Unit 8 Acids-Bases Multiple Choice

In pure water, some of the molecules ionize according to the equation H2O ⇌ H+ + OH− The extent of the ionization increases with temperature. A student heats pure water and records the measured pH at 50°C as 6.6. Based on this information, which of the following mathematical relationships gives the pOH of pure water at 50°C ?

pOH = pH

A 0.10 M solution of a weak monoprotic acid has a pH equal to 4.0. The ionization constant, Ka, of the acid is

1 x 10^-7

The graph opposite shows the titration curve for an aqueous solution of a weak base, B , with HCl as the titrant. Based on the graph, which of the following best estimates the pKa of HB+?

10.8

The pH of a solution prepared by the addition of 10. mL of 0.002 M KOH(aq) 10. mL of distilled water is closest to

11

What is the pH of a 1.0 x 10^-2 -molar solution of HCN? (For HCN, Ka=4.0x10^-10)

4 - 7

Which of the following accounts for the observation that the pH of pure water at 37°C is 6.8?

At 37 ° C the autoionization constant for water, Kw , is larger than it is at 25 ° C.

The table above shows the values of Ka for four weak acids. Which of the following pairs of chemical species, when combined in equimolar amounts, results in a buffer with a pH closest to 7.5 ?

HClO and ClO−

At 25°C, aqueous solutions with a pH of 8 have a hydroxide ion concentration, [OH—], of

1 x 10^-6 M

The pH of 0.1 M ammonia solution is approximately

11

For which acid is the value of the acid-dissociation constant, Ka , the smallest?

Acid 1

The point at which the concentrations of the weak acid and its conjugate base are approximately equal

B

The point at which the moles of the added strong base are equal to the moles of the weak acid initially present

C

Which of the following dissolves in water to form an acidic solution

CH3COOH

Which of the following is a strong electrolyte in aqueous solution?

HBr(g)

The net ionic equation for the reaction that occurs during the titration of nitrous acid with sodium hydroxide is

HNO2 + OH-→ NO2- + H2O

At which point on the titration curve is [A-] closest to twice that of [HA]?

T

Which of the following correctly ranks the three monoprotic acids listed in the table opposite from the weakest to the strongest?

Z < Y < X

Nitrous acid reacts with ammonia according to the balanced chemical equation shown above. If 50.mL of 0.20 M HNO2(aq) and 50.mL of 0.20 M NH3 (aq) are mixed and allowed to reach equilibrium at 25°C , what is the approximate [NH3] at equilibrium?

0.00010 M

A student performs an acid-base titration and plots the experimental results in the graph opposite. Which of the following statements best explains the experimental findings?

A weak acid was titrated with a strong base, as evidenced by the equivalence point at pH > 7.

How can 100. mL of sodium hydroxide solution with a pH of 13.00 be converted to a sodium hydroxide solution with a pH of 12.00 ?

By diluting the solution with distilled water to a total volume of 1.00 L

A buffer solution is made up of acetic acid (CH3 COOH) and sodium acetate (NaCH3COO). The major equilibria in the buffer system are represented above. Which of the following equilibria could be used to support the claim that the addition of a small amount of NaOH to the buffer will result in only a very small change in pH ?

CH3COOH(aq) + OH- (aq)⇄ CH3COO- (aq) + H2O(l)

Each particle diagram shown is a representation of an aqueous solution of one of the acids listed in the table. The molarity of the acids in the solutions is the same. Based on the information, which particle diagram best corresponds to the indicated acid?

Diagram 2 corresponds to HIO.

On the basis of the information above, a buffer with a pH = 6.5 can best be made by using

H2PO4- + HPO4^2-

Which of the following is a weak acid in aqueous solution?

H2S

The acid ionization equilibrium for HF is represented by the chemical equation above. A student claims that the pH of a solution that contains 0.100 M HF(aq) and 0.100 MNaF(aq) will change only slightly when small amounts of acids or bases are added. Which of the following pairs of equations can the student use to justify the claim?

HF(aq) + OH− (aq) → H2O(l) + F− (aq) and F− (aq) + H3O+ (aq)→HF (aq) + H2O (l)

Mixtures that would be considered buffers include which of the following?

II only

To determine the concentration of a NaOH(aq) solution, a student titrated a 50. mL sample with 0.10 M HCl(aq) . HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) The reaction is represented by the equation above. The titration is monitored using a pH meter, and the experimental results are plotted in the graph opposite. At the point labeled R on the pH curve, which of the following ions are present in the reaction mixture at a concentration greater than 0.01 M ?

