Question 1

Which of the following will give a buffer with a pH near 9.24 when the acid and conjugate base are mixed in equimolar proportions? Acid K_a NH₄⁺ (from ammonia) 5)69 × 10^-10 HC₂H₃O₂ (acetic acid) 1)75 × 10^-5 HF (hydrofluoric acid) 6)8 × 10^-4 CH₃CH₂NH₃⁺ (from ethylamine) 2)12 × 10^-11

Accepted Answer

The pKa of NH4+ is approximately 9.24, which matches the desired pH for the buffer.

Question 2

What is the hydronium-ion concentration of a solution formed by combining 400.mL of 0.21 M HNO₃ with 600.mL of 0.10 M NaOH at 25°C?

Accepted Answer

The answer of What is the hydronium-ion concentration of a...

Question 3

What is the hydronium-ion concentration of a buffer solution that is 0.45 M Na₂HPO₄ and 0.30 M NaH₂PO₄? For H₃PO₄,Ka₁ = 6.9 × 10^-3,Ka₂ = 6.2 × 10^-8,and Ka₃ = 4.8 × 10^-13.

Accepted Answer

The answer of What is the hydronium-ion concentration of a...

Question 4

What molar ratio of acetic acid to sodium acetate is required to create a buffer solution having a pH of 4.93 at 25°C? K_a for HC₂H₃O₂ is 1.8 × 10^-5.

Accepted Answer

The Henderson-Hasselbalch equation is used to calculate the pH of a buffer solution: pH = pKa + log([A-]/[HA]). Given pH = 4.93 and pKa = -log(1.8 × 10^-5) = 4.74, solving for the ratio [A-]/[HA] gives approximately 0.65.

Question 5

Which of the following mixtures will be a buffer when dissolved in 1 L of water?

Accepted Answer

A buffer solution requires a weak acid and its conjugate base or a weak base and its conjugate acid. Option B includes the weak acid HC₂H₃ and its conjugate base HC₂S- in the ratio of 2:1, making it a buffer solution. The other options either have only strong acids and bases or only one weak acid or base, which cannot make a buffer solution.

Question 6

What is the hydronium-ion concentration in a solution resulting from mixing 133 mL of 0.100 M HCN and 80 mL of 0.100 M KOH at 25°C? K_a for HCN = 4.9 × 10^-10 at 25°C.

Accepted Answer

The balanced chemical equation for the reaction between HCN and KOH is:

HCN + KOH → H2O + KCN

From the equation, we can see that the same number of moles of HCN and KOH react to form water and KCN.

First, we need to calculate the number of moles of HCN and KOH in the solution:

moles HCN = 0.133 L * 0.100 mol/L = 0.0133 mol
moles KOH = 0.080 L * 0.100 mol/L = 0.0080 mol

Since the same number of moles of HCN and KOH react, the limiting reactant is KOH (it has fewer moles). Thus, all the KOH will be consumed by the reaction, and 0.0080 mol of HCN will also react.

The initial concentration of HCN was 0.100 M, but only 0.0080 mol of HCN remained after the reaction, so the concentration of HCN in the final solution is:

[HCN] = 0.0080 mol / (0.133 L + 0.080 L) = 3.2 × 10^-2 M

Next, we need to calculate the concentration of hydronium ions (H3O+) in the solution. The reaction between HCN and KOH produces CN- (cyanide) ions, which can react with water to form HCN and OH- (hydroxide) ions through a reversible reaction:

CN- + H2O ⇌ HCN + OH-

The equilibrium constant for this reaction is Kw/Ka, where Kw is the ion product constant of water (1.0 × 10^-14 at 25°C) and Ka is the acid dissociation constant of HCN (4.9 × 10^-10 at 25°C). Thus:

Kw/Ka = (1.0 × 10^-14) / (4.9 × 10^-10) = 2.04 × 10^-5

At equilibrium, the product of the concentrations of HCN and OH- ions is equal to this constant:

[HCN][OH-] = Kw/Ka = 2.04 × 10^-5

We know that the initial concentration of OH- ions is zero, and that some HCN has reacted to form OH- ions. So we can use the equilibrium expression to calculate the concentration of H3O+ ions (since [H3O+][OH-] = Kw):

[H3O+] = Kw / [OH-] = Kw / (Kw/Ka/[HCN]) = Ka/[HCN] = 4.9 × 10^-10 / 3.2 × 10^-2 = 1.5 × 10^-8 M

Therefore, the hydronium-ion concentration in the solution is 1.5 × 10^-8 M.

