Gateway General Chemistry 5/6/30 Exam 3 November, 005 6:0-7:40 individual or 8:0-9:40 7:40-8:00; 9:40-0:00 team problem Name Lab GSI (Ryan or Tom) Discussion GSI (Ryan or Tom) A periodic table is attached for use with the exam. You may remove it and do not need to turn it in with the exam. Page Questions Possible points -5 5 Score 3 6-7 8 4 8-9 6 5 0 9 6 6 7 4 8 3 9-0 Team 0 Total /50
Please write the correct answer in the box next to each question ( point each).. Consider the gas-phase equilibrium A B. In a series of experiments, different initial amounts of A and B are mixed together, and the mixture in each case allowed to come to equilibrium. Which one of these experiments would yield values for the amounts of A and B present at equilibrium different from all the other experiments? a. 3.0 moles A, 4.5 moles B ) C b. 4.5 moles A, 3.0 moles B c..5 moles A, 4.5 moles B d. 7.5 moles A, no B e. 0.5 moles A, 7.0 moles B. A weak acid is 5% ionized at equilibrium. Therefore we can say that the ionization reaction is -favored, because. a. product; the amount of products << the amount of reactants b. reactant; the amount of products << the amount of reactants ) B c. reactant; the amount of products >> the amount of reactants d. product; the amount of products >> the amount of reactants e. neither; not enough information is available to reach a conclusion 3. Consider the gas-phase equilibrium A B. Certain amounts of A and B are mixed in a vessel. As they come to equilibrium a. the forward reaction rate declines and the reverse reaction rate rises. 3) E b. the reverse reaction rate declines and the forward reaction rate rises. c. both forward and reverse reaction rates decline. d. both forward and reverse reaction rates rise. e. either a. or b. above, but we cannot say which without more information. 4. Once the reaction quotient, Q, has been determined for a reaction mixture, comparison with the value of the equilibrium constant, K, will determine a. if the mixture is at equilibrium or not. b. if the mixture has an excess of either products or reactants compared to equilibrium. c. if the mixture will react to the left, to the right, or not at all. d. Both a and b. 4) E e. All of a, b, and c. 5. Which reaction illustrates water acting as a base? a. Cu(HO)4 + + 4NH3 Cu(NH3)4 + 4HO b. HCO3 HO + CO c. NH3 + HO NH4 + + OH- d. HPO4 - + HO OH- + H3PO4 e. HSO4 - + HO H3O+ + SO4-5) E
6) ( point) For the reaction A 3B Kc =.37. If the concentrations of A and B are equal at equilibrium, what is the value of that concentration? K c =.37 = [B] 3 /[A] = x 3 /x = x.37m 7) (7 points) Give the equilibrium: HCN (aq) + H O (l) H 3 O + (aq) + CN - (aq) H >0; K a = 4.0 x0-0 What happens to the concentration of hydrogen cyanide [HCN] when the following stresses are placed on the system at equilibrium? (Circle the correct description of the [HCN] as a result of the stress described) a) Temperature is raised [HCN] b) NaCl is added c) NaOH is added d) NaCN is added e) HCl is added f) Water is added g) The following are mixed together: x0-4 M HCN x0-4 M CN - 5 x0-3 M H 3 O + Q = [H 3 O + ][CN - ]/[HCN] = (x0-4 * x0-4 )/(5x0-3 ) = x0-6 Q>K 7 ( point each box) 3
