SUPPLEMENTAL QUESTIONS FOR CHEM 1A – EXAM #3 The following questions are provided to supplement your studying efforts for the first exam. I suggest you use these problems as “practice exams,” so try them WITHOUT your notes or text available, do them in a quiet place with all distractions silenced, and give yourself some type of time limit in which to finish a set number of them. This will best simulate the exam environment. Based on class progress, the set of questions below may not be an exhaustive list – I will make you aware of any changes to this question set if any needs to be made. When performing numerical calculations, pay attention to your significant figures!!! Questions in bold type are questions I consider to be more challenging. 1. The formula C2H6O has two isomers. Draw the structure of each isomer (in any form you like: Lewis, condensed, or line) and determine which of them you predict to have the lower boiling point. Provide a brief explanation of your choice. 2. You are given two glass boxes, one containing a green liquid and the other a blue solid. You have been told both boxes are at room temperature, and the external pressure being applied to the substance within the boxes is equal. Are the following statements true or false regarding these substances? Provide a brief explanation of your choice. a. The blue solid has the lower melting point. b. The green substance is a macromolecular solid. c. If you are told one of the substances is CH3(CH2)4CH3 and the other is cobalt(II) nitrate, the green liquid is cobalt(II) nitrate. 3. Draw a particulate level representation of the two substances in part #2 as they would appear at the given conditions. Use an open circle (¢) to indicate the species as a particle. 4. Osmium is melted and allowed to slowly cool to form a crystalline solid structure. In one experiment conditions are set so as to attempt to force the osmium atoms to assume a body-centered cubic unit cell, and in a second experiment an attempt is made to force the osmium into a face-centered cubic unit cell. Osmium has a radius of 135 pm. Determine the expected density of osmium for each trial. 5. A sample of vanadium has a measured density of 6.22 g/mL. The laboratory’s X-ray crystallographer reports this sample has crystallized with a coordination number of 12, and that the crystal reflects 2.735 angstrom x-rays in phase via second order diffraction at an angle of 46.2°. Use this information to estimate Avogadro’s number to three significant figures. You should know immediately if your answer is correct. Remember, one angstrom = 10−10 m.
6. Below is given the unit cell for an ionic species containing iron and sulfur. Using this unit cell, determine how many mols of this iron sulfide are present in 7.34 g of the solid.
7. The formula C3H8O has three isomers (let’s call them A, B, and C) in which no ring structures are present and the carbons are saturated (each carbon has four single bonds within the structure). Liquid forms of A and B have similar surface tension and viscosity values, whereas a liquid form of C has a lower value for these two properties. From this observation, determine plausible condensed formulas for each isomer. Hint: you may not be able to assign a letter to each individual isomer. Give a brief justification of your answer. 8. Below is shown a vapor pressure visualization of a liquid at a point where a physical equilibrium has been established. The grey rectangle at the bottom represents the liquid and the number of spheres present represents the amount of vaporized particles at physical equilibrium. Use this to answer the questions that follow.
a) Draw a vapor pressure visualization of this substance if the temperature was to be increased. Give a brief justification of your drawing. Hint: there is more than one correct answer, only supply one plausible drawing. b) Draw a vapor pressure visualization of this substance if the external pressure inside the box was decreased. Give a brief justification of your drawing. Hint: there is more than one correct answer, only supply one plausible drawing.
