College Chemistry - Problem Drill 22: and Bases

Question No. 1 of 10 Instructions: (1) Read the problem and answer choices carefully (2) Work the problems on paper as needed (3) Pick the answer (4) Go back to review the core concept tutorial as needed. + 1. A strong HA dissociates in completely to yield H3O and its conjugate A-, with no more HA left. A weak acid dissociates only partially, where the acid and its conjugate base are in equilibrium. Which of the following is true for the dissociation of a weak acid?

A. Ka is large. B. The equilibrium lies far to the right. Question C. The equilibrium lies far to the left. + D. [H3O ] >> [HA]. E. None of the above is true.

A. Incorrect.

A large Ka means a great deal of dissociation—not true for a weak acid.

B. Incorrect. Equilibrium lying to the right means a great deal of dissociation—not true for a weak acid.

C. Correct. Good job! Equilibrium lying to the left means there is not much dissociation—that is true for a weak acid. Feedback D. Incorrect. 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 with the presence of HA in the equilibrium with A-.

A. Ka 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.

Solution The correct answer is (C).

RapidLearningCenter.com  Rapid Learning Inc. All Rights Reserved Question No. 2 of 10 Instructions: (1) Read the problem and answer choices carefully (2) Work the problems on paper as needed (3) Pick the answer (4) Go back to review the core concept tutorial as needed. 2. pH is a measure of acidity or basicity of a solution. Its value is the negative logarithm of the hydronium concentration. Pure water has its pH close to 7 at room temperature. What is the pH of a solution which has [OH-] = 3.410-5 M?

(A) 2.90 (B) 3.40 Question (C) 4.47 (D) 5.00 (E) 9.53

A. Incorrect. - First solve for hydronium concentration with Kw and take the –log function for [H ] to solve for pH.

B. Incorrect.

- First solve for hydronium concentration with Kw and take the –log function for [H ] to solve for pH.

C. Incorrect. - First solve for hydronium concentration with Kw and take the –log function for [H ] to solve for pH. Feedback D. Incorrect. - First solve for hydronium concentration with Kw and take the –log function for [H ] to solve for pH.

E. Correct.

Good job! First solve for hydronium concentration with Kw and take the –log function for [H-] to solve for pH.

- + -14 [OH ][H3O ] = 1.0  10 + -14 - -14 -5 -10 [H3O ] = 1.0x10 /[OH ] = 1.0x10 /3.4x10 = 2.9 x 10 M + -10 pH = -log[H3O ] = -log(2.9 x 10 ) = 9.53 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).

Solution

RapidLearningCenter.com  Rapid Learning Inc. All Rights Reserved Question No. 3 of 10 Instructions: (1) Read the problem and answer choices carefully (2) Work the problems on paper as needed (3) Pick the answer (4) Go back to review the core concept tutorial as needed. 3. In general, an acid HA ionizes into its conjugate base A- and H- with the + - ionization equilibrium HA(aq)  H (aq) + A (aq). The acid- Ka = [H+][A-]/[HA]. Which acid will give a lower pH when they are 0.1M? -5 Acetic acid (HC2H3O2) Ka = 1.8  10 -4 Nitrous acid (HNO2) Ka = 4.5  10 -1 Iodic acid (HIO3) Ka = 1.7  10

(A) Acetic acid

Question (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.

A. Incorrect. + The larger the Ka, the more acid will dissociate with more [H ], i.e. lower pH and more acidic.

B. Incorrect. + The larger the Ka, the more acid will dissociate with more [H ], i.e. lower pH and more acidic.

C. Correct. + Good job! The larger the Ka, the more acid will dissociate with more [H ], i.e. lower pH and more acidic.

Feedback D. Incorrect. + The larger the Ka, the more acid will dissociate with more [H ], i.e. lower pH and more acidic.

E. Incorrect.

You can determine the answer to this question - the larger the Ka, the more acid will dissociate with more [H+], i.e. lower pH and more acidic.

The larger the Ka, the more the acid dissociates and the lower the pH (more dissociation means more [H+] (or hydronium ion) present and that’s what pH measures). HA(aq)  H+(aq) + A-(aq) + - + Ka = [H ][A ]/[HA], assume x = the equilibrium concentration of [H ] 2 Ka = x /(0.1-x) As we can see, in a general trend, larger K will give larger x value. Solution a

The correct answer is (C).