Na+ and Cl− only

The graph below shows the titration curve that results when 100. mL of 0.0250 M acetic acid is titrated with 0.100 M NaOH. Which of the following indicators is the best choice for this titration?

Phenolphthalein

A student carries out the same titration, but uses an indicator instead of a pH meter. If the indicator changes color slightly past the equivalence point, what will the student obtain for the calculated concentration of the acid?

Slightly more than 0.0800 M

Acid-dissociation constants of two acids are listed in the table above. A 20. mL sample of a 0.10 M solution of each acid is titrated to the equivalence point with 20. mL of 0.10 M NaOH. Which of the following is a true statement about the point?

Solution 1 has a higher pH at the equivalence point because CH3CO2H has the stronger conjugate base.

(CH3)2NH(aq) + H2O(l) ⇋ (CH3)2NH2+(aq) + OH- (aq) Kb=5.4x10^-4 at 25 degrees C The equilibrium for the reaction between (CH3)2NH, a weak base, and water is represented by the equation above. The table shows the pH of three solutions of (CH3)2NH (aq) at 25 degrees C. Based on the information given, which of the following is true?

Solutions with a higher concentration of (CH3)2NH have a higher pH because to reach equilibrium more OH− is produced.

H2O(l) + H2O(l) ⇋ H3O+ (aq) + OH— (aq) At 5.0°C , the value of Kw pKw is 14.73. Based on this information, which of the following is correct for pure water at this temperature?

[H3O+]=square root 1.9x10^-15

A student mixes 40.mL of 0.10 M HBr(aq) with 60.mL of 0.10 M KOH(aq) at 25 degrees C. What is the [OH-] of the resulting solution?

[OH-]= 0.020 M

When a strong acid is titrated with a strong base using phenolphthalein as an indicator, the color changes abruptly at the endpoint of the titration and can be switched back and forth by the addition of only one drop of acid or base. The reason for the abruptness of this color change is that

a large change in pH occurs near the endpoint of the titration

Which of the following is the correct mathematical expression to use to calculate the pH of a 0.10 M aqueous Ba(OH)2 solution at 25°C ?

pH = 14.00 + log (0.20)

Which of the following statements about the pH of 0.010 M HClO4 is correct?

pH = 2.00 , because [H+] = 1.0 × 10^−2 M

The equilibrium for the reaction between (CH3)2NH, a weak base, and water is represented by the equation below. The table shows the pH of three solutions of (CH3)2NH (aq) at 25 degrees C. Which of the following equations can be used to correctly calculate the pH at 25°C that is 0.100 M (CH3)2NH (aq) and 0.100 M (CH3)2NH2Cl (aq)?

pH=-log(1.0x10^-14/5.4x10^-4)

The graph below shows the titration curve that results when 100. mL of 0.0250 M acetic acid is titrated with 0.100 M NaOH. What part of the curve corresponds to the optimum buffer action for the acetic acid/acetate ion pair?

point V

The point at which the pH is closest to that of the strong base being added

E

An unknown acid is dissolved in 25 mL of water and titrated with 0.100 M NaOH. The results are shown in the titration curve below. Which of the following could be the unknown acid?

Glycolic acid, pKa=3.8

Addition of sulfurous acid (a weak acid) to barium hydroxide (a strong base) results in the formation of a precipitate. The net ionic equation for this reaction is

H2SO3(aq) + Ba2+(aq) + 2 OH-(aq) ⇄ BaSO3(s)+ 2 H2O(l)

The stepwise dissociation of the amino acid glycine is represented by the chemical equations above. A student titrates a sample of glycine dissolved in dilute acid with 0.100 M NaOH(aq). The data is plotted on the following graph. Based on the data, which of the following species has the highest concentration in an aqueous solution of glycine with a pH of 7?

H3N+CH2COO- (aq)

A solution is prepared by adding 100 mL of 1.0 M HC2 H3O2(aq) to 100 mL of 1.0 M NaC2H3O2(aq). The solution is stirred and its pH is measured to be 4.73. After 3 drops of 1.0 M HCl are added to the solution, the pH of the solution is measured and is still 4.73. Which of the following equations represents the chemical reaction that accounts for the fact that acid was added but there was no detectable change in pH?

