[base] [acid] ph = pka + log

Transcription

1 Fall 2013 CCBC-Catonsville (Wed 10/30/13) Use your time wisely. Do not get stuck on one question. NO CREDIT WILL BE GIVEN UNLESS WORK IS SHOWN CLEARLY. UNITS MUST BE INCLUDED IN YOUR SETUPS. Answers must be written the space provided when a blank is provided. Although scratch paper is available, only work shown on this exam paper will be graded. Write clearly. If I cannot read what you wrote you will get no credit. As usual, watch your units and significant figures. TOTAL YOUR SCORE SCORE POSSIBLE Page 1 27 Page 2 31 Page 3 18 Page 4 16 Page 5 8 TOTAL 100 Bonus p.5 7 Adjusted Total Current Course Average (based on 3 Exams, 12 Quizzes, 11 Wiley HW, 6 Paper HW) = R = L atm mol R = J mol K -1-1 K -1-1 ph = pka + log [base] [acid] K a at 25 C HIO x 10 1 HCO 2 H 1.8 x 10 4 HNO x 10 4 HF 6.8 x 10 4 HOCN 3.5 x 10 4 HC 2 H 3 O x 10 5 HC 4 H 7 O x 10 5 HClO 3.0 x 10 8 HCN 6.2 x H 2 O x K b at 25 C C 4 H 9 NH x 10 4 CH 3 NH x 10 4 NH x 10 5 N 2 H x 10 7 C 6 H 5 NH x 10 10

2 Fall 2013 Page 1 MULTIPLE CHOICE (3 pts each): Circle only one answer. 1. The reaction 2NO (g) + O 2 (g) 2NO 2 (g) has an enthalpy of reaction of kj. The gas, NO, is colorless and the gas, NO 2, is reddish brown. The system is allowed to equilibrate at a certain temperature to give a mixture with a brown color. If the temperature is increased what is expected to happen to the color? A. It should turn darker. B. It should turn lighter. C. Color should not change. 2. Consider the system at equilibrium for 3C (s) + 3H 2 (g) CH 4 (g) + C 2 H 2 (g) The result of decreasing the volume will be A. more CH 4 (g) and C 2 H 2 (g) will be formed to restore the equilibrium. B. more C 2 H 2 (g) is consumed to restore the equilibrium C. more 3 H 2 (g) is formed to restore the equilibrium D. no further change occurs 3. The conjugate base of H 2 AsO 4 is... A. H 2 AsO 4 B. H 3 AsO 4 C. HAsO 4 D. HAsO 4 2 E. AsO For the system NH 2 OH + CH 3 NH 3 CH 3 NH 2 + NH 3 OH + the position of the equilibrium lies to the left. Which is the strongest acid in the system? + A. NH 2 OH B. CH 3 NH 3 C. CH 3 NH 2 D. NH 3 OH + 5. Which is the stronger acid? A. HClO 2 B. HClO 3 6. Which is the stronger acid? A. HI B. HCl 7. Which is most acidic? A. HOI B. HOCl 8. Which diagram below illustrates best how molecules of HNO 2 dissolve in water? H is represented by a square, N by a triangle and oxygen by a circle. Water molecules are not shown. Circle one of these: Diagram A Diagram B Diagram C Diagram D 9. Which is acting as the Lewis acid in the reaction SO 2 + OH HSO 3? A. SO 2 B. OH C. HSO 3 D. none of the above

3 Fall 2013 Page Which solution should be the most acidic? A. Zn(NO 3 ) 2 B. Cd(NO 3 ) 2 C. Ga(NO 3 ) 3 D. Hg(NO 3 ) 2 E. Al(NO 3 ) Calculate the ph of a M solution of Ca(OH) 2 whose temperature is 25.0 C. A B C D E Which of the following salts will form a basic solution upon dissolving in water? A. NH 4 NO 3 B. NaNO 2 C. NaCl D. KBr E. AlCl What is the pk b of CN? A. 4.9 x 10 4 B. 4.9 x C. 9.1x10 10 D. 9.3 E. 2.0x Which pair would you use in order to produce a buffer solution with ph = 4.8? A. HCO 2 H/HCO 2 B. HC 4 H 7 O 2 / C 4 H 7 O 2 C. HCN/CN + D. NH 3 /NH (3 pts) A buffer contains equimolar concentration of C 4 H 9 NH 2 and C 4 H 9 NH 3 Cl. Write the net ionic equation to show how the buffer acts when HCl is added to it. 16. (3 pts) Write the equation for the hydration of the Fe 2+ ion by six water molecules: 17. (3 pts) Write the equation to show the reaction of the hydrated Fe(II) ion (product from the question above) acting as a Brønsted acid as it reacts with one water molecule. 18. (3 pts) Complete and balance the equation below where HS is acting as the Brønsted base: HSO 4 + HS 19. (4 pts) Give the equilibrium constant expression for the reaction 6CO 2 (g) + 6 H 2 O (l) C 6 H 12 O 6 (s) + 6O 2 (g) K c =

4 Fall 2013 Page (6 pts) For the reaction, 2SO 2 (g) + O 2 (g) 2SO 3 (g), at 450 K the equilibrium constant, K c, has a value of A system was charged to give these initial concentrations: [SO 3 ] = M, [O 2 ] = M, [SO 2 ] = M. Will the system proceed to the left or to the right? No credit will be given if work is not shown CLEARLY. ANS. 21. (6 pts) For the reaction 2 CO 2 (g) 2CO (g) + O 2 (g), K c = 26.2 at 827 C. What is the value of K P at the same temperature? Ans. 22. (6 pts) Using this data, 2NO (g) + Cl 2 (g) 2NOCl (g) K c = 3.20 x NO 2 (g) 2NO (g) + O 2 (g) K c = 15.5 calculate a value for K c for the reaction, NOCl (g) + ½ O 2 (g) NO 2 (g) + ½ Cl 2 (g) No credit is given if work is not shown clearly. Ans.

5 Fall 2013 Page (8 pts) A M solution of an acid has a ph = What is the value of the ionization constant K a for this acid? Show your work. Ans. 24. (8 pts) Calculate the ph of a 0.14 M HNO 2 solution that is 5.7 % ionized. Ans.

6 Fall 2013 Page (8 pts) You have ml of a buffer solution containing.30 M acetic acid and 020 M sodium acetate. a) What is the ph of this buffer solution? b) In the buffer solution described above, how many moles of acetic acid and how many moles of acetate are present? # mole acetic acid = # mol acetate = c) Write the chemical equation for the ionization of acetic acid in water. d) What will the ph of this solution be after the addition of 20.0 ml of 1.00 M NaOH solution? Hint: How many moles of NaOH has been added? What will it do to the species in the buffer solution? Bonus Points: (1 pt) Make sure you name is on every page and your answers are on the blanks when provided. (2 pts) Give the answer to the correct number of sig. fig.: ln x = 0.328x10 2 What is x? Ans. (2 pts) What is the definition of the Lewis acid? A Lewis acid is... (2 pts) Give the full structure of acetic acid, showing all the bonds and lone pairs.