HOUR EXAM #3

I pledge on my honor that I have neither given nor received improper aid on this examination.

You will have exactly two hours to complete your exam.

Before you begin, CAREFULLY READ AND FOLLOW THE INSTRUCTIONS BELOW.

Instructions

1. Check to see that your exam contains all seven pages.
2. Show all work neatly and completely to receive partial credit.
3. Write your answers clearly and legibly. Answers not clearly written will be marked wrong.
4. Include units with answers.
5. Direct any questions to the supervising instructor or graduate student proctors.

• K_w = 1.0 x 10^-14
• K_p = K_c(RT)^Dn
• DT_b = iK_bm
• DT_f = -iK_fm
• PV = nRT
• P₁ = c₁P°₁
• P₂ = k_Hc₂
• R = 0.08206 L•atm/mol•K

I. True/False check the appropriate blank (2 points each 10 points total)

	True	False
1. The freezing point of a 0.2 molal solution of sucrose is lower than a 0.2 molal solution of NaCl.	________	________
2. The pH of a 0.01 M acetic acid solution differs from the pH of a 0.01 M hydrochloric acid solution.	________	________
3. When the value of the reaction quotient, Q, is greater than the equilibrium constant, K, the reaction will form more product.	________	________
4. The pH at the equivalence point in the titration of the weak base NH3 with HCl is greater than 7.0.	________	________
5. When stepwise chemical reactions are summed, the stepwise equilibrium constants are also summed.	________	________

II. Select the one best match for the following terms. (2 points each, 12 points total)

	Match
6. NOT a colligative property (choose from P through S)	________
7. % by mass of ethanol in a 0.20 mole fraction of ethanol (C2H5OH, molar mass = 46 g/mol) in water (18 g/mol)	________
8. pOH of an aqueous 0.01 M HCl solution	________
9. pH of an aqueous 0.01 M CaCl2 solution	________
10. Conjugate base of K2HPO4	________
11. Products are favored when	________

A) 64	F) 1 x 10-14/.01	K) K < 1	P) Boiling point elevation
B) 39	G) pH = 7	L) K3PO4	Q) Osmotic pressure
C) 20	H) pH > 7	M) KH2PO4	R) Freezing point depression
D) 12	I) pH < 7	N) H3PO4	S) Vapor pressure raising
E) 2	J) K > 1

III. Multiple choice. For the following questions pick the best possible answer. (5 points each, 25 points total. If you circle two answers, one of which is correct, you will receive 2 points.)

13. If the equilibrium constants for the following reactions are K₁ and K₂ as show,

P₄ (g) + 6 Cl₂ (g) 4 PCl₃ (g) ; K₁

PCl₃ (g) + Cl₂ (g) PCl₅ (g) ; K₂

Then the equilibrium constant for the reaction below is _____?

P₄ (g) + 10 Cl₂ (g) 4 PCl₅ (g)

1. K₁K₂
2. 4K₁K₂
3. K₁(K₂/4)
4. K₁(K₂)⁴
5. K₁ + 4K₂

14. The reaction, HX (aq) + F^-(aq) HF (aq) + X^- (aq) has K = 0.024. Thus, the stronger acid and stronger base are?

1. HX and F^-.
2. HX and X^-.
3. HF and F^-.
4. HF and X^-
5. Not enough information given to determine the answer

15. The vapor pressure of a dilute solution of a non-volatile solute is?

1. greater than that of the pure solvent.
2. less than that of the pure solvent.
3. equal to the prevailing atmospheric pressure.
4. equal to that of the pure solvent.
5. equal to that of the pure solute.

16. Which of the following indicates the weakest acid?

1. K_a = 3.24 x 10^-3
2. pK_a = -3.85
3. pK_a = 7.53
4. K_a = 1.7 x 10^-5
5. pK_a = 3.85

17. Which of the following is a true statement?

1. At equilibrium, the total concentration of products equals the total concentration of reactants.
2. Equilibrium is achieved when forward and backward rates of reaction are equal.
3. There is only one set of equilibrium concentrations that fulfills the K value.
4. Equilibrium is the result of the cessation of all chemical change.
5. All of the above

IV. Short answer / fill in the blank.

18. (11 pts) Consider the following endothermic reaction:

heat + 2 SO₃ (g) 2 SO₂ (g) + O₂ (g)

When 1.00 atm of SO₃ gas is placed initially into a 1.00 L flask at 298 K and allowed to equilibrate, the pressure of SO₃ is 0.80 atm.

19. (5 pts) When 50.00 mL of 0.100 M acetic acid (K_a = 1.8 x 10^-5) is neutralized with 50.00 mL of 0.100 M NaOH, the pH of the resulting solution is not 7.0.

• (2 pt) Is the pH greater than 7 or less than 7?
  
  
  
  
• (3 pts) Explain why the pH is not equal to 7 in three sentences or less.
  
  
  
  

20. (6 pts) What would be the ratio of [CH₃CO₂H]/[CH₃CO₂^-] needed to make a buffer solution with a pH of 4.00? CH₃CO₂H is acetic acid (K_a = 1.8 x 10^-5).





[CH₃CO₂H]/[ CH₃CO₂^-] =

 21. (5 pts) A solution of 4.0023 g of a biological compound in 0.250 L of water has an osmotic pressure of 0.330 atm at 298 K. What is the molar mass of the compound (assuming no ionization or dissociation of the compound)?





Molar mass =

22. (6 pts) At 100°C, the equilibrium constant (K) is 81.0 for the reaction shown below.

H₂ (g) + Si₂H₆ (g) 2 SiH₄ (g)

Initially the two reactant gases, at a partial pressure of 0.600 atm each, are sealed in a flask at 100°C. What is the partial pressure of SiH₄ (g) at equilibrium?





P_SiH4 =

23. (6 pts total) Consider a 500 mL solution of NaOH whose pH =11.70.

24. (6 pts) Volatile liquids A and B make up an ideal mixture. The vapor pressures of pure A and pure B at 300 K are: P°_A(300) = 0.38 atm, P°_B(300) = 0.22 atm. When a mixture of 0.5 moles of A and 0.8 moles of B is made, what will the partial pressure of A above the mixture be? What will the mole fraction of A in the vapor above the mixture be?





P_A =


c_A (vapor phase)=

25. (8 pts) Consider 50.0 mL of 0.1000 M hydrazoic acid, HN₃ (K_a = 1.9 x 10^-5)?