E. Incorrect. Look carefully there is a statement that is true about weak acid dissociation.

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1 AP Chemistry - Problem Drill 21: Acids and Bases No. 1 of Which of the following is true for the dissociation of a weak acid? A. K a is large. B. The equilibrium lies far to the right. C. The equilibrium lies far to the left. D. [H 3 O + ] >> [HA]. E. None of the above is true. A large K a means a great deal of dissociation not true for a weak acid. Equilibrium lying to the right means a great deal of dissociation not true for a weak acid. C. Correct. Equilibrium lying to the left means there is not much dissociation that is true for a weak acid. A large amount of hydronium is characteristic of a strong acid rather than a weak acid. E. Incorrect. Look carefully there is a statement that is true about weak acid dissociation. Weak acid: An acid that dissociates very little. A. K a for a weak acid is very small. A is wrong B. Equilibrium lies to the left for weak acids. B is wrong C. Equilibrium lies to the left for weak acids. C is correct D. For weak acids, the concentration of the intact acid is greater than the dissociated acid. D is wrong The correct answer is (C).

2 No. 2 of The ph of a solution which has [OH -1 ] = M is? (A) 2.90 (B) 3.40 (C) 4.47 (D) 5.00 (E) 9.53 First solve for hydronium concentration and then for ph. First solve for hydronium concentration and then for ph. First solve for hydronium concentration and then for ph. First solve for hydronium concentration and then for ph. E. Correct. You successfully solved for ph. [OH - ][H 3 O + ] = Neutral is when [OH - ] = [H3O + ] = Whichever species is greater than 10-7 controls the ph. In this problem, [OH] > 10-7 and therefore the ph will be basic. The correct answer is (E).

3 No. 3 of Which acid will give a lower ph when they are 0.1M? Acetic acid K a = Nitrous acid K a = Iodic acid K a = (A) acetic acid (B) nitrous acid (C) iodic acid (D) they will all be the same at 1.0 M (E) cannot be determined from the given information The larger the K a, the more acid will dissociate. The larger the K a, the more acid will dissociate. C. Correct. The larger the K a, the more acid will dissociate. The larger the K a, the more acid will dissociate. E. Incorrect You can determine the answer to this question the larger the K a, the more acid will dissociate. The bigger the K a, the more the acid dissociates and the lower the ph (more dissociation means more hydronium ion present and that s what ph measures). The correct answer is (C).

4 No. 4 of Which of the following is true for a buffered solution? (A) The solution resists changes in [H 3 O + ]. (B) The solution will not change its ph very much even if a concentrated acid is added. (C) The solution will not change its ph very much even if a strong base is added. (D) All of these. (E) None of these. Buffer solutions do resist ph changes, but what else do they do? Buffer solutions do resist ph changes, but what else do they do? Buffer solutions do resist ph changes, but what else do they do? D. Correct. Buffer solutions do resist ph changes, even if concentrated acids or strong bases are added. E. Incorrect! Look closely there is a true statement for buffers in this question. All of those statements are true. The correct answer is (D).

5 No. 5 of Adding which of the following would increase the solubility of Ca(OH) 2? (A) NaOH (B) HCl (C) Ca(NO 3 ) 2 (D) NH 3 (E) Ca(OH) 2 Adding a common ion decreases solubility. B. Correct. The acid will neutralize the hydroxide, effectively removing a product of the solubility equilibrium and pushing the reaction to the right. Adding a common ion decreases solubility. Adding a base will not increase the solubility of a base. E. Incorrect. Adding more of a compound will not increase the solubility. Adding NaOH and Ca(NO 3 ) 2 are adding a common ion this decreases solubility. Adding HCl will allow the OH from Ca(OH) 2 and the H from HCl to form water. This is removing ions and will allow more to dissolve (Le Chatelier s Principle) NH 3 is a base. Adding NH 3 will raise the ph, creating more OH -1, which is a common ion and will decrease the solubility. The correct answer is (B).

