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1 Unit How do we analyze a system? Modeling and Measurement Chemistry XXI Lab Session 4 Chemistry XXI How do we use models to derive properties? Your Challenge Five volatile solvents have been found in different containers in an illegal drugs lab. Based on prior experiences, you infer the potential identity of these substances: Butanone Ethanol Chemistry XXI C4H8O C2H6O Hexane Cyclohexane Ethyl Acetate C6H14 C6H12 C4H8O2 How can you determine the identity of the solvent in each container? Modeling Modeling is a powerful tool not only to explain and make predictions about the properties of substances, but to guide the experimental determination of such properties. In lecture, you have been analyzing the particulate model of matter. Chemistry XXI How can we use it to derive properties useful in the identification of unknown substances? Ideal Gas Model At low P and T much higher than their boiling point, the behavior of many gases can be modeled assuming that the system is composed of moving, non interacting particles. Why? Chemistry XXI Basic Relationships What does the ideal gas model predict? P P P T N V Chemistry XXI This ideal behavior of gases is described by the following relationship: Proportionality Constant NT P=k V Equation of State Ideal Gas NT P=k V The number of particles in a real sample can be expressed in terms of the mass (M) of the system: M N= m1 Total mass Mass of 1 particle Chemistry XXI MT P=k m1V MT m1 = k PV This relationship could be used to determine m1 for a substance (a differentiating characteristic) based on the measurement of P, T, V, and M for a sample of gas. Basic Relationships The mass of 1 particle is a very small number. Thus, we prefer to use the mass of a large, but fixed, number of particles as the differentiating characteristic. The reference number is Avogadro s number NA = 6.022 x 1023 particles (1 mole of substance) Then, mN = N A m1 , the MOLAR MASS, is the A differentiating characteristic most commonly used. Chemistry XXI MT m1 = k PV If mN A m1 = NA mN A MT MT = N Ak =R PV PV atm L R = 0.082057 mol K Your Ideas According to our model: mN A MT =R PV So, what do we need to measure to determine mNA for a gas sample? How can these properties be measured? Available resources: Sample of known substance (acetone, K2); Chemistry XXI Balance, thermometer, barometer; Hot and cold baths; Glassware; Discuss in your groups the actual procedure you propose to follow. Your Plan mN A MT =R PV How are you planning to generate the sample of gas? How are you planning to measure its temperature? How are you planning to measure its pressure? How are you planning to measure its mass? How are you planning to measure its volume? Chemistry XXI WHY? Where are the most likely sources of error? How can you plan to avoid or minimize them? Skill Building Tests Apply your ideas and experimental skills to determine the molar mass of the known substance provided. Available resources: Sample of known substance (acetone, K2); Balance, thermometer, barometer; Hot and cold baths; Glassware; Chemistry XXI Make sure to repeat your measurements at least two times! You have 45 minutes Share Results Present your measurements and calculations to the class. Decide as a group which data should be shared to facilitate analysis. the Chemistry XXI Some Reflections As a group Discuss the difference, if any, between the average experimental result and the actual molar mass of the gas. How do you explain these differences? Discuss what the standard deviation tells you about the precision of the average value. Chemistry XXI How could you modify your experimental procedure to improve the accuracy and/or the precision of your results? How could you modify your model to improve its usefulness in identifying unknown substances? Your Challenge Five volatile solvents have been found in different containers in an illegal drugs lab. Based on prior experiences, you infer the potential identity of these substances: Butanone (Tb = 80.0 oC) Cyclohexane (Tb = 80.7 oC) C4H8O C6H12 C2H6O C4H8O2 C6H14 Ethanol (Tb = 78.4 oC) Chemistry XXI Ethyl Acetate (Tb = 77.1 oC) Hexane (Tb = 69.0 oC) How would you face this challenge? What questions do you propose to answer? Beginning Questions What question would your group like to investigate to face the challenge? Discuss with the whole group the best strategy to find the identity of the five unknowns. Group Question 1 2 3 4 5 6 Chemistry XXI Your Real Challenge Design and implement an experimental procedure that will allow you to answer your question. Available resources: Samples of unknown substances; Balance, thermometer, barometer; Hot and cold baths; Glassware; You have 60 minutes Chemistry XXI Claims and Evidence Based on the results of your experiments, present your major claims and the evidence that you have to support the answer to your question. Is the group answering their beginning question? Are their claims clear? Chemistry XXI Is the evidence reliable? Is the evidence appropriate to support the claims? How do you explain your results? Final Reflections What did you learn from doing your experiment? How would you improve what you did? How have your ideas changed as a result of this lab? Chemistry XXI What do you not completely understand? What new questions do you have? Your Report Beginning questions (2 p): What questions guided your explorations? Safety Considerations (2 p): What did you do to stay safe in the lab? Procedures and Tests (2 p): What experiments did you do to answer your questions? Chemistry XXI Data, calculations, and representations (6 p): What observations did you make? What data did you collect? What calculations and representations helped you make sense of the data? Your Report Claims (2 p): What can you claim to answer your questions? Evidence and Analysis (6 p): How did you interpret your results to support your claims? Reflections and additional questions (10 p): What did you learn? What do you not completely understand? How have your ideas changed as a result of this lab? What new questions do you have? How would you improve what you did? Chemistry XXI Your Report Post-Laboratory Questions: 1. Discuss what your results indicate about the use of the ideal gas law to determine the molar mass of any gas. Your discussion should include your judgment of the practicality of the technique and the validity of the basic assumptions (ideal gas behavior). Chemistry XXI 3. Suggest at least one change that you think would produce better results.

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