[cubejunkies]

My Chemistry teacher today gave us this set of problems today as a "quiz" for our test on Tuesday, and he generated it using a program which he makes all of his tests from. Therefore, he told us that the "quiz" we got today would have extremely similar problems with the text that's on Monday, however, the only down side is that he won't grade these quizzes or post an answer key anywhere, so I really would like to know which of the problems on this "quiz" I got right and wrong, that way I can better prepare for the real deal on Monday. Feel free to check only a few of the problems, as this is quite the lengthy quiz relatively speaking, but if you want to check all of my answers, that's ok too. For starters, however, I didn't get 6, 20, or 26 at all, explanations for why are inside the problem below.

Thanks!

1. Calculate the concentration of chloride ions in a saturated lead (II) chloride (K_sp = 2.4 x 10^-4) solution.

7.8 x 10^-2 M

2. Identify the conjugate base of HSO₄^- in the reaction
          H₂PO₄^- + HSO₄^-    H₃PO₄ + SO₄^2-

SO₄^2-

3. Which one of the following salts will form an acidic solution on dissolving in water?

A. KOH
B. NaCN
C. NaF
D. FeCl₃
E. LiBr

4. In the reaction H₂CO₃ + H₂O    HCO₃^- + H₃O⁺, the Bronsted acids are

H₃O⁺ and H₂CO₃

5. Which solution will have the lowest pH?

A. 0.10 M NaCl
B. 0.10 M HCN
C. 0.10 M NaOH
D. 0.10 M H₂CO₃
E. 0.10 M HNO₃

6. What is the pH of a 0.014 M Ca(OH)₂ solution?

(NOTE: I didn't know how to do this one because I know that Ca(OH)₂ is only slightly soluble in water, but a K_sp value wasn't given, and he said any equilibrium constants we needed would be provided. Is this perhaps a teacher mistake, or is there an alternate way of solving this problem without a K_sp value?)

7. What is the pH at the equivalence point in the titration of 100 mL of 0.10 M HCl with 0.10 M NaOH?

7.0

8. Acid strength decreases in the series: HCl > HSO₄^- > HCN. Which of these species is strongest base?

A. HCN
B. SO₄^2-
C. CN^-
D. Cl^-

9. Consider the weak bases below and their K_b values:
C₆H₇O     K_b= 1.3 x 10^-10
C₂H₅NH₂     K_b= 5.6 x 10^-4
C₅H₅N     K_b= 1.7 x 10^-9

Arrange the conjugate acids of these weak bases in order of increasing acid strength

C₂H₅NH₃⁺ < C₅H₅NH⁺ < C₆H₇OH

10. What is the pH of a 0.20 M solution of NH₄Cl? [K_b(NH₃) = 1.8 x 10^-5]

4.98

11. Which one of these salts will form a neutral solution on dissolving in water?

A. KNO₂
B. NH₄NO₃
C. NaCl
D. NaCN
E. FeCl₃

12. Which one of the following combinations cannot function as a buffer solution?

A. HNO₃ and NaNO₃
B. HNO₂ and NaNO₂
C. HCN and KCN
D. HF and NaF
E. NH₃ and (NH₄)₂SO₄

13. Which one of the following is a buffer solution?

A. 0.20 M CH₃COOH
B. 0.10 M KCN
C. 0.40 M HCN and 0.10 M KCN
D. 1.0 M HNO₃ and 1.0 M NaNO₃
E. 0.50 M HCl and 0.10 M NaCl

14. Which of the following yields an acidic solution when dissolved in water?

A. Ca(OH)₂
B. K₂O
C. NaCl
D. LiOH
E. NO₂

15. Which of these species will act as a Lewis acid?

A. NH₄⁺
B. NH₃
C. H₂O
D. BF₃
E. F^-

16. The pOH of a solution is 10.40 Calculate the hydrogen ion concentration in the solution.

2.5 x 10^-4 M

17. Which one of these salts will form a basic solution upon dissolving in water?

A. NH₄NO₃
B. NaCL
C. NaNO₂
D. AlCl₃
E. KBr

18. Calculate the pH of a buffer solution prepared by dissolving 0.20 mol of NaCNO and 1.0 mol of HCNO in enough water to make 1.0 L of solution. [K_a(HCNO) = 2.0 x 10^-4]

3.0

19. Consider a buffer solution prepared from HOCl and NaOCL. What is the net ionic equation for the reaction that occurs when NaOH is added to this buffer?