6 No. 6 of NH 3 NH NH 2 In the liquid ammonia reaction, NH + 4 acts as a. (A) amphoteric species (B) an intermediate (C) the conjugate acid of NH 3 (D) the conjugate base of NH 3 (E) none of these Amphoteric means a species can act as an acid or base. An intermediate is something that is produced and then reacted away in a reaction mechanism. C. Correct. It is the conjugate acid. NH +1 4 would be an acid, not a base. E. Incorrect. Look closely there is a correct term for how the ammonium ion is acting. Amphoteric: acts as acid and base. Intermediate: species that is produced and then reacted away during a series of steps. Conjugate acid: what s left after a base accepts a proton. Conjugate base: what s left when an acid donates a proton. The correct answer is (C).

7 No. 7 of For the titration of a weak acid with a strong base to a desired end-point near ph 7.0, which indicator would be the best choice? Indicator PH range of color change A Methyl Orange B Methyl red C Bromothymol Blue D Phenolphthalein E Alizarin The products will be basic. The products will be basic. The products will be basic. D. Correct. The products will be basic but not very basic, so choose the basic ph closest to neutral. E. Incorrect! The products will be basic but not very basic, so choose the basic ph closest to neutral. When reacting a strong base + a weak acid, the resulting solution will be basic. There are two basic indicators listed the ph of the resulting solution won t be very basic, but rather it will be closer to neutral than 14. The correct answer is (D).

8 No. 8 of How does the concentration of [H 3 O + ] and [OH - ] compare in a basic solution? (A) [H 3 O + ] > [OH - ] (B) [H 3 O + ] < [OH - ] (C) [H 3 O + ] = [OH - ] (D) There are no hydronium ions in a basic solution. (E) Cannot be determined from given information. Bases produce hydroxide in water. B. Correct. Bases produce hydroxide in water. Bases produce hydroxide in water. Even in a basic solution, there will still be hydronium ions present due to the autoionization of water. E. Incorrect! Bases produce hydroxide in water. A basic solution will have more hydroxide than hydronium. The correct answer is (B).

9 No. 9 of What is the ph if [H 3 O + ] = ? (A) 3.4 (B) (C) 4.47 (D) (E) 5.00 The hydronium concentration has a power of 10 = 5, so the ph will be around 5. The hydronium concentration has a power of 10 = 5, so the ph will be around 5. C. Correct. The hydronium concentration has a power of 10 = 5, so the ph will be around 5. In fact, the ph will be a bit less than 5, because 3.4 is a bit greater than 1. The hydronium concentration has a power of 10 = 5, so the ph will be around 5. E. Incorrect! The hydronium concentration has a power of 10 = 5, so the ph will be around 5. But since the hydronium concentration is not , the ph will not be exactly 5. Use estimations! ph = -log [H 3 O +1 ] You can estimate the log of a number by the power of 10. Power of 10 is 5, so the ph will be in the vicinity of 5. ph won t be negative ---- The log is negative. The ph is positive. The correct answer is (C).

10 No. 10 of Which acid would most likely result in a buffer with ph of 5 when added in equal concentrations with its salt? Acetic acid Ka = Nitrous acid Ka = Iodic acid Ka = (A) acetic acid (B) nitrous acid (C) iodic acid (D) they will all be the same at 1.0 M (E) cannot be determined from this information A. Correct. A weak acid and its salt create a buffer. A weak acid and its salt create a buffer. A weak acid and its salt create a buffer. A weak acid and its salt create a buffer. E. Incorrect! You can determine the answer from this information the pka of an weak acid is the ph of the buffer. When added in equal concentrations, a weak acid and its salt produce a buffer with ph = pka. You can estimate pka by looking at the power of 10 (pka = -log Ka). The acid with a pka closest to 5 would be Acetic acid. The correct answer is (A).