RapidLearningCenter.com  Rapid Learning Inc. All Rights Reserved Question No. 4 of 10 Instructions: (1) Read the problem and answer choices carefully (2) Work the problems on paper as needed (3) Pick the answer (4) Go back to review the core concept tutorial as needed. 4. A buffer is an aqueous solution of a mixture of a weak acid and its conjugate base or a and its . It is used to prevent changes in the pH of a solution. Which of the following is true for a ?

+ (A) The solution resists changes in [H3O ]. (B) The solution will not change its pH very much even if a small amount of strong acid is added. (C) The solution will not change its pH very much even if a small amount of Question strong base is added. (D) All of the above. (E) None of these.

A. Incorrect. Buffer solutions do resist pH changes, but what else do they do?

B. Incorrect. Buffer solutions do resist pH changes, but what else do they do?

C. Incorrect. Buffer solutions do resist pH changes, but what else do they do?

Feedback D. Correct. Buffer solutions do resist pH changes, even if a small amount of strong acid or strong base is added.

E. Incorrect. Look closely—there is a true statement for buffers in this question.

A buffer is an aqueous solution that can maintain a nearly constant pH when diluted, or when strong acids or bases are added. A buffer solution consists of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid.

The correct answer is (D).

Solution

RapidLearningCenter.com  Rapid Learning Inc. All Rights Reserved Question No. 5 of 10 Instructions: (1) Read the problem and answer choices carefully (2) Work the problems on paper as needed (3) Pick the answer (4) Go back to review the core concept tutorial as needed.

5. An acid ionization constant, Ka, is a quantitative measure of the strength of an acid in solution. Larger the value, stronger the acid. It is the equilibrium constant for a chemical reaction known as the dissociation to its conjugate base and ion. Calculate the hydrogen ion concentration [H+] of a 1.0 M solution of + - -10 hydrocyanic acid HCN. The ionization constant Ka of HCN  H + CN is 6.2x10 .

(A) 1.0x10-3 M (B) 2.5 x 10-5 M Question (C) 2.5 M (D) 3.0 x 102 M (E) 6.2 x10-10 M

A. Incorrect. Set up the equilibrium constant expression. The [H+] and [CN-] are the same and the HCN final concentration at equilibrium should be close to its original concentration since this is a weak acid with a very small Ka.

B. Correct. Good job! Set up the equilibrium constant expression. The [H+] and [CN-] are the same and the HCN final concentration at equilibrium should be close to its original concentration since this is a weak acid with a very small Ka.

C. Incorrect. Set up the equilibrium constant expression. The [H+] and [CN-] are the same and the HCN final concentration at equilibrium should be close to its original

Feedback concentration since this is a weak acid with a very small Ka. D. Incorrect. Set up the equilibrium constant expression. The [H+] and [CN-] are the same and the HCN final concentration at equilibrium should be close to its original concentration since this is a weak acid with a very small Ka. E. Incorrect. Set up the equilibrium constant expression. The [H+] and [CN-] are the same and the HCN final concentration at equilibrium should be close to its original concentration since this is a weak acid with a very small Ka.

Both H+ and CN- are the products of ionization, as their initial concentrations are zero. Their equilibrium concentrations must be the same.

HCN  H+ + CN- 1.0-x x x

+ - 2 -10 Ka = [H ][CN ]/[HCN] = x /(1.0-x) = 6.2x10 Since x is much smaller than 1.0, we can assume: Solution x2/1.0 = 6.2x10-10 and x = 2.5x10-5 Hence [H+] = 2.5x10-5 M

The correct answer is (B).

RapidLearningCenter.com  Rapid Learning Inc. All Rights Reserved Question No. 6 of 10 Instructions: (1) Read the problem and answer choices carefully (2) Work the problems on paper as needed (3) Pick the answer (4) Go back to review the core concept tutorial as needed. 6. An acid and a base which differ only by the presence or absence of a proton are called a conjugate acid-base pair. The ability of some chemicals to act either as an acid or a base is called amphoterism. The self-ionization of water is an example of + - water being able to react either as an acid or a base (H2O + H2O  H3O + OH ). In + - + the liquid reaction 2 NH3  NH4 + NH2 , NH4 acts as a ___.

