Chem. 1C Midterm 2 Version B May 11, 2016

Transcription

1 Chem. 1C Midterm 2 Version B May 11, 2016 First initial of last name Name: Print Neatly. You will lose 1 point if I cannot read your name or perm number. Perm Number: All work must be shown on the exam for partial credit. Points will be taken off for incorrect or no units. Calculators are allowed. Cell phones may not be used for calculators. On fundamental and short answer problems you must show your work in order to receive credit for the problem. If your cell phone goes off during the exam you will have your exam removed from you. Fundamentals (of 36 possible) ( )+( ) = Problem 1 (of 17 possible) Problem 2 (of 17 possible) Multiple Choice (of 30 possible) Midterm Total (of 100 possible) 1

2 Fundamental Questions Each of these fundamental chemistry questions is worth 6 points. You must show work to get credit. Little to no partial credit will be awarded. Make sure to include the correct units on your answers. 1) 6 pts When a certain weak-field forms an octahedral complex with Ru 2+ cation, the energies of the valence d orbitals on the ruthenium atom are split according to this electron box diagram: Use the diagram to answer the following questions. How many unpaired d electron spins does the ruthenium atom have? 4 Is the complex paramagnetic or diamagnetic? Paramagnetic Predict the color of the complex? (You must show work to receive credit) blue or green red or violet orange or yellow white or transparent To be promoted from the t2g to the eg orbitals the electron must absorb 225 of energy. Determine energy needed to promote one electron Determine wavelength associated with that energy nm light means that the light is absorbed in the green/yellow area of the spectrum. Therefore, the sample will appear red/violet. 2) 6 pts What is the 1) oxidation number (on the metal), 2) the electron configuration for Co x (x is the oxidation number found in 1) and 3) coordination number for [Co(ox)3] 3-? 1) Co 3+ 2) [Co 3+ ]=[Ar]3d 6 3) 6 2

3 3) 6 pts What is the name or formula of the following? potassium diaquabis(oxalato)chromate(ii) K2[Cr(C2O4)2(H2O)2] [CoCl2(NH3)2(en)]NO3 diaminedichloroethylenediaminecobalt(iii) nitrate chlorobis(ethylenediamine)thiocyanocobalt(iii) tetrachlorocadmiumate(ii) [CoCl(SCN)(en)2]2[CdCl4] 4) 6 pts Three bottles of aqueous solutions are discovered in an abandoned lab. The solutions are green, yellow, and purple. It is known that three complex ions of chromium(iii) were commonly used in the lab: [Cr(H2O)6] 3+, [Cr(NH3)6] 3+, and [Cr(H2O)4Cl2] +. Determine the likely identity of each of the colored solutions Color of Solution Color Absorbed Energy Yellow Violet Highest Energy Green Red Lowest Energy Purple Yellow Order of crystal field splitting from lowest to highest. Cl - < H 2 O < NH 3 Therefore: Yellow Solution = [Cr(NH3)6] 3+, Purple Solution = [Cr(H2O)6] 3+, Green Solution = [Cr(H2O)4Cl2] + 5) 12 pts If you have an aqueous solution that is 10.% NaCl by mass, calculate the following: 1) Molarity of NaCl 2) Mole fraction of NaCl 3) Molality of NaCl The density of the NaCl solution is 1.07 and the density of water is Assume 100. g sample 10. g NaCl and 90. g H2O

4 Short Answer Questions Each of the following short answer questions are worth the noted points. Partial credit will be given. You must show your work to get credit. Make sure to include proper units on your answer. 1a) 8 pts What volume of ethylene glycol (C2H6O2), a non electrolyte must be added to 15.0 L of water to produce an antifreeze solution with a freezing point of C? (The density of ethylene glycol is 1.11, and the density of water is 1.00? Need to calculate Calculate m C2H 6O 2 Calculate Calculate mass of C2H6O2 15, Calculate , b) 3 pts What is the boiling point of this solution? c) 6 pts What is the osmotic pressure of this solution at 75 C? Assume volumes are additive. : Π Π

5 2) Consider the apparatus in which A and B are two 1.00 L flasks joined by a stopcock C. The volume of the stopcock is negligible. Initially A and B are evacuated, the stopcock C is closed. Then 35.0 g of chloroform, CHCl 3 is placed in flask A and 35.0 g of acetone, CH 3 COCH 3, is placed in flask B. The system is allowed to come to equilibrium at 25 C with the stopcock closed. The vapor pressures of chloroform and acetone at 25 C are 195 torr and 222 torr respectively. 2a) 2 pts What is the pressure in each of the flask at equilibrium? The pressure in flask A (chloroform) 195 torr and the pressure in flask B (acetone) 222 torr. 2b) 12 pts The stopcock is now opened. What will be the final composition (given by the mole fraction) of the gas and liquid phases in each flask once the system reaches equilibrium, assuming ideal behavior? Determine the final pressure of the solution Determine the moles of chloroform and acetone Find the vapor pressure of the solution Determine the pressure due to the chloroform Determine the pressure due to the acetone Determine the total pressure Determine the mole fraction of chloroform in the gas phase Determine the mole fraction of acetone in the gas phase The moles of chloroform and acetone that transferred to the liquid phase are small compared to the moles of liquid initially ( mol and mol of chloroform and acetone respectively in the gas phase). Therefore, the initial liquid mole values can be used. Determine the mole fraction of chloroform in the liquid phase Determine the mole fraction of acetone in the gas phase c) 3 pts Acetone and chloroform solutions show negative deviations from Raoult s law. Therefore, the vapor pressure of the system will be: greater than less than the same as the ideal vapor pressure. 5

6 Multiple Choice Questions On the ParScore form you need to fill in your answers, perm number, test version, and name. Failure to do any of these things will result in the loss of 1 point. Your perm number is placed and bubbled in under the ID number. Do not skip boxes or put in a hyphen; unused boxes should be left blank. Bubble in your test version (B) under the test form. Note: Your ParScore forms will not be returned to you, therefore, for your records, you may want to mark your answers on this sheet. Each multiple choice question is worth 5 points. 1. We can predict the solubility of a compound by looking at the sign of the enthalpy of solution. A) True B) False 2. A liquid-liquid solution is called an ideal solution if I. it obeys PV = nrt. II. it obeys Raoult's law. III. solute-solute, solvent-solvent, and solute-solvent interactions are very similar. IV. solute-solute, solvent-solvent, and solute-solvent interactions are quite different. A) I, II, III B) II, IV C) II, III D) I, II E) None of the above 3. For the complex ion [Ru(NCS)2(NH3)4] +, how many different isomers of all types are possible? A) 2 B) 6 C) 4 D) 3 E) None of the above 4. Rank the following compounds according to increasing solubility in water. I. CH3 CH2 CH2 CH3 II. CH3 CH2 O CH2 CH3 III. CH3 CH2 OH IV. CH3 OH A) I < III < IV < II B) I < II < IV < III C) I < II < III < IV D) III < IV < II < I E) None is correct. 6

7 5. Would you expect a solution of hexane (C6H14) and chloroform (CHCl3) to be relatively ideal, to show a positive deviation, or to show a negative deviation with respect to Raoult's law? A) positive deviation B) negative deviation C) relatively ideal 6. How many of the following coordination compounds will form a precipitate when treated with an aqueous solution of AgNO3. [CrCl2(NH3)3] [Cr(NH3)6]Cl3 A) 3 B) 2 C) 0 D) 1 E) 4 [CrCl(NH3)5](OH)2 Na3[Cr(CN)6] 7