Question 1

What is the pH of a solution that consists of 0.50 M H₂C₆H₆O₆ (ascorbic acid)and 0.75 M NaHC₆H₆O₆ (sodium ascorbate)? [K_a (H₂C₆H₆O₆)= 6.8 × 10^-5]

Accepted Answer

First, we need to consider the reaction between ascorbic acid and sodium ascorbate: H₂C₆H₆O₆ + NaH₆O₆ → H2₂O6 + Na₆ The reaction produces H2₂O6 (ascorbate ion) and Na₆ (sodium ion) from the reactants ascorbic acid and sodium ascorbate, respectively. The Ka value for ascorbic acid is given as 6.8 × 10^-5. Let x be the concentration of H2₂O6 (ascorbate ion) formed in the reaction. Then, the final concentration of H₂C₆H₆O₆ in solution is 0.50-x M and the final concentration of NaH₆O₆ in solution is 0.75-x M. The reaction consumes equal amounts of ascorbic acid and sodium ascorbate, so the concentration of H2₂O6 (ascorbate ion) formed is also x M. The Ka expression for ascorbic acid is: Ka = [H+][H₂C₆H₆O₆]/[H₂C]eq where [H₂C]eq is the equilibrium concentration of ascorbic acid. We assume that [H₂C]eq is equal to the initial concentration of ascorbic acid because the reaction involves only a small fraction of the reactants. Since x is small compared to the initial concentrations, we can approximate (0.50 - x) as 0.50 and (0.75 - x) as 0.75. Ka = [H+][0.50]/[0.50]eq Taking the negative logarithm of both sides, -pKa = pH = -log(Ka) = -log([H+]) - log([0.50]/[0.50]eq) We know that pKa = 5 + log([H₂C]/[H2₂O6]). At equilibrium, the amount of H2₂O6 produced is equal to the amount of H₂C consumed. Therefore, [H₂C]/[H2₂O6] = 1. Thus, pKa = 5. Substituting in the values: pH = 5 - log([H+]) - log([0.50]/[0.50]eq) pH = 5 + log([0.50]eq/[0.50]) - log(Ka) pH = 4.34 Therefore, the pH of the solution is 4.34. The best choice is C.

Question 2

Which of the following is correct for the equivalence point in an acid-base titration?

Accepted Answer

At the equivalence point in an acid-base titration, the amounts of acid and base combined are in a stoichiometric ratio. This means that all of the acid has been neutralized by the base, or vice versa, and the solution contains only the salt and water formed by the reaction. The pH at the equivalence point depends on the strength of the acid and/or base being titrated, as well as the concentrations of the solutions being used, and can be either acidic or basic. Therefore, options A, C, D, and E are incorrect.

Question 3

A 50.00-mL solution of 0.10 M HNO₂ is titrated with a 0.10 M KOH solution.After 25.00 mL of the KOH solution is added,what is the pH in the titration flask? [K_a(HNO₂)= 4.5 × 10^-4]

Accepted Answer

At the point where 25.00 mL of 0.10 M KOH has been added to 50.00 mL of 0.10 M HNO2, the reaction has reached the halfway point to the equivalence point because the volumes of acid and base are equal and their concentrations are the same. At this halfway point, the pH equals the pKa of the acid. The pKa can be calculated from the given Ka value for HNO2 (4.5 × 10^-4) as follows: pKa = -log(Ka) = -log(4.5 × 10^-4) ≈ 3.35.

Question 4

A buffer is prepared by adding 1.00 L of 1.0 M HCl to 750 mL of 1.5 M NaHCOO.What is the pH of this buffer? [K_a (HCOOH)= 1.7 × 10^-4]

Accepted Answer

The answer of A buffer is prepared by adding 1.00...

Question 5

A buffer is prepared by adding 150 mL of 1.0 M NaOH to 250 mL of 1.0 M NaH₂PO₄.How many moles of HCl must be added to this buffer solution to change the pH by 0.18 units?