The best choice is D because it correctly calculates the concentration of hydronium ions in the solution.

Question 7

What is the pH of a solution that is 0.048 M in HA and also 0.0032 M in NaA? (K_a = 4.1 × 10^-6)

Accepted Answer

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

Question 8

Which of the following is the most effective buffer system for a pH value of 4.45?

Accepted Answer

The answer of Which of the following is the most...

Question 9

What is the hydrogen-ion concentration of a solution that is 0.052 M in acetic acid and 0.052 M in sodium acetate at 25°C? The acid-ionization constant of acetic acid is 1.8 × 10^-5 at 25°C.

Accepted Answer

The solution is a buffer, and the hydrogen-ion concentration can be calculated using the Henderson-Hasselbalch equation. Since the concentrations of acetic acid and sodium acetate are equal, the pH is equal to the pKa, which is -log(1.8 × 10^-5) = 4.74. Therefore, the hydrogen-ion concentration is 10^-4.74 ≈ 1.8 × 10^-5 M.

Question 10

What is the pH of a buffer made by combining 181 mL of 0.34 M NaC₂H₃O₂ with 170 mL of 0.28 M HC₂H₃O₂? The K_a of acetic acid is 1.75 × 10^-5.

Accepted Answer

The pH of the buffer can be calculated using the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]), where [A-] is the concentration of the conjugate base and [HA] is the concentration of the acid. First, calculate the moles of NaCH3COO and HCH3COO: Moles of NaCH3COO = 0.34 M * 0.181 L = 0.06154 mol Moles of HCH3COO = 0.28 M * 0.170 L = 0.0476 mol The total volume of the solution = 181 mL + 170 mL = 351 mL = 0.351 L The concentration of NaCH3COO ([A-]) = 0.06154 mol / 0.351 L = 0.175 M The concentration of HCH3COO ([HA]) = 0.0476 mol / 0.351 L = 0.136 M pKa = -log(1.75 × 10^-5) = 4.76 pH = 4.76 + log(0.175/0.136) = 4.76 + log(1.287) = 4.76 + 0.11 = 4.87, which rounds to 4.88.

Question 11

What is the pH of a solution that is 0.14 M in acetic acid,HC₂H₃O₂,and 0.50 M in sodium acetate,NaC₂H₃O₂,at 25°C? K_a of acetic acid is 1.8 × 10^-5.

Accepted Answer

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

Question 12

A certain weak base B has a base-ionization constant K_b of 6.6 × 10^-4 at 25°C.If strong acid is added to a solution of B,at what pH will [B] = [BH⁺]?

Accepted Answer

The equation for the ionization of the weak base B is:

B + H2O ↔ BH+ + OH-

The equilibrium constant for this reaction is the base-ionization constant, which has a value of 6.6 × 10-4 at 25°C. We can use this equilibrium constant to find the pH at which [B] = [BH+].

Let x be the concentration of [BH+] (and [OH-] by conservation of charge), then the concentration of [B] is also x. The concentration of [H+] is initially zero (as we are starting with a weak base in water), but as we add strong acid, the concentration of [H+] will increase. Let [H+] be y.

The equation for the reaction between strong acid and the weak base is:

H+ + B ↔ BH+

The equilibrium constant for this reaction is:

Ka = [BH+][H+] / [B]

We can write the acid dissociation constant for the strong acid as:

Ka' = [H+][A-] / [HA]

where [A-] and [HA] are the concentrations of the conjugate base and acid, respectively.

Since we are adding strong acid, we can assume that initially [A-] = 0 and [HA] = y (the concentration of the strong acid added). Therefore:

Ka' = [H+]y / y = [H+]

Substituting this into the expression for Ka gives:

Ka = x( [H+] / 6.6 × 10-4 ) = [H+] ( x / 6.6 × 10-4 )

At equilibrium, we have:

[H+] + [OH-] = Kw / [H+] = 1.0 × 10-14 / [H+]

where Kw is the ion-product constant of water.

Substituting this and x = [BH+] = [B] into the expression for Ka gives:

[H+] = 6.6 × 10-4 / x

Substituting this and the equation for [OH-] into the equation for Kw:

6.6 × 10-4 / x × x / (1.0 × 10-14 / 6.6 × 10-4 x) = 1.0 × 10-14

x2 = Kw / Ka = 1.52 × 10-11

x = [BH+] = [B] = sqrt(1.52 × 10-11) = 3.9 × 10-6

Therefore, pOH = -log[OH-] = -log(3.9 × 10-6) = 5.4

pH = pKw - pOH = 14.0 - 5.4 = 8.6

So the best choice is B (rounded to one decimal place).