H3O+(aq) + C2H3O2- (aq) -> HC2H3O2 (aq) + H2O (l)

At point R in the titration, which of the following species has the highest concentration?

HA

The Lewis electron-dot diagrams of the HClO3 molecule and the HClO2 molecule are shown opposite at the left and right, respectively. Which of the following statements identifies the stronger acid and correctly identifies a factor that contributes to its being the stronger acid?

HClO3 (aq) is the stronger acid because the additional electronegative oxygen atom on the chlorine atom stabilizes the conjugate base.

The reaction of a weak base, B , with HCl is represented by the equation above. The graph above (Q1) shows the titration curve for 25.0mL of an aqueous solution of B titrated with 0.100MHCl. Based on the graph, which of the following best estimates the initial concentration of the solution of the weak base?

0.20 M

A solution prepared by mixing 10 mL of 1 M HCl and 10 mL of 1.2 M NaOH has a pH of

13

The graph opposite shows the titration curve that resulted when a sample of 0.1 M monoprotic acid was titrated with a solution of NaOH. Based on the graph, the pKa of the acid is closest to

4.0

A student prepares a lactic acid-sodium lactate buffer solution by mixing 40.mL of 0.50 M HC3 H5O3(aq) with 200.mL of 1.0 M NaC3H5O3(aq) . The pKa of HC3H5O3 is 3.08. What is the pH of the resulting solution?

4.08

The chemical equation above represents the acid ionization equilibrium for HC4H7O2 for which pKa=4.8. Which of the following is the best estimate for the pH of a buffer prepared by mixing 100.mL of 0.20 M HC4H7O2 with 100 mL of 1.10 M NaC4H7O2?

4.5

The acid-dissociation constants of HC3 H5O3(aq) and CH3 NH3 (aq) are given in the table above. Which of the following mixtures is a buffer with a pH of approximately 3?

A mixture of 100. mL of 0.1 M HC3H5O3 and 50. mL of 0.1 M NaOH

The four acids are, in no particular order, chlorous, hydrochloric, lactic, and propanoic. Of the following species, which has the greatest concentration in a 1.0 M solution of acid 1 at equilibrium?

Acid 1

The pH of solutions of four acids prepared at various concentrations were measured and recorded in the table above. The four acids are, in no particular order, chlorous, hydrochloric, lactic, and propanoic. Which of the four acids listed in the table is hydrochloric acid?

Acid 2

The pH of solutions of four acids prepared at various concentrations were measured and recorded in the table above. The four acids are, in no particular order, chlorous, hydrochloric, lactic, and propanoic. If equal volumes of the four acids at a concentration of 0.50 M are each titrated with a strong base, which will require the greatest volume of base to reach the equivalence point?

All the acids will require the same volume of base to reach the equivalence point.

5 H2O2(aq) + 2 MnO4 — (aq) + 6 H+ (aq) → 2 Mn2+ (aq) + 8 H2O(l) + 5 O2(g) In a titration experiment, H2O2(aq) reacts with aqueous MnO4- (aq) as represented by the equation above. The dark purple KMnO4 solution is added from a buret to a colorless, acidified solution of H2O (aq) in an Erlenmeyer flask. (Note: At the end point of the titration, the solution is a pale pink color.) Which of the following best describes what happens to the pH of the H2O2 solution as the titration proceeds

As H+ ions are consumed, the solution becomes less acidic and the pH increases.

2 H2O(l)⇋ H3O+ (aq) + OH— (aq) The autoionization of water is represented by the equation above. Values of pKw at various temperatures are listed in the table below. Based on the information above, which of the following statements is true?

As the temperature increases, the pH of pure water decreases.

The pH of a 0.01 M HNO2(aq) solution is in which of the following ranges? (For HNO2(aq), Ka=4.0x10^-4)

Between 2 and 3

A solution containing HCl and the weak acid HClO2 has a pH of 2.4. Enough KOH(aq) is added to the solution to increase the pH to 10.5. The amount of which of the following species increases as the KOH(aq) is added?

ClO2- (aq)

A 50.0 mL sample of an acid, HA, of unknown molarity is titrated, and the pH of the resulting solution is measured with a meter and graphed as a function of the volume of 0.100 M NaOH added. At which point on the titration curve is [A-] closest to twice that of [HA]?

