Chem Midterm 4 May 14, 2009

Transcription

1 Chem Midterm 4 May 14, 009 Name All work must be shown on the exam for partial credit. Points will be taken off for incorrect or no units and for the incorrect number of significant figures. Only a non graphing calculator is allowed. On short 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. Page (of possible) Page (of 16 possible) Page 4 (of 1 possible) Page 5 (of 1 possible) Page 6 (of 8 possible) Multiple Choice (of 0 possible) Subtotal 1

2 1a) pts What is the conjugate base of HCl? Cl - 1b) pts What is the conjugate base of H SO 4? HSO 4-1c) pts What is the conjugate acid of NH? NH 4 1d) pts What is the conjugate acid of NaOH? H O ) 9 pts Complete and balance the following reactions: HBr(aq) NaOH(aq) NaBr(aq) H O(l) 6HI(aq) Al(s) H (g) AlI (aq) HCl(aq) NaHCO (aq) H O(aq) CO (g) NaCl(aq) ) 5 pts What volume of M HCL is required to reach the endpoint in the complete titration of 5.00 ml of 0.50 M NaOH? HCl(aq) NaOH(aq) NaCl(aq) H O(l) n M = V n = MV = ( 0.50M )( L) = molNaOH 1molHCl molNaOH = molHCl 1molNaOH n M = V n molHCl v = = = L M 0.100M units

3 4a) pts What is the ph of a solution with an [H O concentration of M? 4 ph = log[ H O = log[ =.17 4b) pts If the poh is 5.17 what is the OH - concentration? ph = log[ OH 5.17 = log[ OH OH = M 4c) 6 pts The concentration of OH - in a sample of seawater is M. Calculate the concentration of H O ions, and classify the solution as acidic, neutral, or basic. 14 = = [ H O [ OH K w [ H or O poh = log[ OH 14 = ph = 14 poh = = 8.70 ph = log[ H O 8.70 = log[ H H O Basic = [ OH ph poh O = M = = log[ = = 5.0 units 5) What is the equilibrium constant for the following reactions? 5a) pts P 4 (s) 5O (g) P 4 O 10 (s) 1 K eq = 5 [ O 5a) pts Cr O - 7 (aq) H O(l) CrO - 4 (aq) H (aq) [ H [ CrO4 K eq = [ CrO7

4 6) 4 pts When the reaction SO (g) O (g) SO (g) is at equilibrium at 800 K the concentrations of SO is M, the concentration of O is M, and the concentration of SO is M. What is the equilibrium constant? K eq [ SO [ M = = [ SO [ O [.0 10 M [.5 10 = M 4 7) Consider the following reaction at equilibrium C(s) H O(g) CO(g) H (g) Predict the effect of: 7a) pts Adding C to the reaction mixture. No change. As long as there is some solid it will not change equilibrium to add of subtract it. 7b) pts Condensing H O and removing it from the reaction mixture. More reactants form (left) 7c) pts Adding CO to the reaction mixture More reactant form (left) 7d) pts Removing H from the reaction mixture. More products form (right) 4

5 8) 4 pts What is the molar solubility of Mg(OH)? Ksp = Mg(OH) (aq) Mg (aq) OH - (aq) K [ Mg [ OH K sp sp S = = [ Mg = S = S = S[S K 4 sp = [ OH = 4S = mol L units 9) For each of the following reactions, identify the element being oxidized, the element being reduced, the oxidizing agent, and the reducing agent. 9a) 4 pts N (g) O (g) NO(g) N 0 lost e - N was oxidized N (g) is reducing agent O 0 - gained e - O was reduced O (g) is oxidizing agent 9b) 4 pts CO(g) O (g) CO (g) C 4 lost e - C was oxidized CO(g) is reducing agent O 0 - gained e - O was reduced O (g) is oxidizing agent 5

6 10) Balance the following redox equations: 10a) 4 pts Mg(s) Cr (aq) Mg (aq) Cr(s) Mg(s) Cr (aq) Mg (aq) Cr(s) Mg 0 lost e- Mg is oxidized Cr 0 gained e- Cr is reduced Oxidation ½ reaction Mg Mg e - Reduction ½ reaction e - Cr Cr (Mg Mg e - ) Mg Mg 6e - (e - Cr Cr) 6e - Cr Cr Balanced Reactions: Mg(s) Cr (aq) Mg (aq) Cr(s) _ 10b) 4 pts Al(s) Ag (aq) Al (aq) Ag(s) Al(s) Ag (aq) Al (aq) Ag(s) Al 0 lost e- Al is oxidized Ag 1 0 gained e- Ag is reduced Oxidation ½ reaction Al Al e - Reduction ½ reaction e - Ag Ag Al Al e - Al Al e - (e - Ag Ag) e - Ag Ag Balanced Reactions: Al(s) Ag (aq) Al (aq) Ag(s) _ 6

7 Multiple Choice Questions CIRCLE THE CORRECT ANSWER 1) pts The Bronsted-Lowery definition of a base is: A) A proton donor. B) A proton acceptor. C) Produces H in solution. D) Produces OH - in solution. Answer: B ) pts Consider a M solution of HNO. Which of the following statements is NOT true? A) This solution would turn litmus to red. B) This solution could neutralize a base. C) This solution has a ph of D) This solution could dissolve a metal. ) pts Which solution below is considered to have basic character? A) poh = 7 B) ph = C) poh = 4 D) poh = 1 4) pts Which of the following statements are TRUE of buffer solutions? 1. A buffer solution can be made by mixing equal concentration of acetic acid and sodium acetate.. A buffer solution can be made by mixing equal concentrations of hydrochloric acid and sodium chloride.. A buffer solution resists changes in ph when small quantities of acid or base are added. A) 1 and only B) and only C) 1 and D) All of 1,, and E) None of 1,, and 7

8 5) pts Which of the following changes will increase the reaction rate? A) An increase in the concentration of the products. B) A decrease of the reaction temperature. C) Allowing more time for the reaction. D) An increase in the concentration of reactants. E) All of the above Answer: D 6) pts Which of the following is TRUE for a system that is in dynamic equilibrium? A) The forward reaction goes to 100% completion. B) The reaction rate of the forward reaction approaches zero. C) The concentration of products is equal to the concentration of the reactants. D) Both the forward and reverse reactions come to a halt. Answer: B 7) pts Which of the following is TRUE of a system for which K eq <<1? A) It will take a long time to reach equilibrium. B) It will take a short time to reach equilibrium. C) The equilibrium favors the reverse reaction. D) The equilibrium favors the forward reaction. 8) pts Which compound is the most soluble? A) Cus, K sp = B) Pbs K sp = C) AgCl, K sp = D) AgI, K sp = E) Not enough information 9) pts The oxidation number of Cr in Cr O - 7 is A) B) - C) 7 D) 6 Answer: D 8

9 10) pts In an electrochemical cell, which of the following statements is FALSE? A) The anode is the electrode where oxidation occurs. B) A salt bridge provides electrical contact between the half-cells. C) The cathode is the electrode where reduction occurs. D) A spontaneous electrochemical cell is called a galvanic cell. E) All of the above are true. Answer: E 9