Question 13

What mass of sodium hydroxide must be added to 60.0 mL of 0.978 M acetic acid in order to create a buffer with a pH of 4.59? K_a for acetic acid is 1.8 × 10^-5.

Accepted Answer

To create a buffer with a specific pH using acetic acid and sodium hydroxide, we use the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]), where pKa = -log(Ka), [A-] is the concentration of the acetate ion, and [HA] is the concentration of acetic acid. Given that the Ka for acetic acid is 1.8 × 10^-5, pKa = -log(1.8 × 10^-5) ≈ 4.74. The desired pH is 4.59, so we rearrange the equation to solve for the ratio [A-]/[HA]: 4.59 = 4.74 + log([A-]/[HA]), which gives log([A-]/[HA]) = -0.15, and thus [A-]/[HA] ≈ 0.71. Since the initial concentration of acetic acid is 0.978 M, and we want the ratio of [A-] to [HA] to be 0.71 after adding NaOH, we can set up the equation 0.978x / (0.978 - 0.978x) = 0.71, where x is the fraction of acetic acid that has been converted to acetate. Solving for x gives x ≈ 0.415. Therefore, 0.415 * 0.978 M = 0.406 M of acetic acid is converted to acetate. The volume of the solution is 60.0 mL = 0.060 L, so the moles of acetate produced are 0.406 mol/L * 0.060 L = 0.02436 mol. Since the molar mass of NaOH is approximately 40 g/mol, the mass of NaOH needed is 0.02436 mol * 40 g/mol = 0.9744 g, which rounds to 0.96 g.

Question 14

A weak acid,HF,is in solution with dissolved sodium fluoride,NaF.If HCl is added,which ion will react with the extra hydrogen ions from the HCl to keep the pH from changing?

Accepted Answer

When HCl is added, it will donate extra hydrogen ions (H+) to the solution. 
As HF is a weak acid, it will only partially dissociate into H+ and F-. 
Therefore, the added H+ ions will react with the F- ions from the dissolved NaF to form HF, which will help maintain the pH of the solution. 
So, the ion that will react with the extra hydrogen ions to keep the pH from changing is F- from NaF, making option D the correct answer.

Question 15

Which of the following solutions will not yield an effective NaH₂PO₄/Na₂HPO₄ buffer?

Accepted Answer

The answer of Which of the following solutions will not...

Question 16

Which of the following would be the best mole ratio of conjugate acid to conjugate base for a large buffer capacity?

Accepted Answer

A 1:1 ratio of conjugate acid to conjugate base provides the largest buffer capacity because it allows the solution to effectively neutralize both added acids and bases.

Question 17

Suppose a buffer solution is made from formic acid (HCHO₂)and sodium formate (NaCHO₂).What is the net ionic equation for the reaction that occurs when a small amount of sodium hydroxide is added to the buffer?

Accepted Answer

The net ionic equation for the reaction of sodium hydroxide with formic acid in a buffer solution is the reaction between the hydroxide ion (OH-) and formic acid (HCOOH), producing the formate ion (HCOO-) and water (H2O). This reflects the action of the buffer in neutralizing added base.

Question 18

Suppose a buffer solution is made from formic acid (HCHO₂)and sodium formate (NaCHO₂).What is the net ionic equation for the reaction that occurs when a small amount of hydrochloric acid is added to the buffer?

Accepted Answer

The answer of Suppose a buffer solution is made from...

Question 19

Which of the following solutions would show the greatest change in pH upon the addition of 10.0 mL of 1.0 M NaOH to 1.0 L of the solution?

Accepted Answer

The answer of Which of the following solutions would show...

Question 20

Which of the following mixtures will be a buffer when dissolved in a liter of water?

Accepted Answer

A buffer solution must contain a weak acid and its conjugate base or a weak base and its conjugate acid. In option D, H₃- is a weak acid and can act as a buffer with its conjugate base ₃PO₄-. NaOH can act as a base and can be used to adjust the pH of the buffer solution. None of the other options contain an appropriate weak acid/conjugate base or weak base/conjugate acid pair.

Quiz 16: Acid-Base Equilibria

What is the hydronium-ion concentration of a solution formed by combining 400.mL of 0.21 M HNO₃ with 600.mL of 0.10 M NaOH at 25°C?