T

Which of the following pairs of mathematical expressions can be used to correctly calculate the pH and pOH of a 0.0015 M KOH(aq) solution at 25°C ?

pH=14.00-(-log(0.0015)) and pOH=-log(0.0015)

The equilibrium for the acid ionization of HC2 If 10.0mL of 0.20 M HC2 HO3 is represented by the equation above. HO3 react with 5.0mL of 0.10 M NaOH, which of the following could be used to calculate the correct pH of the resulting solution?

pH=pKa+log[((0.0050x0.10)/0.0150)/((0.0100x0.20)-(0.0050x0.10)/0.0150))]

Data collected during the titration of a 20.0 mL sample of a 0.10 M solution of a monoprotic acid with a solution of NaOH of unknown concentration are plotted in the graph opposite. Based on the data, which of the following are the approximate pKa of the acid and the molar concentration of the NaOH?

pKa= 4.7 and [NaOH] = 0.10 M

A 40.0mL sample of the weak base C5H11N was titrated with 1.00 M HCl at 25 degrees C. Based on the resulting titration curve shown opposite, which of the following pairs provide the best estimates for the pKb and Kb of C5H11N ?

pKb≈ 3.0 and Kb≈1× 10^−3

In pure water, some of the molecules ionize according to the equation H2O ⇌ H+ + OH− The extent of the ionization increases with temperature. The graph opposite shows the pH of pure water at different temperatures. Which of the following represents the variations in the pOH of pure water under the same conditions?

the graph that goes down from left to right in a straight line (looks like \)

When 200. mL of 2.0 M NaOH(aq) is added to 500. mL of 1.0 M HCl(aq) , the pH of the resulting mixture is closest to

1.0

Caffeine (C8H10 O2) is a weak base with a Kb value of 4 x 10-4. The pH of a 0.01 M solution of caffeine is in the range of

11-12

A 0.10 M solution of a weak monoprotic acid has a pH equal to 4.0. The ionization constant, Ka , of the acid is

1x10^-8

A 0.35 g sample of Li(s) is placed in an Erlenmeyer flask containing 100 mL of water at 25°C. A balloon is placed over the mouth of the flask to collect the hydrogen gas that is generated. After all of the Li(s) has reacted with H2O(l), the solution in the flask is added to a clean, dry buret and used to titrate an aqueous solution of a monoprotic acid. The pH curve for this titration is shown in the diagram above. On the basis of the pH curve, the pKa value of the acid is closest to

5

An acetate buffer solution is prepared by combining 50. mL of 0.20 M acetic acid, HC2 H3O2(aq), and 50. mL of 0.20 M sodium acetate, NaC2H3O2 (aq). A 5.0 mL sample of 0.10 M NaOH(aq) is added to the buffer solution. Which of the following is a correct pairing of the acetate species present in greater concentration and of the pH of the solution after the NaOH (aq) is added? (The pKa of acetic acid is 4.7)

C2H3O2- pH > 4.7

The table above includes the Lewis diagrams of the conjugate acids of some weak nitrogenous bases. Based on the relative stability of the conjugate acids, which of the following bases has the largest value for Kb ?

CH3NH2

The equilibrium for the acid ionization of HC2H3O2 is represented by the equation above. A student wants to prepare a buffer with a pH of 4.76 by combining 25.00mL of 0.30 M HC2H3O2 with 75.00mL of 0.10 M NaC2H3O2. While preparing the buffer, the student incorrectly measures the volume of NaC2 H3O2 so that the actual volume used is 76.00mL instead of 75.00mL. Based on the error, which of the following is true about the buffer prepared by the student?

The buffer solution will have a slightly higher capacity for the addition of acids than for the addition of bases because the amount of C2H3O2-added was higher than the amount of HC2H3O2 added.

The acid ionization equilibrium for the weak acid HF is represented by the equation above. To prepare a buffer with a pH = 3.50 , a student needs to mix 250.mL of 0.100 M HF and 250.mL of 0.100 M KF. If the student mistakenly mixes 250.mL of 0.0500 M HF and 250.mL of 0.0500 M KF , which of the following is the result of this error?

The buffer will have a lower capacity because of the smaller number of moles of HF and F− available to react if an acid or base is added.

