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Unformatted text preview: Name: BLANK Chem 230 Exam #2
October 27, 2008 Discussion Section #: Use only the space provided to answer the questions. Ifyou need more work space, start your
answer on the back ofa page and then write only your final answer in the space provided.
Anything written in the margins and on the back ofpages will not be graded. Make sure your final answer is clearly marked. The exam totals to 150 points. Multiple Choice:
Mark all multiple choice clearly, by both circling the correct choice and writing the letter. Each
multiple choice question is worth 3points. 1) How many moles of COClz (Ksp = 64) are needed to raise the boiling point of 2 kg of water by
3 CC? For water, kb = 0.5 K kg mol'l. a) 2 mol b) 3 mol c) 4 mol d) 6 mol e) None of the above 2) For a constant number of moles of a particular gas, which of the following results in the
largest magnitude AS (i.e., ignore the sign)? a) Isothermally increasing the volume from 1 L to 2 L b) Isothermally decreasing the pressure from 2 atm to 1 atm c) Raising the temperature from 150 K to 300 K under constant volume d) Lowering the temperature from 300 K to 150 K under constant pressure e) All of the above are the same 3) For the reaction COz(g) + H20(l) = H+(aq) + HCOg'(aq), what are the correct units for an
experimentally determined equilibrium constant? a) mol L‘1 b) mol2 L'2 c) mol L‘1 atm'1 d) mol2 L'2 atm'1 e) None of the above Name: BLANK 4) A patent is ﬁled for a new material that has negative absolute entropy. Which 1aw(s) of
thermodynamics suggest this is a fraud? a) The 1St b) The 211d c) The 3rd (1) Both the 211d & 3rd e) All three 1) None of them 5) If a titration was performed on a weak acid with a pKa of 4, at what pH would the equivalence
point be seen? a) less than 7 b) exactly 7 c) greater than 7 6) For a system composed of 6 random letters A-Z, which of the following is the state with the
most entropy? a) AAAAAA b) JKLLKJ c) YYZZYY d) AEIOUY e) They are all the same 7) Mg(OH)2 has K5510”. If 0.1 mol of Mg(OH)2 was added to l L of water, what would the
approximate pH of the water be, assuming the water was initially pH 7? a) 4 b) 6 c) 8 d) 10 e) None of the above Name: BLANK 8) Solution A is made from 1 L of water and 0.1 moles of Na(CH3COO), and Solution B is
made from 1 L of water and 0.1 moles of Na2(SO4). Assuming both salts completely dissolve
and behave ideally, which of the following is a M statement? a) Solution A has higher osmotic pressure than Solution B b) Solution A has a lower vapor pressure than Solution B c) Solution A has a higher freezing point than Solution B (1) Solution A has a higher boiling point than Solution B e) None of the above 9) Which of the G vs T plots shown below indicates a solute that has strong interactions with the solvent?
A Pure solvent "~._
‘.. """ ..
"~., Temperature Temperature Temperature 10) H3PO4 has a Kar of 102. If the pH of a solution is 3 and the combined concentration of all
forms of HxPO4 is 1 mol L‘l, what is the best estimate of the concentration of H2PO4‘? a) 1 mol L‘1 b) 0.9 mol L‘1 c) 10‘2 mol L‘1 d) 0.5 mol L‘1 e) None of the above 11) At a particular concentration of solute, an ideal solution is predicted to have a pressure of
6x10‘6 atm at 300K and 7x10‘6 atm at 350 K. A real salt is added to water; the solution cools, and
some solid does not dissolve. The osmotic pressure is calculated to be 6X10‘6 atm at 300 K.
What will the osmotic pressure be at 350 K, assuming the volume of the solution does not
change? a) less than 6x10‘6 atm b) 6X10‘6 atm c) between 6X10‘6 atm and 7x10‘6 atm d) 7X10‘6 atm e) greater than 7x10‘6 atm Name: BLANK 12) The reaction COz(g) + H20(l) r—‘ H+(aq) + HCO3'(aq) is exothermic. Which of the following
is a fali statement? a) Addition of HCl will increase the pressure of C02 b) Cooling the reaction will increase the concentration of H+ c) Heating the reaction will decrease the pressure of C02 d) Increasing the volume of the container will decrease the concentration of H+ e) None of the above 13) Which of the following is a spontaneous process? a) Any process with ASSys > 0 b) Two gases in the same container maintaining different temperatures c) The precipitation of NaCl (KSp = 36) when 0.1 moles are added to 1 L of water
d) All of the above e) None of the above 14) The reaction HF + NH3 r—‘ F' + NH4+ has K=107. For NHI, pKa =9. What is the pKa of HF?
a) 3 b) 2 c) -3 d) 5 e) None of the above 15) Treating both salts as ideal, what is the correct relationship for the processes of dissolving
NlClz and NlC12*4H20 in water? a) AS NlClz > AS NlClz*4H20 b) AS NlClz < AS NlClz*4H20 c) AS NlClz = AS NlClz*4H20 d) There is insufficient information to say 16) 0.5 moles of a salt are added to 1 L of water. The solution warms, and some solid remains.