(A) Amphoteric species (B) An intermediate Question (C) The conjugate acid of NH3 (D) The conjugate base of NH3 (E) None of the above

A. Incorrect. Amphoteric means a species can act as an acid or base.

B. Incorrect. An intermediate is something that is produced and then reacted away in a reaction mechanism.

C. Correct. + Good job! Apply the definition of a conjugate acid. NH4 is the conjugate acid of NH . Feedback 3 D. Incorrect. + NH4 would be an acid, not a base.

E. Incorrect. Look closely—there is one correct answer above for how the ammonium ion acts.

Rewrite the reaction as: + - NH3 + NH3  NH4 + NH2

Compare this with the self-ionization of water: + - H2O + H2O  H3O + OH

+ Drawing on the parallel, you can tell NH4 is obviously a conjugate acid of NH3.

Amphoteric: acts as an acid or a 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? Solution Conjugate base: what’s left when an acid donates a proton?

The correct answer is (C).

RapidLearningCenter.com  Rapid Learning Inc. All Rights Reserved Question No. 7 of 10 Instructions: (1) Read the problem and answer choices carefully (2) Work the problems on paper as needed (3) Pick the answer (4) Go back to review the core concept tutorial as needed. 7. To select an indicator for a titration, look for an indicator with the color change pH range as close as possible to its equivalent point. For a titration of a strong acid with a strong base, the equivalent point is at pH=7. For a titration of a weak acid with a strong base, the equivalent point will shift to more basic, and vice visa. For the titration of a weak acid with a strong base to a desired end-point near pH 7.0 (but not at pH 7), which indicator would be the best choice?

(A) Methyl Orange with pH 3.2-4.4

Question (B) Methyl Red with pH 4.8-6.0 (C) Bromothymol Blue with pH 6.1-7.6 (D) Phenolphthalein with pH 8.2-10.0 (E) Alizarin with pH 11.0-12.4

A. Incorrect. The products will be basic. This indicator is for an acid product in solution.

B. Incorrect. The products will be basic. This indicator is for a slightly acidic solution.

C. Incorrect. The products will be basic due to the strong base with the weak acid. Its equivalent point should be slightly above pH 7, but not exactly at 7. This range 6.1-7.6 would be ideal for a titration of a strong acid with a strong base.

Feedback D. Correct. Good job! The products will be basic—but not very basic, so choose the slightly 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 a strong base reacts with 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).

Solution

RapidLearningCenter.com  Rapid Learning Inc. All Rights Reserved Question No. 8 of 10 Instructions: (1) Read the problem and answer choices carefully (2) Work the problems on paper as needed (3) Pick the answer (4) Go back to review the core concept tutorial as needed. 8. The concept of common ion effect involves how the solubility of a salt changes when some ion that is common to both added substance and the salt in question.

Adding which of the following would increase the solubility of Ca(OH)2?

(F) NaOH (G) HCl Question (H) Ca(NO3)2 (I) NH3 (J) Ca(OH)2

A. Incorrect. Adding a common ion decreases solubility.

B. Correct. The acid will neutralize the , effectively removing a product of the “solubility equilibrium” and pushing the reaction to the right.

C. Incorrect.

Adding a common ion decreases solubility. Feedback D. Incorrect. 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(NO3)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 and will allow more to dissolve (Le Chatelier’s Principle) -1 NH3 is a base. Adding NH3 will raise the pH, creating more OH , which is a common ion and will decrease the solubility.

Therefore, the effect of common ion is to decrease the solubility of the salt.

The correct answer is (B).

Solution

RapidLearningCenter.com  Rapid Learning Inc. All Rights Reserved Question No. 9 of 10 Instructions: (1) Read the problem and answer choices carefully (2) Work the problems on paper as needed (3) Pick the answer (4) Go back to review the core concept tutorial as needed.

9. Carbonic acid H2CO3 is a diprotic acid with two protons to donate, first to - -7 2- -11 HCO3 (K1 = 4.5x10 ) and second to carbonate CO3 (K2 = 4.7x10 ). - 2- Calculate the pH value of a buffer solution of 0.10 M HCO3 and 0.15 M CO3 .

