CH 223 Midterm 2 Review - Spring 2023

BufferSolutions

100

Which of the following solutions would make the best buffer system?

a. 0.10 M NaCl and 0.10 M HCl

b. 1.0 M CH₃CH₂COOH and 1.0 M NaCH₃CH₂COO

c. 0.10 M NaOH and 0.10 HCl

d. 1.0 M HNO₃ and 1.0 NaNO₃

e. 1.0 M HCN and 1.0 M NaCl

b. 1.0 M CH₃CH₂COOH and 1.0 M NaCH₃CH₂COO

200

What is the pKa for the acid HA if a solution of 0.65 M HA and 0.85 M NaA has a pH of 4.75?

pK_a = 4.63

300

Which of the following solutions has the greatest buffering capacity?

a. 0.528 M CH₃COOH and 0.197 M NaCH₃COO

b. 0.365 M CH₃COOH and 0.527 M NaCH₃COO

c. 0.817 M CH₃COOH and 0.862 M NaCH₃COO

d. 0.124 M CH₃COOH and 0.131 M NaCH₃COO

c. 0.817 M CH₃COOH and 0.862 M NaCH₃COO

400

The pH of blood is 7.35. It is maintained in part by the buffer system composed of carbonic acid (H₂CO₃) and the bicarbonate (HCO₃^-) ion. What is the ratio of [bicarbonate]/[carbonic acid] at this pH? The K_a of carbonic acid is 4.2 x 10^-7.

[A^-]/[HA] = 9.397

500

Buffer solutions with the component concentrations shown below were prepared. Which of them should have the lowest pH?

a. [CH₃COOH] = 0.25 M, [CH₃COO^-] = 0.25 M            

b. [CH₃COOH] = 0.75 M, [CH₃COO^-] = 0.75 M            

c. [CH₃COOH] = 0.75 M, [CH₃COO^-] = 0.25 M            

d. [CH₃COOH] = 0.25 M, [CH₃COO^-] = 0.75 M

e. [CH₃COOH] = 1.00 M, [CH₃COO^-] = 1.00 M

c. [CH₃COOH] = 0.75 M, [CH₃COO^-] = 0.25 M

Titration -Conceptual

100

What determines pH after the equivalence point of a weak base/strong acid titration?

The concentration of excess strong acid determines pH after the equivalence point.

200

What is the equivalence point of a titration?

Equivalence point: when the amount of titrant added is exactly enough to fully neutralize the analyte.

300

If the pH is >7 at the equivalence point, what is true about the acid/base character of the analyte?

It is a weak acid.

400

What are the four main strategies to solve for pH over the four main areas of interest of a titration?

Starting point: ICE table for dissociation of weak acid or base

Buffer region: calculate change in conjugate pair concentrations --> Henderson-Hasselbalch

Equivalence point: ICE table for dissociation of conjugate acid or base

Overshoot region: calculate concentration of excess titrant --> -log for pH

500

At what point in a titration does pH = pK_a, and why is this true?

pH = pK_a at the half equivalence point, because half of the analyte has been converted to its conjugate form and the ratio inside the log of the Henderson-Hasselbalch equation reduces to 1, making the log equal to 0 (pH = pK_a + log(1) --> pH = pK_a).

Titration -Computational

100

What is the pH of a 2.0 L solution of 0.30 M HCN (K_a = 6.2 x 10^-10) after addition of 722.0 mL 0.90 M NaOH?

pH = 12.26

200

What is the pH when 20.0 mL of 0.20 M HCl is added to 60.0 mL of 0.133 M hydrazine (N₂H₄, K_b = 1.3 x 10^-6)?

pH = 8.11

300

400.0 mL of 0.640 M formic acid (CH₂O₂, K_a = 1.8 x 10^-4) is titrated with 0.10 M NaOH. What is the pH of the solution after 310.0 mL of base has been added?

pH = 2.88

400

A 50.0 mL sample of an aqueous HCO₃ solution is titrated with 0.345 M NaOH. The equivalence point is reached with 83.5 mL of base. What was the initial concentration of the HCO₃?

[HCO₃]_initial = 0.576 M

500

Calculate the pH of the solution resulting from the addition of 381.0 mL 0.90 M NaOH to 600.0 mL of a 0.340 M H₂CO₃ solution_.

pH = 13.15

Misc. Subjects

100

If HF is dissolved in water such that the concentration of HF is 0.3M, what is the pH of the solution?

K_a HF = 7.2 x 10^-4

pH = 1.83

200

Three moles of NH₃ is dissolved in 1.6 L of water, what is the pH of the solution?

K_b NH₃ = 1.8 x 10^-5

11.8

300

If deltaS_univ is negative, is the forward reaction associated with it spontaneous or nonspontaneous?

Nonspontaneous

400

What is the change in entropy of a system if 341J of heat flows into a system at 185K?

1.84 J/K

500

In a solution of NH₃ and NH₄Cl, what is the reaction when acid is added and when a base is added?

NH₄₊ + OH^- ⇄  NH₃ + H₂O

NH₃ + H₃O⁺ ⇄ NH₄⁺ + H₂O

Entropy

100

The second law of thermodynamics tells us…

a. the entropy of the universe is constant

b. entropy is neither created nor destroyed

c. the universe proceeds toward a state of lower entropy

d. the universe proceeds toward a state of higher entropy

e. the entropy of the universe may increase or decrease

d. the universe proceeds toward a state of higher entropy

200

Which of the following is true?

a. the reverse of a spontaneous reaction is also spontaneous

b. a non-spontaneous process can be caused to occur

c. a highly spontaneous process always occurs rapidly

d. a spontaneous process moves a system out of equilibrium

e. a spontaneous process always occurs without any external influence

b. a non-spontaneous process can be caused to occur

300

Under which of the following conditions would one mole of argon (Ar) have the lowest entropy, S?

a. 14 ᵒC and 5 L

b. 87 ᵒC and 15 L

c. 87 ᵒC and 5 L

d. 14 ᵒC and 15 L

a. 14 ᵒC and 5 L

400

In which of the following pairs will the entropy of the first substance be greater than that of the second?

a. 1 mole of F₂(g); 1 mole of Cl₂(g)

b. 1 mole of I₂(s); 1 mole of I₂(g)

c. 1 mole of CaCO₃(s); 1 mole of CaO(s) plus 1 mole of CO₂(g)

d. 1 mole of H₂(g) at 25ᵒ C; 1 mole of H₂(g) at 50ᵒ C

e. 1 mole of O₃(g); 1 mole of O₂(g)

e. 1 mole of O₃(g); 1 mole of O₂(g)

500

Calculate ΔSᵒ for the reaction
SiCl₄(g) + 2Mg(s) <---> 2MgCl₂(s) + Si(s)

Substance     ||     Sᵒ (J/K*mol)

SiCl₄(g)     ||     330.73

Mg(s)     ||     32.68

MgCl₂(s)     ||     89.62

Si(s)     ||     18.83

ΔSᵒ = -198.02 J/K