CHEMISTRY 20B SAMPLE MIDTERM EXAM

These questions are samples only and do not reflect the variety of topics that have been covered in class and the textbook. Other types of questions on material covered in the class and homework could appear on the midterm. The summer 2010 midterm exam will primarily cover material in Oxtoby from chapters 9 through the middle of chapter 12 plus lecture material that may go beyond the book. The midterm will consist of 4 multi-part questions over 2 hours. Do not come to the exam if you are auditing the course. NAME ________________________________________ STUDENT ID # _______________________ INSTRUCTIONS: This exam consists of 3 questions on 6 single-sided pages. This is a closed-book exam; the front side only of one 8.5x11 note sheet is allowed. You may use a simple calculator. Show all of your work; no credit will be given for an answer alone. Give units for all numerical answers and employ significant figures. SOME HELPFUL CONSTANTS AND CONVERSIONS: Gas Constant : R = 8.314 J K-1 mol-1 = 0.08206 L atm K-1 mol-1 Avogadros Number : N0 = 6.0221 x 1023 mol-1 Boltzmanns Constant: kB = 1.3807 x 10-23 J K-1 = 1.3807 x 10-16 erg K-1 Plancks Constant: h = 6.626 x 10-34 J s = 6.626 x 10-27 erg s Gravitational Acceleration g = 980 cm s-2 = 9.80 m s-2 1 atm = 1.013 x 105 Pa

Question Value Score 1 20 2 30 3 30

Total 80

Question 1 (20 points) The following questions pertain to aqueous solutions. The normal boiling point of water is 100.0 C, and the normal freezing point of water is 0.0 C. The freezing point depression constant for water is kf = 1.86 K kg mol-1, and the boiling point elevation constant for water is kb = 0.512 K kg mol-1. The mass density of water is 1.00 g mL-1. 1. (a) Suppose that 0.10 moles of sodium chloride (NaCl) salt are added to 1.0 L of pure

liquid water. What is the new normal freezing point in C of the salt solution? 1. (b) Suppose that 100.0 mL of a homogeneous aqueous solution contains 3.0 g of an

unknown material that dissolves but does not dissociate. The normal boiling point of the solution is measured to be 101.0 C. What is the molar mass (i.e. molecular weight) of the unknown material?

Question 2 (30 points) The vanadic ion V3+ forms green salts and is a good reducing agent. In a neutral aqueous solution, it is transformed into the colorless vanadic hydroxide ion, V(OH)4+. The salt, vanadic sulfate V2(SO4)3 , can be oxidized in aqueous solution according to the following incomplete and unbalanced equation fragment:

V2 SO4( )3(aq) + ... V(OH)4+(aq) + SO42(aq) + ...

2. (a) What is the oxidation number of vanadium on the left hand side of the equation

fragment? What is the oxidation number of vanadium on the right hand side? 2. (b) Using the equation fragment as a guide, complete and balance the reaction for the

oxidation of vanadium sulfate using common species that commonly participate in aqueous redox reactions of electrolytes (e.g. hydronium ions, hydroxide ions, water molecules, electrons). Add only species that are absolutely necessary.

2. (c) Suppose you are given 0.540 g of an unknown substance X and have a 0.200 M aqueous solution of vanadium sulfate. Suppose each molecule of X can accept one electron. You would like to know the molecular weight of X, so you add the vanadium sulfate solution to X in a beaker a little bit at a time and stir thoroughly while watching. What kind of color change do you expect to see as you increase the vanadium sulfate solution? If the solution changes color after 15.0 mL have been added to substance X, what is the molecular weight of X?

Question 3 (30 points) Consider a solution of liquid methanol (CH3OH) mixed with liquid propanol (CH3CH2CH2OH). At 40.0 C, the two liquids are fully miscible, the vapor pressure above pure liquid methanol is 303 torr, and the vapor pressure above pure liquid propanol is 44.6 torr. 3. (a) Which of the two alcohols is more volatile? Briefly explain using 1-2 sentences. 3. (b) Assuming ideal solution behavior, what is the liquid composition of a solution of

methanol and propanol at 40.0 C if the total vapor pressure above the solution is 202 torr? You may express your answer as a mole fraction of methanol and a mole fraction of propanol.

3. (c) For the total vapor pressure of 202 torr given in part (b), what is the mole fraction of methanol in the vapor phase at 40.0 C? Is this higher or lower than the mole fraction of methanol in the liquid solution? Briefly explain why your answer is sensible.

3. (d) Briefly explain a simple process by which you could separate out highly pure

methanol from the solution of methanol and propanol at 40.0 C.