(A) 3.40 (B) 5.31

Question (C) 10.51 (D) 4.47 (E) 7.00

A. Incorrect.

This is a buffer solution involving the secondary ionization of H2CO3. Set up the - + 2- ionization constant expression of HCO3 + H2O  H3O + CO3 and solve for the + equilibrium concentration [H3O ].

B. Incorrect.

This is a buffer solution involving the secondary ionization of H2CO3. Set up the - + 2- ionization constant expression of HCO3 + H2O  H3O + CO3 and solve for the + equilibrium concentration [H3O ].

C. Correct.

Good job! This is a buffer solution involving the secondary ionization of H2CO3. Set - + 2- up the ionization constant expression of HCO3 + H2O  H3O + CO3 and solve for + the equilibrium concentration [H3O ]. Feedback D. Incorrect.

This is a buffer solution involving the secondary ionization of H2CO3. Set up the - + 2- ionization constant expression of HCO3 + H2O  H3O + CO3 and solve for the + equilibrium concentration [H3O ].

E. Incorrect.

This is a buffer solution involving the secondary ionization of H2CO3. Set up the - + 2- ionization constant expression of HCO3 + H2O  H3O + CO3 and solve for the + equilibrium concentration [H3O ].

+ H2CO3 is the polyprotic acid with 2x H of primary and secondary dissociations.

This is a buffer solution with the secondary dissociation of H2CO3. - + 2- -11 HCO3 + H2O  H3O + CO3 … K2 = 4.7x10

+ 2- - + -11 K2 = [H3O ][CO3 ]/[HCO3 ] = [H3O ][0.15]/[0.10] =4.7x10 + -11 Solve for [H3O ] = 3.1x10 + -11 pH = -log [H3O ] = -log (3.1x10 ) = 10.51

Solution Alternatively, simply apply the Henderson-Hasselback Equation (which is practically the same as above): 2- - pH = pKa + log([base]/[acid]) = -log k2 + log ([CO3 ]/[HCO3 ]) = 10.51

The correct answer is (C).

RapidLearningCenter.com  Rapid Learning Inc. All Rights Reserved Question No. 10 of 10 Instructions: (1) Read the problem and answer choices carefully (2) Work the problems on paper as needed (3) Pick the answer (4) Go back to review the core concept tutorial as needed. 10. A conjugate acid is defined as the acid formed when a base gains a proton. Similarly, a conjugate base is formed when an acid loses a proton. Which of the following does NOT represent a conjugate acid-base pair?

+ - (A) H3O and OH - (B) H2O and OH (C) HCl and Cl- - 2- Question (D) HSO3 and SO3 - 2- (E) HCO3 and CO3

A. Correct. + + Good job! The acid H3O loses a proton (H ) and yields its conjugate base H2O (not OH-). Therefore, these two are a NOT a conjugate acid-base pair.

B. Incorrect. + - The acid H2O loses a proton (H ) and yields its conjugate base OH . Therefore, these two are a conjugate acid-base pair.

C. Incorrect. The acid HCl loses a proton (H+) and yields its conjugate base Cl-. Therefore, these two are a conjugate acid-base pair.

Feedback D. Incorrect. 2- - The base SO3 is a conjugate of a weak acid (HSO3 ). Therefore, these two are a conjugate acid-base pair.

E. Incorrect. 2- - The base CO3 is a conjugate of a weak acid (HCO3 ). Therefore, these two are a conjugate acid-base pair.

Apply Brønsted-Lowry definitions of acid and base. An acid donates a proton (H+) and a base accepts a proton (H+).

Conjugate acid-base pairs are compounds that differ by the presence of one proton, or H+. All acids have a conjugate base, which is formed when their proton has been donated; likewise, all bases have a conjugate acid, formed after they have accepted a proton.

+ + - The acid H3O loses a proton (H ) and yields its conjugate base H2O (not OH ). The - + + Solution base OH accepts a proton (H ) to give its conjugate acid H2O (not H3O ). + - Therefore, the combination of H3O and OH does not represent a conjugate acid- base pair.

The correct answer is (A).

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