Accepted Answer

The answer of A buffer is prepared by adding 150...

Question 6

A buffer is prepared by adding 300.0 mL of 2.0 M NaOH to 500.0 mL of 2.0 M CH₃COOH.What is the pH of this buffer? [K_a(CH₃COOH)= 1.8 × 10^-5]

Accepted Answer

The answer of A buffer is prepared by adding 300.0...

Question 7

Which best describes the pH at the equivalence point of a titration of a weak acid with a strong base?

Accepted Answer

At the equivalence point of a titration of a weak acid with a strong base, the solution is basic because the conjugate base of the weak acid remains in solution. This results in a pH greater than 7 but less than 14.

Question 8

What is the [H₃O⁺] in a solution that consists of 1.5 M NH₃ and 2.5 M NH₄Cl? K_b = 1.8 × 10^-5

Accepted Answer

The answer of What is the [H₃O⁺] in a solution...

Question 9

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

Accepted Answer

At the equivalence point of a strong acid-strong base titration, the pH will be 7 because the amount of H+ ions from the acid will equal the amount of OH- ions from the base, resulting in neutralization and the formation of water. Therefore, the correct answer is C) 7.0.

Question 10

A 20.0-mL sample of 0.25 M HNO₃ is titrated with 0.15 M NaOH.What is the pH of the solution after 30.0 mL of NaOH have been added to the acid?

Accepted Answer

The reaction between HNO3 (a strong acid) and NaOH (a strong base) is a neutralization reaction, where 1 mole of HNO3 reacts with 1 mole of NaOH to produce NaNO3 and H2O. The moles of HNO3 initially present are $20.0 \, \text{mL} \times 0.25 \, \text{M} = 5.0 \, \text{mmol}$. The moles of NaOH added are $30.0 \, \text{mL} \times 0.15 \, \text{M} = 4.5 \, \text{mmol}$. After the reaction, there are $5.0 \, \text{mmol} - 4.5 \, \text{mmol} = 0.5 \, \text{mmol}$ of HNO3 left unreacted in a total volume of $20.0 \, \text{mL} + 30.0 \, \text{mL} = 50.0 \, \text{mL}$. The concentration of HNO3 is then $0.5 \, \text{mmol} / 50.0 \, \text{mL} = 0.01 \, \text{M}$. Since HNO3 is a strong acid, its pH can be calculated directly from its concentration: $\text{pH} = -\log[0.01] = 2.00$.

Question 11

An acetic acid buffer containing 0.50 M CH₃COOH and 0.50 M CH₃COONa has a pH of 4.74.What will the pH be after 0.0020 mol of HCl has been added to 100.0 mL of the buffer?

Accepted Answer

First, we need to determine the initial concentrations of CH3COOH and CH3COO- before any HCl is added. Since the buffer is made up of equal concentrations of the weak acid CH3COOH and its conjugate base CH3COO-, we know that:

[CH3COOH] = 0.50 M
[CH3COO-] = 0.50 M

The Henderson-Hasselbalch equation can then be used to calculate the initial pH of the buffer:

pH = pKa + log([CH3COO-]/[CH3COOH])
pKa of CH3COOH is 4.74, so:
4.74 = 4.74 + log([CH3COO-]/[CH3COOH])
log([CH3COO-]/[CH3COOH]) = 0
[CH3COO-]/[CH3COOH] = 1
[CH3COO-] = [CH3COOH] = 0.50 M

Therefore, the initial pH of the buffer is 4.74.