The equilibrium for the reaction between (CH3)2NH, a weak base, and water is represented by the equation below. The table shows the pH of three solutions of (CH3)2NH (aq) at 25 degrees C. A student mixes 100.mL of 0.200 M (CH3)2NH(aq) with 100.mL of 0.200 M (CH3)2NH2Cl(aq) and claims that if a small amount of strong base is added to the mixture, then the resulting change in pH of the mixture will be smaller than the change in pH that would result from adding the same amount of strong base to 200.mL of 0.200 M (CH3)2NH(aq) . Which of the following best explains whether or not the student's claim is correct?

The claim is correct because the mixture contains a significant amount of the acid (CH3)2NH2+ (aq) , which can react with and partially neutralize the added strong base, thereby reducing the change in pH . In 0.200 M (CH3)2NH (aq), the concentration of the acid (CH3)2NH2 (aq) is very small; therefore,the added strong base is not neutralized and the change in pH will be larger.

Which of the following is the best particulate representation of the species (other than H2 O) that are present in significant concentrations in the solution at point U in the titration?

The diagram with 3 Na+, 2 A-, and 1 OH-

The table above provides the chemical structures for weak bases and their ionization constant, Kb. Based on the data, which of the following provides the best reason for the trend in base strengths?

The different electronegativities of H, I, and Br

The equation above represents the acid ionization equilibrium for HF. To prepare a buffer with pH≈3.50, 4.20g of NaF(s) should be added to 500.0mL of 0.100MHF(aq) The buffer is accidentally prepared using 90% pure NaF(s) instead of 99% pure NaF(s) .Assume that the impurities in the NaF(s) samples are inert. Which of the following explains how the error affects the pH and capacity of the buffer?

The pH is slightly lower than 3.50 and it has a lower capacity for the addition of acids because less than 4.20g of NaF(s) was added.

The pH of solutions of four acids prepared at various concentrations were measured and recorded in the table above. The four acids are, in no particular order, chlorous, hydrochloric, lactic, and propanoic. A 25 mL sample of a 1.0 M solution of acid 1 is mixed with 25 mL of 0.50 M NaOH. Which of the following best explains what happens to the pH of the mixture when a few drops of 1.0 M HNO3 are added?

The pH of the mixture stays about the same, because the conjugate base of acid 1 reacts with the added H3O+ ions.

HCN (aq) + H2O (l) <-> H3O+ (aq) + CN- (aq) Ka=[H3O+][CN-]/[HCN]=6.2x10^-10 The equilibrium reaction shown above represents the partial ionization of the weak acid HCN (aq). A 2.00 M HCN (aq) solution has a pH≈4.95. If 0.05g (0.010mol) of NaCN(s) is added to 100mL of 0.200 M HCN (aq), which of the following explains how and why the pH of the solution changes?

The pH will be higher than 4.95 because adding CN− will disrupt the equilibrium, resulting in an increased production of HCN that decreases the concentration of H3O+.

Benzoic acid, HC7 H5O2 , has a pKa of 4.20 and a molar solubility of 0.0278M. Sodium benzoate, NaC7 a molar solubility of 4.16M. H5O2 , has Several 50.0mL samples of 2.00 M NaC7 H5O2(aq) are treated with 3.00M HCl, the pH is recorded, and any solid crystals are filtered, dried, and weighed. The data from the experiment are given in the table above. Which of the following best explains the experimental results?

When pH < pKa, [HC7H5O2] > [NaC7H5O2] and HC7H5O2(s) precipitates from the solution.

A student pours a 10.0mL sample of a solution containing HC2 H3O2 (pKa = 4.8) and NaC2H3O2 into a test tube. The student adds a few drops of bromocresol green to the test tube and observes a yellow color, which indicates that the pH of the solution is less than 3.8. Based on this result, which of the following is true about the relative concentrations of HC2H3O2 and NaC2H3O2 in the original solution?

[HC2H3O2]>[NaC2H3O2]

The ionization constant for acetic acid is 1.8x10^-5; that for hydrocyanic acid is 4 x 10-10 In 0.1 M solutions of sodium acetate and sodium cyanide, it is true that

[OH—] of the sodium acetate solution is less than that of the sodium cyanide solution

NH3 is a weak base that reacts with water according to the chemical equilibrium represented above. The table provides some information for two NH3(aq) solutions of different concentration at 25°C . Which of the following is true about the more concentrated 0.30 M NH3(aq) , and why?

[OH−] = 2.3 ×10−3 M and pOH < 2.78 , because a higher [OH−] corresponds to a lower pOH for 0.30 M NH3(aq) compared to 0.15 M NH3(aq).