Acid and Base Quiz

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 above 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?

A (same graph) down

B(aq)+HCl(aq)⇄HB+(aq)+Cl−(aq) The reaction of a weak base, B, with HCl is represented by the equation above. The graph 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?

A 0.20M

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

A 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+.

A student pours a 10.0mL sample of a solution containing HC2H3O2(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?

A [HC2H3O2]>[NaC2H3O2]

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

The graph above 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+?

B 10.8

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

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

The pH of a 0.01 M HNO2 (aq) solution is in which of the following ranges? (For HNO2 (aq), Ka = 4.0 x 10-4)

B Between 2 and 3

H3N+CH2COOH(aq)+H2O(l)⇄H3N+CH2COO−(aq)+H3O+(aq) H3N+CH2COO−(aq)+H2O(l)⇄H2NCH2COO−(aq)+H3O+(aq) 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 0.100MNaOH(aq). The data are 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?

B H3N+CH2COO−(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 pH meter and graphed as a function of the volume of 0.100 M NaOH added. Which of the following is the best particulate representation of the species (other than H2O) that are present in significant concentrations in the solution at point U in the titration?

B OH- appears

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. 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?

B Slightly more than 0.0800 M

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

C 1 x 10-6 M

The value of Kw for water at 0°C is 1×10−15. What is the pOH of water at 0°C?

C 7.5

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 ?

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

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?

C ClO2 -(aq)

Samples of NaF(s) and NH4Cl(s) are dissolved in separate beakers that each contain 100mL of water. One of the salts produces a slightly acidic solution. Which of the following equations best represents the formation of the slightly acidic solution?

C NH4+(aq)+H2O(l)⇄NH3(aq)+H3O+(aq)

B(aq)+H2O(l)⇄HB+(aq)+OH−(aq)Kb=[HB+][OH−][B] The table above provides the chemical structures for weak bases and their ionization constants, Kb. Based on the data, which of the following provides the best reason for the trend in base strengths?

C The different electronegativities of H, I, and Br

HF(aq)+H2O(l)⇄H3O+(aq)+F−(aq) 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?

C 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.

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 0.30MNH3(aq), and why?

C [OH−]=2.3×10−3M and pOH<2.78, because a higher [OH−] corresponds to a lower pOH for 0.30MNH3(aq) compared to 0.15MNH3(aq).

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

C pH=14.00−(−log(0.0015)) and pOH=−log(0.0015)

A 0.08M solution of CH3COOH(pKa=4.74) is titrated with 0.10MNaOH(aq). What is the pH at the equivalence point of the titration and why?

C pH>7, because CH3COO−(aq) is a weak base.

Caffeine (C8H10N4O2) 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

D 11-12

The pH of a solution made by combining 150.0 mL of 0.10 M KOH(aq) with 50.0 mL of 0.20 M HBr(aq) is closest to which of the following?

D 12

Based on the information in the table above, which of the following pairs of solutions can be used to create a buffer with a pH of 9.3 ? (Assume all solutions are 1.0M.)

D 50.0mL of NH3(aq) and 50.0mL of NH4Cl(aq)

An acetate buffer solution is prepared by combining 50. mL of 0.20 M acetic acid, HC2H3O2(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.)

D Acetate C2H3O2- Species pH >4.7

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

D Between 4 and 7

NH3(aq)+CH3NH3+(aq)⇄NH4+(aq)+CH3NH2(aq) Keq=0.041 Which of the following species is the strongest base in the reaction represented above?

D CH3NH2(aq)

HF(aq)+H2O(l)⇄H3O+(aq)+F−(aq) Ka=6.3×10−4 at 25°C The acid ionization equilibrium for HF is represented by the chemical equation above. A student claims that the pH of a solution that contains 0.100MHF(aq) and 0.100MNaF(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?

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

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°C. (CH3)2NH(aq)+H2O(l)⇄(CH3)2NH2+(aq)+OH−(aq)Kb=5.4×10−4at25°C Based on the information given, which of the following is true?

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

Is a strong electrolyte in aqueous solution

E HBr(g)