Next, we need to determine the new concentrations of CH3COOH and CH3COO- after 0.0020 mol of HCl is added to the buffer. Since HCl is a strong acid, it will react completely with CH3COO- to form CH3COOH and Cl-, according to the following equation:

CH3COO- + HCl → CH3COOH + Cl-

We can use stoichiometry to determine the amount of CH3COO- that reacts with the HCl:

0.0020 mol HCl x (1 mol CH3COO-/1 mol HCl) = 0.0020 mol CH3COO-

This means that 0.0020 mol of CH3COO- will be converted to CH3COOH, and the new concentrations of both species can be calculated:

[CH3COOH] = 0.50 M + (0.0020 mol / 0.100 L) = 0.52 M
[CH3COO-] = 0.50 M - 0.0020 mol / 0.100 L = 0.48 M

Now we can use the Henderson-Hasselbalch equation again to calculate the new pH of the buffer:

pH = pKa + log([CH3COO-]/[CH3COOH])
pKa of CH3COOH is still 4.74, so:
pH = 4.74 + log(0.48/0.52) = 4.71

Therefore, the final pH of the buffer after HCl is added is 4.71. The correct answer is B.

Question 12

What is the pH of a buffer that consists of 0.20 M Na H₂PO₄ and 0.40 M Na₂HPO₄? [K_a(Na H₂PO₄)= 6.2 × 10^-8]

Accepted Answer

The answer of What is the pH of a buffer...

Question 13

Which is the best acid to prepare a buffer with the lowest pH?

Accepted Answer

The lower the pH of a buffer, the stronger the acid component of the buffer needs to be. Among the given options, the acid with the lowest pKa (highest acidity) is HOC₆H₄OCOOH with a pKa of 1.0 × 10^-3. This acid would be the best choice to prepare a buffer with the lowest pH.

Question 14

A 50.0-mL sample of 0.50 M HCl is titrated with 0.50 M NaOH.What is the pH of the solution after 28.0 mL of NaOH have been added to the acid?

Accepted Answer

The answer of A 50.0-mL sample of 0.50 M HCl...

Question 15

A 20.00-mL sample of 0.3000 M HBr is titrated with 0.15 M NaOH.What is the pH of the solution after 40.3 mL of NaOH have been added to the acid?

Accepted Answer

The answer of A 20.00-mL sample of 0.3000 M HBr...

Question 16

What is the pH of a buffer that consists of 0.45 M CH₃COOH and 0.35 M CH₃COONa? [K_a(CH₃COOH)= 1.8 × 10^-5]

Accepted Answer

The answer of What is the pH of a buffer...

Question 17

What is the [H₃O⁺] in a buffer that consists of 0.30 M HCOOH and 0.20 M HCOONa? [K_a(HCOOH)= 1.7 × 10^-4]

Accepted Answer

The answer of What is the [H₃O⁺] in a buffer...

Question 18

Which best describes the pH at the equivalence point of a titration of a weak base with a strong acid?

Accepted Answer

The equivalence point of a titration between a weak base and a strong acid is acidic, hence the pH will be less than 7. This is because the strong acid fully dissociates in water, contributing excess H+ ions, which lowers the pH.

Question 19

Which is the best choice for preparing a buffer with a pH of 3.5?

Accepted Answer

The best choice for preparing a buffer with a pH of 3.5 is the one whose acid dissociation constant (Ka) is closest to the hydrogen ion concentration corresponding to that pH. The pKa (which is the negative logarithm of Ka) should be close to the desired pH for optimal buffering capacity. Option D, with a pKa of 3.54 (since pKa = -log(2.9 × 10^-4)), is closest to the desired pH of 3.5, making it the best choice for a buffer at this pH.

Question 20

Which of the following is an exact or approximate value of the pH at the equivalence point of a titration of a strong acid with a strong base?

Accepted Answer

At the equivalence point of a titration of a strong acid with a strong base, the moles of acid and base are equal, meaning that the solution is neutral. Therefore, the pH at the equivalence point is exactly 7.0.

Quiz 17: Acid-Base Equilibria and Solubility Equilibria

What is the pH of a solution that consists of 0.50 M H₂C₆H₆O₆ (ascorbic acid) and 0.75 M NaHC₆H₆O₆ (sodium ascorbate) ? [K_a (H₂C₆H₆O₆) = 6.8 × 10^-5]

Which of the following is correct for the equivalence point in an acid-base titration?

