2. What mass of an aqueous 22.9% sodium chloride solution contains 99.5 g of water?

Transcription

1 CHEM 1412 MIDTERM EXAM (100 pts total) ANSWER KEY Student s Name PART A (20 multiple choice questions, 3 pts each): 1. The solubility of a gas in a liquid can always be increased by: a) decreasing the pressure of the gas above the solvent. b) increasing the pressure of the gas above the solvent. c) increasing the temperature of the solvent. d) decreasing the polarity of the solvent. e) decreasing the temperature of the gas above the solvent. 2. What mass of an aqueous 22.9% sodium chloride solution contains 99.5 g of water? a) 129 g b) 29.6 g c) g d) 22.8 g e) 99.5 g 3. Which of the following solutes, dissolved in 1000 g of water, would provide a solution with the lowest freezing point? a) mol of barium chloride, BaCl2 b) mol of urea, CO(NH2)2 c) mol of calcium sulfate, CaSO4 d) mol of acetic acid, CH3COOH e) mol of ammonium nitrate, NH4NO3 1 of 10

2 4. Which of the following solutions has the lowest osmotic pressure? a) 0.10 M Al(NO3)3 b) 0.20 M C6H12O6 c) 0.15 M Ba(NO3)2 d) 0.10 M CaCl2 e) 0.15 M Na2S 5. Ozone reacts with nitrogen dioxide to produce oxygen and dinitrogen pentoxide according to the following chemical equation: O3(g) + 2 NO2(g) O2(g) + N2O5(g) The rate law for this reaction is Rate = k[o3][no2]. If concentration is measured in moles per liter and time is measured in seconds, what are the units of k? a) L mol 1 s b) L 2 mol 2 s 1 c) L mol 1 s 1 d) mol L 1 s 1 e) mol 2 L 2 s 1 6. In a first-order reaction, the half-life is 137 minutes. What is the rate constant? a) s 1 b) 5790 s 1 c) s 1 d) s 1 e) s 1 2 of 10

3 7. The following data were obtained for the hypothetical reaction 2A + B products. [A]0 (M) [B]0 (M) Initial Rate (M/s) What is the overall order of this reaction? a) 3 b) 1/2 c) 0 d) 4 e) 1 8. For the hypothetical reaction A products, the concentration of A was monitored over time. From the following graph, what is the rate constant for the decomposition of A? a) s 1 b) s 1 c) s 1 d) s 1 e) s 1 3 of 10

4 9. The Ostwald process converts ammonia, NH3, to nitric oxide, NO, by reaction with oxygen in the presence of a catalyst at high temperatures. In a test of the process a reaction vessel is initially charged with 3.90 mol NH3(g) and 4.90 mol O2(g), sealed, and heated at a fixed high temperature. When equilibrium is established the reaction mixture is analyzed and found to contain 2.90 mol NO(g). What is the quantity of NH3(g) in the equilibrium reaction mixture? 4 NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(g) a) 1.00 mol NH3(g) b) 6.80 mol NH3(g) c) 3.90 mol NH3(g) d) 1.28 mol NH3(g) e) 2.00 mol NH3(g) 10. A 4.50-mol sample of HI is placed in a 1.00-L vessel at 460 C, and the reaction system is allowed to come to equilibrium. The HI partially decomposes, forming mol H2 and mol I2 at equilibrium. What is the equilibrium constant Kc for the following reaction at 460 C? ½ H2(g) + ½ I2(g) HI(g) a) b) c) d) 11.1 e) of 10

5 11. For the reaction 2 NO(g) + O2(g) 2 NO2(g) at 750 C, what is the relationship between Kc and Kp? a) Kc = Kp b) Kc = Kp (RT) 1 c) Kc = Kp = 1.0 d) Kc = Kp (RT) ¾ e) Kc = Kp (RT) Consider the following reaction: 2 HF(g) H2(g) + F2(g), Kc = Given that 1.00 mol of HF(g), mol of H2(g), and mol of F2(g) are mixed in a 5.00-L flask, determine the reaction quotient, Q. a) Q = b) Q = c) Q = d) Q = 2.12 e) none of these 13. What is Kc for the following equilibrium? For phosphoric acid, H3PO4, Ka1 = , Ka2 = , and Ka3 = Self-ionization constant of water Kw = HPO4 2 (aq) + OH (aq) PO4 3 (aq) + H2O(l) a) b) c) 491 d) 48 e) of 10

