Key: Concept explorations are comprehensive problems that provide a framework that will enable you to explore and learn many of the critical concepts and ideas in each chapter. If you master the concepts associated with these explorations, you will have a better understanding of many important chemistry ideas and will be more successful in solving all types of chemistry problems. These problems are well suited for group work and for use as in-class activities.
1.23. Physical and Chemical Changes
Say you are presented with two beakers, beaker A and beaker B, each containing a white, powdery compound. • a. From your initial observations, you suspect that the two beakers contain the same compound. Describe, in general terms, some experiments in a laboratory that you could do to help prove or disprove that the beakers contain the same compound. • b. Would it be easier to prove that the compounds are the same or to prove that they different? Explain your reasoning. • c. Which of the experiments that you listed above are the most convincing in determining whether the compounds are the same? Justify your answer. • d. A friend states that the best experiment for determining whether the compounds are the same is to see if they both dissolve in water. He proceeds to take 10.0 g of each compound and places them in separate beakers, each containing 100 mL of water. Both compounds completely dissolve. He then states, “Since the same amount of both substances dissolved in the same volume of water, they must both have the same chemical composition.” Is he justified in making this claim? Why or why not?
1.24 Significant Figures
Part 1 • a. Consider three masses that you wish to add together: 3 g, 1.9 g, and 3.8 g. These numbers represent measured values. Add the numbers together and report your answer to the correct number of significant figures. • b. Now perform the addition in a stepwise fashion in the following manner. Add 3 g and 1.9 g, reporting this sum to the correct number of significant figures. Next, take the number from the first step and add it to 3.8 g, reporting this sum to the correct number of significant figures. • c. Compare your answers from performing the addition in the two distinct ways presented in parts a and b. Does one of the answers represent a “better” way of reporting the results of the addition? If your answer is yes, explain why your choice is better. • d. A student performs the calculation (1.7 × 5.163 g) + 2.09 g and, being mindful of significant figures, reports an answer of 11 g. Is this the correct answer? If not, what might this student have done incorrectly? • e. Another student performs the calculation (5 × 5.163 g) + 2.09 and reports an answer of 32.1g. Is this the correct answer? If not, what might this student have done incorrectly? • f. Yet another student performs the calculation (5.00 × 5.163 g) + 2.09 and reports an answer of 30 g. Is this the correct answer? If not, what did this student probably do incorrectly? • g. Referring to the calculations above, outline a procedure or rule(s) that will always enable you to report answers using the correct number of significant figures.
Part 2 • a. A student wants to determine the volume of 27.20 g of a substance. He looks up the density of the material in a reference book, where it is reported to be 6.3342 g/cm3. He performs the calculation in the following manner: o 27.20 g × 1.0 cm3/6.3 g = 4.32 cm3 • Is the calculated answer correct? If not, explain why it is not correct. • b. Another student performs the calculation in the following manner: o 27.2 g × 1.00 cm3/6.33 g = 4.32 cm3 • Is this a “better” answer than that of the first student? Is this the “best” answer, or could it be…