A 50.00-mL solution of 0.10 M HNO₂ is titrated with a 0.10 M KOH solution.After 25.00 mL of the KOH solution is added,what is the pH in the titration flask? [K_a(HNO₂) = 4.5 × 10^-4]

A buffer is prepared by adding 1.00 L of 1.0 M HCl to 750 mL of 1.5 M NaHCOO.What is the pH of this buffer? [K_a (HCOOH) = 1.7 × 10^-4]

A buffer is prepared by adding 150 mL of 1.0 M NaOH to 250 mL of 1.0 M NaH₂PO₄.How many moles of HCl must be added to this buffer solution to change the pH by 0.18 units?

A buffer is prepared by adding 300.0 mL of 2.0 M NaOH to 500.0 mL of 2.0 M CH₃COOH.What is the pH of this buffer? [K_a(CH₃COOH) = 1.8 × 10^-5]

Which best describes the pH at the equivalence point of a titration of a weak acid with a strong base?

What is the [H₃O⁺] in a solution that consists of 1.5 M NH₃ and 2.5 M NH₄Cl? K_b = 1.8 × 10^-5

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

A 20.0-mL sample of 0.25 M HNO₃ is titrated with 0.15 M NaOH.What is the pH of the solution after 30.0 mL of NaOH have been added to the acid?

An acetic acid buffer containing 0.50 M CH₃COOH and 0.50 M CH₃COONa has a pH of 4.74.What will the pH be after 0.0020 mol of HCl has been added to 100.0 mL of the buffer?

What is the pH of a buffer that consists of 0.20 M Na H₂PO₄ and 0.40 M Na₂HPO₄? [K_a(Na H₂PO₄) = 6.2 × 10^-8]

Which is the best acid to prepare a buffer with the lowest pH?

A 50.0-mL sample of 0.50 M HCl is titrated with 0.50 M NaOH.What is the pH of the solution after 28.0 mL of NaOH have been added to the acid?

A 20.00-mL sample of 0.3000 M HBr is titrated with 0.15 M NaOH.What is the pH of the solution after 40.3 mL of NaOH have been added to the acid?

What is the pH of a buffer that consists of 0.45 M CH₃COOH and 0.35 M CH₃COONa? [K_a(CH₃COOH) = 1.8 × 10^-5]

What is the [H₃O⁺] in a buffer that consists of 0.30 M HCOOH and 0.20 M HCOONa? [K_a(HCOOH) = 1.7 × 10^-4]

Which best describes the pH at the equivalence point of a titration of a weak base with a strong acid?

Which is the best choice for preparing a buffer with a pH of 3.5?

Which of the following is an exact or approximate value of the pH at the equivalence point of a titration of a strong acid with a strong base?

Quiz 17: Acid-Base Equilibria and Solubility Equilibria

What is the pH of a solution that consists of 0.50 M H2C6H6O6 (ascorbic acid) and 0.75 M NaHC6H6O6 (sodium ascorbate) ? [Ka (H2C6H6O6) = 6.8 × 10-5]

Which of the following is correct for the equivalence point in an acid-base titration?

A 50.00-mL solution of 0.10 M HNO2 is titrated with a 0.10 M KOH solution.After 25.00 mL of the KOH solution is added,what is the pH in the titration flask? [Ka(HNO2) = 4.5 × 10-4]

A buffer is prepared by adding 1.00 L of 1.0 M HCl to 750 mL of 1.5 M NaHCOO.What is the pH of this buffer? [Ka (HCOOH) = 1.7 × 10-4]

A buffer is prepared by adding 150 mL of 1.0 M NaOH to 250 mL of 1.0 M NaH2PO4.How many moles of HCl must be added to this buffer solution to change the pH by 0.18 units?

A buffer is prepared by adding 300.0 mL of 2.0 M NaOH to 500.0 mL of 2.0 M CH3COOH.What is the pH of this buffer? [Ka(CH3COOH) = 1.8 × 10-5]

Which best describes the pH at the equivalence point of a titration of a weak acid with a strong base?