6 14. Which of the following will give a buffer with a ph near 4.76 when the acid and conjugate base are mixed in equimolar proportions? Acid Ka NH4 + (from ammonia) HC2H3O2 (acetic acid) HF (hydrofluoric acid) CH3CH2NH3 + (from ethylamine) a) acetic acid b) hydrofluoric acid c) ethylamine d) none e) ammonia 15. Suppose a buffer solution is made from formic acid (HCHO2) and sodium formate (NaCHO2). What is the net ionic equation for the reaction that occurs when a small amount of hydrochloric acid is added to the buffer? a) H3O + (aq) + HCHO2(aq) H2O(l) + H2CHO2 + (aq) b) H3O + (aq) + CHO2 (aq) HCHO2(aq) + H2O(l) c) HCl(aq) + CHO2 (aq) HCHO2(aq) + Cl (aq) d) HCl(aq) + OH (aq) H2O(l) + Cl (aq) e) H3O + (aq) + OH (aq) 2 H2O(l) 6 of 10

7 16. Which acid-base combination is depicted by this titration curve? a) Titration of a weak acid with a strong base. b) Titration of a weak base with a strong acid. c) Titration of a strong acid with a strong base. d) Titration of a strong base with a strong acid. e) Not enough information provided. 17. A saturated solution of which of the following salts will have the greatest molar concentration of silver ion? a) Ag2S (Ksp = ) b) AgCl (Ksp = ) c) Ag2CrO4 (Ksp = ) d) Ag2CO3 (Ksp = ) e) Ag4Fe(CN)6 (Ksp = ) 7 of 10

8 18. Which of the following, when added to a saturated solution of AgCl, will cause a decrease in the molar concentration of Ag + relative to the original solution? 1. HCl(g) 2. AgCl(s) 3. MgCl2(s) a) 1 only b) 2 only c) 3 only d) 1 and 3 e) 1, 2, and Sodium chloride is added slowly to a solution that is M in Cu +, Ag +, and Au +. The Ksp values for the chloride salts are , , and , respectively. Which compound will precipitate first? a) AuCl(s) b) All will precipitate at the same time. c) It cannot be determined. d) AgCl(s) e) CuCl(s) 20. Which of the following is not likely to form a complex ion with Al 3+? a) NH4 + b) NH3 c) OH d) H2O e) CH3NH2 8 of 10

9 PART B (4 problems, 10 pts each): For every problem below show ALL work to get full credit. Include formulas where appropriate. Round your answers to proper number of significant figures and record units with every numerical value calculated. Do not round intermediate answers, but do keep track of the last significant digit by underlining it. 1. The rate constants for the first-order decomposition of a compound are s 1 at 43 C and s 1 at 65 C. What is the value of the activation energy for this reaction? (R = 8.31 J/(mol K)) k1 = s 1, T1 = = 316 K k2 = s 1, T2 = = 338 K ln k 2 k 1 = E a R ( 1 T 1 1 T 2 ) R ln k 2 J k 8.31 ln E a = 1 ( 1 T 1 = mol K J 1 1 T ) ( K 1 = K ) mol = 59.6 kj mol 2. In an experiment, 0.42 mol H2 and 0.42 mol I2 are mixed in a 1.00-L container, and the reaction forms HI. If Kc = 49 for this reaction, what is the equilibrium concentration of HI? I2(g) + H2(g) 2 HI(g) Initial (M): Change (M): x x +2x Equilibrium (M): 0.42 x 0.42 x 2x K c = [HI]2 [H 2 ][I 2 ] = (2x) 2 (0.42 x)(0.42 x) = (2x) 2 (0.42 x) 2 = 49 2x = x 2x = x 9x = 2.94 x = M [HI] = 2x = M = 0.65 M 9 of 10

10 3. Consider the titration of ml of M NH3 (Kb = ) with M HNO3. After ml of M HNO3 has been added, what is the ph of the solution? Solution volume after titration: V = ml ml = ml = L mol HNO 3 = L M = moles mol NH 3 = L M = moles K a (NH 4 + ) = K w K b (NH 3 ) K a (NH 4 + ) = [H 3O + ][NH 3 ] [NH 4 + ] [H 3 O mol ] = = M L mol [NH 3 ] = = M L + NH 3(aq) + H 3 O (aq) initial (M): change (M): NH 4(aq) after titration (M): equilibrium (M): x x x = = = x( x) x x = [H 3 O + ] = x = H 2 O (l) = M ph = log[h 3 O + ] = Silver nitrate (AgNO3) is slowly added to a solution containing M Br and M Fe(CN)6 4 until a precipitate just forms. What is the molar concentration of Ag + just as the precipitate forms? AgBr Ksp = and Ag4Fe(CN)6 Ksp = What is the chemical composition of the precipitate? [Ag + ] = K sp(agbr) [Br ] [Ag + ] = ( K sp(ag 4 Fe(CN) 6 ) [Fe(CN) 6 4 ] = = M ) 1 4 = ( ) 1 4 = M If [Ag + ] < M, then no precipitation occurs If M < [Ag + ] < M, then AgBr precipitates, but Ag4Fe(CN)6 does not. If [Ag + ] > M, both AgBr and Ag4Fe(CN)6 precipitate. Since AgNO3 was added just until an initial precipitate forms, its composition must be pure AgBr. 10 of 10