What is the hydronium-ion concentration of a buffer solution that is 0.45 M Na₂HPO₄ and 0.30 M NaH₂PO₄? For H₃PO₄,Ka₁ = 6.9 × 10^-3,Ka₂ = 6.2 × 10^-8,and Ka₃ = 4.8 × 10^-13.

What molar ratio of acetic acid to sodium acetate is required to create a buffer solution having a pH of 4.93 at 25°C? K_a for HC₂H₃O₂ is 1.8 × 10^-5.

Which of the following mixtures will be a buffer when dissolved in 1 L of water?

What is the hydronium-ion concentration in a solution resulting from mixing 133 mL of 0.100 M HCN and 80 mL of 0.100 M KOH at 25°C? K_a for HCN = 4.9 × 10^-10 at 25°C.

What is the pH of a solution that is 0.048 M in HA and also 0.0032 M in NaA? (K_a = 4.1 × 10^-6)

Which of the following is the most effective buffer system for a pH value of 4.45?

What is the hydrogen-ion concentration of a solution that is 0.052 M in acetic acid and 0.052 M in sodium acetate at 25°C? The acid-ionization constant of acetic acid is 1.8 × 10^-5 at 25°C.

What is the pH of a buffer made by combining 181 mL of 0.34 M NaC₂H₃O₂ with 170 mL of 0.28 M HC₂H₃O₂? The K_a of acetic acid is 1.75 × 10^-5.

What is the pH of a solution that is 0.14 M in acetic acid,HC₂H₃O₂,and 0.50 M in sodium acetate,NaC₂H₃O₂,at 25°C? K_a of acetic acid is 1.8 × 10^-5.

A certain weak base B has a base-ionization constant K_b of 6.6 × 10^-4 at 25°C.If strong acid is added to a solution of B,at what pH will [B] = [BH⁺]?

What mass of sodium hydroxide must be added to 60.0 mL of 0.978 M acetic acid in order to create a buffer with a pH of 4.59? K_a for acetic acid is 1.8 × 10^-5.

A weak acid,HF,is in solution with dissolved sodium fluoride,NaF.If HCl is added,which ion will react with the extra hydrogen ions from the HCl to keep the pH from changing?

Which of the following solutions will not yield an effective NaH₂PO₄/Na₂HPO₄ buffer?

Which of the following would be the best mole ratio of conjugate acid to conjugate base for a large buffer capacity?

Suppose a buffer solution is made from formic acid (HCHO₂) and sodium formate (NaCHO₂) .What is the net ionic equation for the reaction that occurs when a small amount of sodium hydroxide is added to the buffer?

Suppose a buffer solution is made from formic acid (HCHO₂) and sodium formate (NaCHO₂) .What is the net ionic equation for the reaction that occurs when a small amount of hydrochloric acid is added to the buffer?

Which of the following solutions would show the greatest change in pH upon the addition of 10.0 mL of 1.0 M NaOH to 1.0 L of the solution?

Which of the following mixtures will be a buffer when dissolved in a liter of water?

Quiz 16: Acid-Base Equilibria

Which of the following will give a buffer with a pH near 9.24 when the acid and conjugate base are mixed in equimolar proportions? Acid Ka NH4+ (from ammonia) 5) 69 × 10-10 HC2H3O2 (acetic acid) 1) 75 × 10-5 HF (hydrofluoric acid) 6) 8 × 10-4 CH3CH2NH3+ (from ethylamine) 2) 12 × 10-11

What is the hydronium-ion concentration of a solution formed by combining 400.mL of 0.21 M HNO3 with 600.mL of 0.10 M NaOH at 25°C?

What is the hydronium-ion concentration of a buffer solution that is 0.45 M Na2HPO4 and 0.30 M NaH2PO4? For H3PO4,Ka1 = 6.9 × 10-3,Ka2 = 6.2 × 10-8,and Ka3 = 4.8 × 10-13.

What molar ratio of acetic acid to sodium acetate is required to create a buffer solution having a pH of 4.93 at 25°C? Ka for HC2H3O2 is 1.8 × 10-5.

Which of the following mixtures will be a buffer when dissolved in 1 L of water?

What is the hydronium-ion concentration in a solution resulting from mixing 133 mL of 0.100 M HCN and 80 mL of 0.100 M KOH at 25°C? Ka for HCN = 4.9 × 10-10 at 25°C.

What is the pH of a solution that is 0.048 M in HA and also 0.0032 M in NaA? (Ka = 4.1 × 10-6)

Which of the following is the most effective buffer system for a pH value of 4.45?