What is the [H3O+] in a solution that consists of 1.5 M NH3 and 2.5 M NH4Cl? Kb = 1.8 × 10-5

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

A 20.0-mL sample of 0.25 M HNO3 is titrated with 0.15 M NaOH.What is the pH of the solution after 30.0 mL of NaOH have been added to the acid?

An acetic acid buffer containing 0.50 M CH3COOH and 0.50 M CH3COONa has a pH of 4.74.What will the pH be after 0.0020 mol of HCl has been added to 100.0 mL of the buffer?

What is the pH of a buffer that consists of 0.20 M Na H2PO4 and 0.40 M Na2HPO4? [Ka(Na H2PO4) = 6.2 × 10-8]

Which is the best acid to prepare a buffer with the lowest pH?

A 50.0-mL sample of 0.50 M HCl is titrated with 0.50 M NaOH.What is the pH of the solution after 28.0 mL of NaOH have been added to the acid?

A 20.00-mL sample of 0.3000 M HBr is titrated with 0.15 M NaOH.What is the pH of the solution after 40.3 mL of NaOH have been added to the acid?

What is the pH of a buffer that consists of 0.45 M CH3COOH and 0.35 M CH3COONa? [Ka(CH3COOH) = 1.8 × 10-5]

What is the [H3O+] in a buffer that consists of 0.30 M HCOOH and 0.20 M HCOONa? [Ka(HCOOH) = 1.7 × 10-4]

Which best describes the pH at the equivalence point of a titration of a weak base with a strong acid?

Which is the best choice for preparing a buffer with a pH of 3.5?

Which of the following is an exact or approximate value of the pH at the equivalence point of a titration of a strong acid with a strong base?

What is the pH of a solution that consists of 0.50 M H₂C₆H₆O₆ (ascorbic acid) and 0.75 M NaHC₆H₆O₆ (sodium ascorbate) ? [K_a (H₂C₆H₆O₆) = 6.8 × 10^-5]

A 50.00-mL solution of 0.10 M HNO₂ is titrated with a 0.10 M KOH solution.After 25.00 mL of the KOH solution is added,what is the pH in the titration flask? [K_a(HNO₂) = 4.5 × 10^-4]

A buffer is prepared by adding 1.00 L of 1.0 M HCl to 750 mL of 1.5 M NaHCOO.What is the pH of this buffer? [K_a (HCOOH) = 1.7 × 10^-4]

A buffer is prepared by adding 150 mL of 1.0 M NaOH to 250 mL of 1.0 M NaH₂PO₄.How many moles of HCl must be added to this buffer solution to change the pH by 0.18 units?

A buffer is prepared by adding 300.0 mL of 2.0 M NaOH to 500.0 mL of 2.0 M CH₃COOH.What is the pH of this buffer? [K_a(CH₃COOH) = 1.8 × 10^-5]

What is the [H₃O⁺] in a solution that consists of 1.5 M NH₃ and 2.5 M NH₄Cl? K_b = 1.8 × 10^-5

A 20.0-mL sample of 0.25 M HNO₃ is titrated with 0.15 M NaOH.What is the pH of the solution after 30.0 mL of NaOH have been added to the acid?

An acetic acid buffer containing 0.50 M CH₃COOH and 0.50 M CH₃COONa has a pH of 4.74.What will the pH be after 0.0020 mol of HCl has been added to 100.0 mL of the buffer?

What is the pH of a buffer that consists of 0.20 M Na H₂PO₄ and 0.40 M Na₂HPO₄? [K_a(Na H₂PO₄) = 6.2 × 10^-8]

What is the pH of a buffer that consists of 0.45 M CH₃COOH and 0.35 M CH₃COONa? [K_a(CH₃COOH) = 1.8 × 10^-5]

What is the [H₃O⁺] in a buffer that consists of 0.30 M HCOOH and 0.20 M HCOONa? [K_a(HCOOH) = 1.7 × 10^-4]