What is the sign of AS" for this process? a) Positive b) Zero c) Negative d) Cannot be determined Name: BLANK Short Answer Most questions can be answered solely with numbers, diagrams, or short phrases. When asked
to “explain” or ’justijjz, " use a maximum of m sentences or a clear diagram. All numerical
answers must have correct units on the final answer. Showing clear work within the space
provided is recommended for calculations to allow for partial credit. Clearly indicate your final
answer by boxing or underlining, if showing work or changing your answer. 17) (18 pts) Answer the following questions on combustion of ethane.
2 C2H6(g) + 7 02(g) —> 4 C02(g) + 6 H20(g)
Energies of Formation 2 C(s, graphite) -- 3 H2(g) —> C2H6(g) AH°1= -100 kJ mol‘1 AS°1= -200 J mol‘1 K‘1
1/2 02(g) + H2(g) —> H20(g) AH°f= -200 kJ mol‘1 AS°f= 0 J mol‘1 K‘1
C(s, graphite) + 02(g) —> C02(g) AH°1= -400 kJ mol‘1 A “f: 0 J mol‘1 K‘1
6 C(s, graphite) -- 3 H2(g) —> C6H6(g) AH°f= 100 kJ mol‘1 AS°f= -100 J mol‘1 K‘1 a) Calculate the free energy of the reaction at 500 K. b) If the combustion occurs in a heat engine with a 250 K cold reservoir, what is the maximum
amount of work that can be done with this reaction? Name: BLANK 18) (16pts) Use the KSp values below to answer the following questions. Ca(C03) 10‘ll Na2(C03) 64
Can 10‘12 NaF 25
Ca(SO3) 10-9 Na2(SO3) 36
Ca3(PO4)2 10‘25 Na3(PO4) 81 21) Rank the calcium salts in order of solubility, from most soluble to least. b) What is the maximum concentration of Ca2+(aq) in a 0.1 mol L‘1 NaF solution? c) 10‘6 mol of Ca(803) are added to l L of water. What is the maXimum number of moles of
Na2(803) that can be added without any solid precipitate appearing? 19) (8 pls) A new drug is a weak acid with pKa of 7. The HA form of the drug has a partition
coefﬁcient of 100 as deﬁned by [Drug](octanol)/[Drug](H20). The A‘ form has a partition
coefﬁcient of 102. The pH of blood is approximately 7.4. Based on this information, predict
which form of the drug will dominate in cell membranes, assuming octanol is a good model. Brieﬂy justify your answer. 20) (8 pls) Identify which two parameters (P, V, n, T) must be held constant to accurately calculate ASfus, and brieﬂy explain why.
#1) #2) Name: BLANK 21) (28 pts) The following reactions occur simultaneously in a single vessel.
2 NOCl(g) r—‘ 2 NO(g) + Clz(g) K = 0.1 Clz(g) + H20(l) r—‘ H+(aq) + Cl'(aq) + HOCl(aq) K = 100
a) Write the equilibrium eXpression for the first reaction. b) Write the equilibrium constant eXpression for the second reaction. c) Write the overall equilibrium constant expression for the vessel. d) If initial conditions in the vessel are 1 atm for all gases and 10'5 mol L‘1 for all aqueous
species, will the total pressure in the vessel increase, decrease, or remain constant? Brieﬂy
justify your response. e) After the system has reacted equilibrium, the total pressure is increased with Ne. Does the
pressure of C12 increase, decrease, or remain unchanged? Brieﬂy justify your response. f) When the system is in equilibrium, the pH of the solution phase is increased. Does the
pressure of NOCl increase, decrease, or remain unchanged? Brieﬂy justify your response. g) When the system is in equilibrium, the volume of the container is decreased. Does the
concentration of HOCl increase, decrease, or remain unchanged? Brieﬂy justify your response. Name: BLANK For problems 22 & 23, use the following information: H3PO4 = H2PO4' + H+ Kal = 10'2 H2C03 = HCOg' + H+ Kal = 10'4
H2PO4' = HPO42' + H+ Kaz = 10'7 HCOg' = COgZ' + H+ Kaz = 10'10
HPO42' = PO43' + H+ Kag = 10'12 NH4+ = NH3 + H+ Ka = 10'9 22) (12 pts) A titration of a 150 ml solution of H2C03 takes 30 ml of 0.1 mol L'1 NaOH to reach
the final equivalence point.
a) What was the initial concentration of H2C03 in the solution? b) What volume of 0.1 mol L'1 H3PO4 would you need to be able to titrate it with the same
amount of NaOH? c) A newly discovered polyprotic acid is titrated as in part a, but only a single equivalence point
is observed, at pH 7. Brieﬂy explain what this indicates about the acid. 23) (12 pts) To fill in the table below, choose the list two acid-base pairs (from the top of the
page) to buffer the desired pH. For each acid-base pair, indicate the acid/base ratio necessary to achieve the indicate pH. The two buffers chosen are independent, not mixed together.
pH Buffer 1 Acid/Base Ratio 1 Buffer 2 Acid/Base ratio 2 3 11 ...
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