c) Draw a vapor pressure visualization of this substance if more of the liquid was to be poured into the container. Give a brief justification of your drawing. d) Assume the drawing as given represents CH3Cl. Draw a vapor pressure visualization of CH3Br at the same temperature and pressure. Give a brief justification of your drawing. Hint: there is more than one correct answer, only supply one plausible drawing. e) Draw a vapor pressure visualization of this substance if a non-volatile solute was added. Give a brief justification of your drawing. Hint: there is more than one correct answer, only supply one plausible drawing. 9. Molecule A can H-bond, while molecule B has only dispersion forces. Draw approximate heating curves for each, starting each in the solid phase and adding sufficient heat to produce first the liquid then vapor phase. Briefly describe the reason for any differences between your two heating curves. You may assume the molar masses for each molecule are approximately equal. 10. A small amount of the following solids or liquids are going to be placed in water. Write the eventual identity of the species in solution. Write “NR” if you think the species would be insoluble. Be sure to include states and any possible charges. a) CaI2(s) → b) (CH3)2CHCH3(l) → c) CH3CH2OH(l) → d) Fe(OH)3(s) → e) (NH4)2CO3(s) → f) BaSO4(s) → g) PbBr2(s) → h) C4H9Cl(l) → 11. Categorize all species from #10 as electrolytes or non-electrolytes and give a qualitative estimate of ΔSmix for each as either “high” or “low.” 12. One of Dr. McKeachie’s go to summer drinks is what he calls a “mineral spritz,” which is Campari (dark red bitters) mixed with chilled Pellegrino (mineral water). Based on its density and alcohol content, Campari is about 4.8 M ethanol in water. Assuming volume is additive when mixing, answer the following questions regarding the mineral spritz. a) The typical recipe is equal volume Campari and mineral water. What is the ethanol molarity of a typical mineral spritz?
b) How many 125 mL mineral spritz can he make using a fifth of Campari? A fifth is defined as 750 mL. Hint: use your answer from part a as the molarity of each mineral spritz made. c) Let’s say he wants to make a 125 mL mineral spritz that is 1.0 M ethanol. What volume of Campari and Pellegrino are required? d) Campari gets its dark red color from the naturally occurring dye carmine, which has an absorption λmax at 500 nm with ε = 0.09244 M−1 cm−1. Because he is a nerd and had some spare time, Dr. McKeachie prepared several mineral spritz as shown in the table below. Complete the table, using the “pure” form of the Beer-Lambert law (A = ε I C; no “+ error” and I = pathlength) for “expected absorbance.” Assume he will use a 1.00 cm cuvette.
Spritz contents [ethanol] expected absorbance @ 500 nm (1) Campari only
(2) 90. mL Campari; 100. mL water (3) 68 mL Campari; 100. mL water (4) 45 mL Campari; 100. mL water (5) 23 mL Campari; 100. mL water
(6) 100. mL water e) An “unknown mineral spritz” has been prepared by dumping random amounts of Campari and Pellegrino together until the total volume is 250. mL. This solution gives an absorbance of 0.246 at 500 nm in a 1.00 cm cuvette. What volume of Campari and Pellegrino were used? Let’s make this easier and assume the absorbance of the unknown is the same as the expected absorbance from the Beer-Lambert Law (again, no “+ error”). f) On special occasions, Dr. McKeachie will make a “champagne spritz” where he replaces mineral water with sparkling wine. Assuming a typical California sparking wine is 2.1 M in ethanol, what is [ethanol] in a champagne spritz? 13. 8.99 g of CHCl3 (chloroform) are mixed with water, and this solution has a final volume of 194 mL. The solution’s density is 1.03 g/mL. Determine the mass percent, mol fraction, molarity and molality of chloroform this solution. You may assume the density of water is 1.000 g/mL. 14. What is the expected vapor pressure of the solution created in #13, in atm, at 29 °C? The vapor pressure of pure water is 30. mm Hg at 29 °C and the vapor pressure of pure chloroform at 29 °C is 110 Torr. 15. Let’s say the vapor pressure of the solution created in #13 is measured as 0.466 psi. Are positive or negative deviations to Raoult’s Law occurring? Does this match what you would expect, given the components of this solution?
16. Determine the boiling and freezing point of the solution created in #13, in °C. 17. More water is added to the solution created in #13, and the osmotic pressure at 37 °C of the new solution is measured as 5.52 atm. How many mL of water were added to the original solution? 18. A small amount of sodium sulfide has been added to 47 g of water. This sample of water is observed to boil at 100.759 °C. What mass of sodium sulfide was placed in the water?