8) ( points) According to Chemical and Engineering News (July 4, 005 p), if current CO emission trends continue, the oceans will become so acidic that corals will cease to thrive. The oceans are a sink for CO from fossil fuels, absorbing about half of emissions. When CO dissolves, it produces carbonic acid, which is corrosive to shells of marine organisms and can interfere with the oxygen supply of marine animals. In the past 00 years, the average ph of the surface seawater has declined from 8.3 to 8.. What is the increase in [H 3 O + ] does this corresponds to? ph = -log [H 3 O + ] 8.3, [H 3 O + ] = 5x0-9 8., [H 3 O + ] = 6x0-9 change in [H 3 O + ] = x0-9 9) (4 points; /3 point per box) Fill in the following table: Write out any and all equilibria taking place when this molecule is dissolved in water. If there is no equilibrium, then write NONE NH 4 Cl Circle the approximate ph of the resulting solution NH 3(aq) + H O (l) NH 4 + + OH - ph <7 ph~7 ph >7 NaCl NONE ph <7 ph~7 ph >7 HNO 3 NONE ph <7 ph~7 ph >7 Ca(OH) NONE ph <7 ph~7 ph >7 KClO 3 ClO 3 - (aq) + H O (l) HClO 3(aq) +OH - (aq) ph <7 ph~7 ph >7 H 3 PO 4 H 3 PO 4(aq) + H O (l) H 3 O + (aq) + H PO 4 - H PO 4 - (aq) + H O (l) H 3 O + (aq) + HPO 4 - HPO 4 - (aq) + H O (l) H 3 O + (aq) + PO 4-3 ph <7 ph~7 ph >7 4 4
0) (9 points) Consider the salt sodium fluoride used to fluorinate drinking water. At levels of ppm used for fluorinated water, there is not enough fluoride to affect the ph. However, at high concentrations ( M), the salt does affect the ph. What is the ph of.0 M solution of NaF. (K a HF = 7. x0-4 ) a) Write out the relevant equilibrium equation that would affect the ph: F - (aq) + H O HF (aq) + OH - (aq) Base acid conj acid conj base b) Label the acid, base, conjugate acid, and conjugate base in the equilibrium above. c) What is the expression for the equilibrium constant of the equilibrium that you wrote in a) K b = [HF][OH - ]/[F - ] d) What is the numerical value of the equilibrium constant? K b = x0-4 /K a =.39 x0 - e) What are the concentrations of all of the ions present in solution (Na +, F -, OH -,, H 3 O + ) at equilibrium? M NaF F - HF OH - I M 0 0 C -x +x +x E -x x x 3.73 x0-6 3.73 x0-6.39 x0 - = x /(-x).39 x0 - = x 3.73 x0-6 = x <<< [Na + ] =.0M [F - ] =.0 M [OH - ] = 3.73 x0-6 [H 3 O + ] =.68 x0-9 4 f) What is the ph? ph = -log[h 3 O + ] = -log (.68x0-9 ) = 8.57 5
) (6 points) Potassium acid phthalate also known as KHP (C 8 H 5 O 4 K) reacts with sodium hydroxide as shown below. KHP can be used to determine the concentration of a solution of sodium hydroxide through titration. KHP O C C O OH O - K + + NaOH O C C O O - Na + O - K + + H O Table : Data from titrations Weight Volume of KHP NaOH used (g) used (ml) 0.703 6.4 0.73 6.89 0.755 6.5 A titration is carried out by weighing dry KHP into a beaker, dissolving it in 75 ml of water, adding an indicator, and then titrating with NaOH until the endpoint is reached. The data obtained from three such titrations is recorded in Table. a) What is the concentration of NaOH? ) 0.703g KHP( mol KHP/04g KHP)( mol NaOH/ mol KHP)(/0.064L) = 0.304 M ) 0.335 M 3) 0.39 M Average = 0.343M b) What indicator could you use in this titration? Describe how you would know when you had reached the end-point. Phenolphthalein; turns from clear to pink 3 c) You can now use the NaOH solution to determine the concentration of acetic acid (CH 3 COOH) in as 0.0 ml sample of vinegar. It takes 0.3 ml of the NaOH solution from part a to reach the end point. What is the molar concentration of acetic acid in vinegar? 0.343 M NaOH (0.3L) (mol acetic acid) = 0.687M acetic acid L ( mol NaOH) (0.000L) 6