OH^- + HOCl  H₂O + OCl^-

20. You have 500.0 mL of a buffer solution containing 0.20 M acetic acid and 0.30 M sodium acetate. What will the pH of this solution be after the addition of 20.0 mL of 1.00 M NaOH solution? [K_a(CH₃COOH) = 1.8 x 10^-5]

I got 4.97 but this isn't one of the choices.

A. 5.07
B. 4.56
C. 4.74
D. 4.41
E. 4.92 (<- I calculated that this is the pH of the buffer system before the addition of NaOH)

21. The molar solubility of manganese (II) carbonate is 4.2 x 10^-6 M. What is the K_sp for this compound?

1.8 x 10^-11

22. Calculate the hydrogen ion conentration in a solution having a pH of 4.60

2.5 x 10^-5 M

23. Write a chemical reaction representing the reaction of a weak acid with a weak base.

HCN (aq) + CH₃NH₂ (aq) CH₃NH₃⁺ (aq) + CN^- (aq)

24. Write the formula for the conjugate base of H₂PO₄^-

HPO₄^2-

25. The K_sp of CaF₂ is 4 x 10^-11. What is the maximum concentration of Ca²⁺ possible in a 0.10 M NaF solution?

8 x 10^-10 M

26. Calculate the pH at the equivalence point for the titration of 0.25 M CH₃COOH with 0.25 M NaOH. (For CH₃COOH, K_a = 1.8 x 10^-5)

I don't know how to do this one without the number of milliliters that was present during the titration, as this affects the amount of salt formed at the equivalence point, which is what is purely responsible for the pH at the equivalence point in this case.

27. If the pH of stomach acid is 1.0, what is the hydroxide ion concentration in this solution?

10^-3 M

28. Identify the conjugate acid-base pairs in the reaction
     HSO₄^- + HF H₂SO₄ + F^-

HSO₄^- and H₂SO₄
HF and F^-

29. If the pH of tomato juice is 4.0, what is the hydroxide ion concentration in this solution?

10^-10 M

[Borek]

6. What is the pH of a 0.014 M Ca(OH)₂ solution?

(NOTE: I didn't know how to do this one because I know that Ca(OH)₂ is only slightly soluble in water, but a K_sp value wasn't given, and he said any equilibrium constants we needed would be provided. Is this perhaps a teacher mistake, or is there an alternate way of solving this problem without a K_sp value?)

0.014 is given as a concentration - I read it as you don't have to worry about K_sp, assume solution is either saturated or below saturation.

20. You have 500.0 mL of a buffer solution containing 0.20 M acetic acid and 0.30 M sodium acetate. What will the pH of this solution be after the addition of 20.0 mL of 1.00 M NaOH solution? [K_a(CH₃COOH) = 1.8 x 10^-5]

I got 4.97 but this isn't one of the choices.

A. 5.07
B. 4.56
C. 4.74
D. 4.41
E. 4.92 (<- I calculated that this is the pH of the buffer system before the addition of NaOH)

Hard to tell not knowing what you did. When you add strong base you remove acid (it becomes neutralized) and you create salt (conjugate base, product of neutralization). Common mistake is to omit one of the changes in the calculations.

26. Calculate the pH at the equivalence point for the titration of 0.25 M CH₃COOH with 0.25 M NaOH. (For CH₃COOH, K_a = 1.8 x 10^-5)

I don't know how to do this one without the number of milliliters that was present during the titration, as this affects the amount of salt formed at the equivalence point, which is what is purely responsible for the pH at the equivalence point in this case.

Initial volumes don't matter as final concentration will depend only on the initial concentration ratio. As both titrant and titrated substance have the same concentration you need to mix the solution in 1:1 ratio, that means diluting 1:2.