Practice Exam 3 – 75 minutes – Chapters 7, 14, 15 (Sec 1-5)
Useful Information
Ka and Kb values are those in the Chem 120 Lab Manual
Constants
R = 8.3145 J/mol.K = 0.08206 L.atm / mol.K
k = 1.38 x 10-23 J/molecule.K
1 J = 1 kg.m2/s2
h = 6.63 x 10-34 J.s
N = 6.022 x 1023
c = 2.998x108 m/s
RH = 2.18 x 10-18 J
Equations
pH + pOH = 14.00
pX = -log X
KaKb = Kw = 1.0 x 10-14
E = hn
c = ln
l = h/mv
p = mv
Dp.Dxh/2p
En = - RH/n2
Multiple Choice (2 points each)
_____1. The titration curves labeled 1 and 2 were obtained by titrating equal
volumes of two different acid samples with portions of the same sodium
hydroxide solution. What conclusions can be drawn about the relative
concentrations and strengths of acids 1 and 2 from these curves?
A) The concentrations are the same but acid 1 is weaker than acid 2.
B) The concentrations are the same but acid 1 is stronger than acid 2.
C) Acid 1 is the same strength as acid 2, but it is less concentrated.
D) Acid 1 is the same strength as acid 2, but it is more concentrated.
_____2. Which will be the same for 100. mL solutions of 0.50 M NH3 and 0.50 M NaOH? Solution Variables:
I. concentration of OH¯ ion
II. volume of 0.20 M HCl required for neutralization
A) I only
B) both I and II
C) II only
D) neither I nor II
_____3. Which of the following responses is true with regard to a 0.1 M solution of a weak acid HA?
A) [H+] < [A-] B) [H+] = [A-] C) [H+] > [A-] D) [OH-] > [A-]
E) pH = 1.0
_____4. The pH of a 0.050 M solution of NaOH is (to the correct number of significant figures)
A) 1.3 B) 1.30 C) 12 D) 12.7 E) 12.70
_____5. The pOH of a solution is 10.589. What is the hydronium ion concentration of the solution?
A) 2.58 x 10–11 B) 3.88 x 10–4 C) 0.533 D) 1.02 E) 2.58 x 103
_____6. What is the value of the K for the following equilibrium?
H2AsO4– + OH– <–––> HAsO42– + H2O
A) Ka1 B) Ka2 C) Ka3 D) Kw/Ka1
E) Kw/Ka2 F) 1/Ka3 G) 1/Ka2 H) Ka1/Kw
I) Ka2/Kw J) Ka3/Kw
Use the titration curve of a weak acid with a strong base below to answer Questions 7– 9.
_____7. Which point indicates the region where the solution behaves as a buffer?
A) A B) B C) C D) D E) E
_____8. Which point indicates the equivalence point of the titration?
A) E B) B C) C D) D E) E
_____9. The pKa of the acid is closest to
A) 1.5 B) 2.5 C) 4.7 D) 7.0 E) 8.7
_____10. A phosphate buffer (H2PO4-/HPO42-) has a pH of 8.3. Which of the following changes will cause the pH to increase?
A) dissolving a small amount of Na2HPO4
B) dissolving a small amount of NaH2PO4
C) adding a small amount of dilute hydrochloric acid
D) adding a small amount of dilute phosphoric acid
_____11. If the value of the equilibrium constant, K, is less than 1 for the following reaction, what is the strongest base in this system?
HOCl(aq) + NO2-(aq) Û OCl-(aq) + HNO2(aq)
A) HOCl B) NO2- C) OCl- D) HNO2
_____12. Heisenberg's Uncertainly Principle states that there is a limit to the accuracy at which we can determine the:
A) position of a particle
B) momentum of a particle
C) acceleration of a particle
D) position and momentum of a particles simultaneously
_____13. Which set of quantum numbers represents a 2p electron?
n l m s
A) 1 0 0 1/2
B) 2 1 1 1/2
C) 2 2 -1 -1/2
D) 4 3 -1 -1/2
_____14. (4 pts) Which of the following combinations would result in a buffer? State all that apply.
A) 100.0 mL each of 0.20 M NaOH & 0.40 M H3PO3
B) 100.0 mL each of 0.50 M NaOH & 0.50 M HCl
C) 50.0 mL each of 0.40 M NaOH & 0.20 M HC2H3O2
D) 50.0 mL each of 0.50 M NaOH & 1.0 M NaF
15. (3 pts) The pHs of the following sodium salts are as stated below. Rank the acids of these salts (HX, HY, and HZ) from strongest acid to weakest acid. Assume all salt solutions have the same molarity.
NaX (pH = 12.7) NaY (pH = 7.2) NaZ (pH = 8.9)
Strongest Acid / Weakest Acid16. (6 pts) Would aqueous solutions of the following substances be acidic, basic, or neutral? Write a balanced chemical equation to justify your answer.
A) KCN ______
B) NH4Cl ______
C) AlCl3 ______
17. (6 pts) In a 0.0500 M solution of a weak monoprotic acid, [H+ ]= 5.7 x 10-3.
What is its Ka?
18. (6 pts) Calculate the pH of a 0.20 M aqueous solution of (CH3)2NH2Cl, dimethylamine chloride.
19. (6 pts) What is the pH of a 0.050 M triethylamine, (C2H5)3N, solution?
(20) 20. Propanoic acid (CH3CH2COOH) has a Ka of 1.34 ´ 10-5. A 25.00 mL sample of 0.1000 mol/L propanoic acid (in flask) is titrated with 0.1000 mol L-1 NaOH solution, added from a buret. Carry out the calculations of the quantities indicated below.
A) The pH after 0.00 mL of NaOH are added
B) The pH after 15.00 mL of NaOH are added
C) The pH after 25.00 mL of NaOH is added.
D) The hydroxide ion concentration after 26.00 mL of NaOH are added
21. (6 pts) A buffer is prepared by adding 150. mL of 0.30 M sodium hydroxide to 100. mL of 0.30 M phosphoric acid. What is the pH of the buffer?
22. (9 pts) For each of the following write the electron configuration, draw the orbital diagram, and state if the species is diamagnetic or paramagnetic. You may use the core notation.
Electron Configuration Orbital Diagram Magnetism
Se2-
Fe
Cr
23. (3 pts) For the following sublevel, identify the n and l quantum numbers, and state the maximum number of electrons that each subshell can hold.
n l maximum # of electrons
4d
24. (5 pts) Calculate the wavelength of light required to break the bond between two chlorine atoms in a chlorine molecule. The Cl-Cl bond energy is 243 kJ/mol
Bonus Problems ( 2 points each)
1. Calculate the pH of 0.25 M NaHCO3.
2. Calculate the pH of 0.20 M dimethylamine fluoride.
Additional Problems on Material – not included on Practice Exam 3.
1. Rank the following ions and atoms from smallest to largest: Al3+, As, Br, Ca
______< ______< ______< ______
2. Answer the following:
A) Which should have the largest difference between the first and second ionization energy: Si, Na, P or Mg? ______
Explain your answer.
B) Which has the largest ionization energy: Na, P, or As? ______
_____3. Which sequence is arranged in order of increasing ionization energies?
A) Be, B, C, N, O
B) B, Be, C, O, N
C) Be, B, C, O, N