) (4 points) For the reaction H (g) + I (g) HI (g) K c = 50.0 at 745K a) Write out the expression for K c K c = [HI] /[H ][I ] b) Circle one: The reaction at 745K is: reactant favored product favored c) When.00 mol I and 3.00 mol H are allowed to come to equilibrium at 745 K in a flask of volume 0.00 L, what amount (in moles) of HI will be produced? H I HI I 0.3 M 0. M 0 C -x -x +x E 0.3-x 0.-x x K c = (x) /(0.3-x)(0.-x) = 50 4x =.5 0x + 50x 0 = 46x - 0 x + 5 x = 0.0963, 0.338 0.9 M HI + 0 L =.9 moles HI produced d) What total amount of HI is present at equilibrium if an additional 3.00 mol H is added to the 0.00 L flask? H I HI I 0.6 M 0. M 0 C -x -x +x E 0.6-x 0.-x x K c = (x) /(0.6-x)(0.-x) = 50 4x = 3 35x + 50x 0 = 46x - 35 x + 3 x = 0.099, 0.66 0.97 M HI + 0 L =.97 moles HI produced 7
3) ( points) The K sp of Ag SO 4 is.7 x0-5. What is the maximum concentration of each of the ions that can be achieved in solution? Ag SO 4(s) Ag + + SO 4 - K sp = [Ag + ][SO 4 - ] =.7 x0-5 = (x) (x) =.7 x0-5 4x 3 = x = 0.06 [Ag + ] = 3. x0 - M [SO 4 - ] =.6x0-8
Gateway General Chemistry 5/6/30 Exam November, 005 (7:40-8:00) Team problem (0 points) Names: Discussion GSI (Ryan or Tom) Attached is the journal article: Al Rmalli, S.W.; Harrington, C.F.; Ayub, M.; Haris, P.I. A biomaterial bases approach for arsenic removal of water. Journal of Environmental Monitoring 005, 7, 79-8. The article describes the preparation of arsenic standards: A 00 mg L _ arsenic(iii) stock solution was prepared from sodium arsenite Na 3 AsO 3 (BDH, Dorset, UK) and a 00 µg L - arsenic(v) stock solution was prepared from a 000 ± 3 µg L - standard As(V) solution (CPI, International, USA). The arsenic solutions used in the experiments were prepared by appropriate dilution of the stock solutions with deionised water. All the other reagents used were analytical or general reagent grade. How many mg of Na 3 AsO 3 would you weigh out and add to a 00 ml volumetric flask in order to make a 00.mL solution that is 00. mg/l in arsenic? 00 mg As ( 9.9 g Na 3 AsO 3 ) (0.L) Weigh out 5.6 g Na 3 AsO 3 into 00 ml flask and fill L ( 74.9 g As) to the mark with DI water How would you prepare 00 ml of a00 µg/l As(V) from a stock solution that was 000 µg/l As(V)? M V = M V 000µg (x) = 00 µg (00 ml) L L X = 0 ml Take 0 ml stock 000 µg/l put into a 00 ml volumetric and dilute to the mark The atomic absorption technique used to determine the concentration of arsenic uses Beer s Law relationship between concentration and absorption. First a calibration curve must be constructed. What was the range of standard concentrations used in the experiment in the paper? 3.0 6.0 µg/l 9
A specific weight (750 mg) of water hyacinth root powder was suspended in 5 ml of tap water spiked with 00 mg /L of arsenite [As(III)] or arsenate [As(V)] and adjusted to ph 6. The appropriate ph of the solution was obtained by addition of either nitric acid (0% v/v) or ammonium hydroxide (0% v/v). Given that ammonium hydroxide is equivalent to ammonia (NH 3 ), why would the researchers have had to use two different solutions to adjust the ph to 6? HNO 3 acid used to lower ph if above 6 NH 3 base used to raise ph if below 6 The authors were surprised by some aspect of the results shown in Figure. What aspect, and explain why they were surprised. Sketch the graph that they expected to see using the same axes as are shown in the paper. (Show the expected shape between the lowest and highest points.) They expected a linear curve. On page 8, the authors almost suggest an equilibrium that is affected by the ph of the solution. Describe this equilibrium in your own words. Include an equilibrium equation in your description. Vary with answer 0