What is the hydrogen-ion concentration of a solution that is 0.052 M in acetic acid and 0.052 M in sodium acetate at 25°C? The acid-ionization constant of acetic acid is 1.8 × 10-5 at 25°C.

What is the pH of a buffer made by combining 181 mL of 0.34 M NaC2H3O2 with 170 mL of 0.28 M HC2H3O2? The Ka of acetic acid is 1.75 × 10-5.

What is the pH of a solution that is 0.14 M in acetic acid,HC2H3O2,and 0.50 M in sodium acetate,NaC2H3O2,at 25°C? Ka of acetic acid is 1.8 × 10-5.

A certain weak base B has a base-ionization constant Kb of 6.6 × 10-4 at 25°C.If strong acid is added to a solution of B,at what pH will [B] = [BH+]?

What mass of sodium hydroxide must be added to 60.0 mL of 0.978 M acetic acid in order to create a buffer with a pH of 4.59? Ka for acetic acid is 1.8 × 10-5.

A weak acid,HF,is in solution with dissolved sodium fluoride,NaF.If HCl is added,which ion will react with the extra hydrogen ions from the HCl to keep the pH from changing?

Which of the following solutions will not yield an effective NaH2PO4/Na2HPO4 buffer?

Which of the following would be the best mole ratio of conjugate acid to conjugate base for a large buffer capacity?

Suppose a buffer solution is made from formic acid (HCHO2) and sodium formate (NaCHO2) .What is the net ionic equation for the reaction that occurs when a small amount of sodium hydroxide is added to the buffer?

Suppose a buffer solution is made from formic acid (HCHO2) and sodium formate (NaCHO2) .What is the net ionic equation for the reaction that occurs when a small amount of hydrochloric acid is added to the buffer?

Which of the following solutions would show the greatest change in pH upon the addition of 10.0 mL of 1.0 M NaOH to 1.0 L of the solution?

Which of the following mixtures will be a buffer when dissolved in a liter of water?

What is the hydronium-ion concentration of a solution formed by combining 400.mL of 0.21 M HNO₃ with 600.mL of 0.10 M NaOH at 25°C?

What is the hydronium-ion concentration of a buffer solution that is 0.45 M Na₂HPO₄ and 0.30 M NaH₂PO₄? For H₃PO₄,Ka₁ = 6.9 × 10^-3,Ka₂ = 6.2 × 10^-8,and Ka₃ = 4.8 × 10^-13.

What molar ratio of acetic acid to sodium acetate is required to create a buffer solution having a pH of 4.93 at 25°C? K_a for HC₂H₃O₂ is 1.8 × 10^-5.

What is the hydronium-ion concentration in a solution resulting from mixing 133 mL of 0.100 M HCN and 80 mL of 0.100 M KOH at 25°C? K_a for HCN = 4.9 × 10^-10 at 25°C.

What is the pH of a solution that is 0.048 M in HA and also 0.0032 M in NaA? (K_a = 4.1 × 10^-6)

What is the hydrogen-ion concentration of a solution that is 0.052 M in acetic acid and 0.052 M in sodium acetate at 25°C? The acid-ionization constant of acetic acid is 1.8 × 10^-5 at 25°C.

What is the pH of a buffer made by combining 181 mL of 0.34 M NaC₂H₃O₂ with 170 mL of 0.28 M HC₂H₃O₂? The K_a of acetic acid is 1.75 × 10^-5.

What is the pH of a solution that is 0.14 M in acetic acid,HC₂H₃O₂,and 0.50 M in sodium acetate,NaC₂H₃O₂,at 25°C? K_a of acetic acid is 1.8 × 10^-5.

A certain weak base B has a base-ionization constant K_b of 6.6 × 10^-4 at 25°C.If strong acid is added to a solution of B,at what pH will [B] = [BH⁺]?

What mass of sodium hydroxide must be added to 60.0 mL of 0.978 M acetic acid in order to create a buffer with a pH of 4.59? K_a for acetic acid is 1.8 × 10^-5.

Which of the following solutions will not yield an effective NaH₂PO₄/Na₂HPO₄ buffer?

Suppose a buffer solution is made from formic acid (HCHO₂) and sodium formate (NaCHO₂) .What is the net ionic equation for the reaction that occurs when a small amount of sodium hydroxide is added to the buffer?

Suppose a buffer solution is made from formic acid (HCHO₂) and sodium formate (NaCHO₂) .What is the net ionic equation for the reaction that occurs when a small amount of hydrochloric acid is added to the buffer?