations
  1. Find the average of each set of three volumes and record these averages in the data table.
  2. Plot your data in a graph. Place the pressure on the x-axis and the volume of the gas on the y-axis. Does the graph indicate a linear relationship (proportional relationship) between the pressures and their corresponding volumes? Explain.
  3. According to Boyle’s law, pressure and volume have an inversely proportional relationship. If this data is correct, you should obtain a linear relationship (straight line) when you plot pressure versus the corresponding inverse of the average volume. Calculate the inverse, 1/Vavg of each volume and record these values in the appropriate column of the data table.
  4. In the same graph, using different color or ink, plot the pressures versus their corresponding 1/Vavgvalues. Do these points suggest a linear relationship? Explain.
  5. The line obtained for the second plot crosses the vertical axis of the graph above the origin, which tells you that there is pressure on the gas even when there are no weights on the piston. Consider 1/V = 0 only when the total pressure on the gas is zero (and the volume is infinitely large). The additional pressure is the atmospheric pressure. To find the pressure in using masses, extend the plot of P versus 1/V to the point where it intersects the horizontal axis. At this point 1/V = 0. The scale distance from this point to the origin is the atmospheric pressure measured in weights. Using your graph, determine this value.
  6. Add the value you found for atmospheric pressure to the pressure in weightsfor each trial and record these values of Ptotal in the table.
  7. Calculate the product of Ptotal x Vavefor each trial and record these values in the data table.
Critical Thinking: Analysis and Conclusion
  1. What patterns does the graphof P versus V suggest?(Interpreting diagrams)
  2. How does the graph of P versus 1/V illustrate Boyle’s Law? (Interpreting diagrams)
  3. Look at the values you calculated in the last column of the data table. How do they compare? (Making comparisons)
  4. What do these values mean in terms of Boyle’s law? (Making inferences)
Critical Thinking: Applications
  1. The total external pressure on the piston when no weights are present is due to the pressure of the atmosphere plus the mass of the piston. Suppose your partner says the external pressure come exclusively from the mass of the piston. How would you disprove this idea? (Designing experiments)
  2. Recall the bicycle pump mentioned in the Introduction to this lab investigation. In order for the pump to work, air pressure must be greater in the pump than in the bicycle tire. Then air will move from the pump to the tire, causing inflation. How does the principle of Boyle’s law come into effect in the operation of a bicycle pump? (Applying concepts)
For the Pull-Out Class
  1. Research the four-step cycle of the operation of the cylinders and pistons in an automobile engine. Find out what happens in each step of the cycle and determine when and how the principle of Boyle’s law applies to the function of the engine.

REFERENCES
Chemistry: Connections to Our Changing World, Prentice Hall. p. 197-202
Chemistry for the New Millennium, Adriana Publishing Co., Inc. p. 85-87