Acids and Bases

Question 1

Bronsted definition of acid

Answer 1

Proton Donor

Question 2

Bronsted definition of base

Answer 2

Proton Acceptor

Question 3

Conjugate Base

Answer 3

Acid after losing proton

Question 4

Conjugate Acid

Answer 4

Base after gaining proton

Question 5

K_w

Answer 5

K_w = [H+][OH-] = 1 x 10^-14

Question 6

At standard conditions, pure water dissociates to achieve [H+] = ? and [OH-] = ?

Answer 6

[H+] = 10^-7 M and [OH-] = 10^-7 M.

Question 7

Definition of pH

Answer 7

pH = -[H+]

Question 8

Acidic / Basic / Neutral pH

Answer 8

Acidic pH < 7

Basic pH > 7

Neutral pH = 7

Question 9

Definition of pOH

Answer 9

pOH = -log[OH-]

Question 10

pH + pOH = ?

Answer 10

14

Question 11

Zwitter ions for amino acids

Question 12

Why do strong acids dissociate completely?

Answer 12

Complete dissociation occurs because the conjugate base anion is highly stable.

Question 13

Why do strong bases dissociate completely in solution?

Answer 13

Complete dissociation occurs because the conjugate acid cation is highly stable.

Question 14

Hydrolysis of weak acid salts produces ? and hydrolysis of weak base salts produces ?

Answer 14

Hydrolysis of weak acid produces conjugate base and hydrolysis of weak base produces conjugate acid

Question 15

Calculate pH of salt of weak acid

Answer 15

• Let’s say a solution contains M molar of CH₃COONa.
• CH₃COO^- + H₂O ↔ CH₃COOH + OH^-
• As M molar of CH₃COO^- start to abstract protons from the solvent:
  
  • [CH₃COO^-] = M - x
  • [CH₃COOH] = x
  • [OH^-] = x
• K_b = K_w/K_a = [CH₃COOH][OH^-] / [CH₃COO^-] = x²/(M - x)
• Because x is very small, K_w/K_a = x²/M → solve for x.
• pOH = -log[OH^-] = -log(x)
• pH = 14 - pOH.

Question 16

Calculate pH of salt of weak base

Answer 16

• Let’s say a solution contains M molar of NH₄Cl.
• NH₄⁺ ↔ NH₃ + H⁺.
• As M molar of NH₄⁺ dissociates:
  
  • [NH₄⁺] = M - x
  • [NH₃] = x
  • [H⁺] = x
• K_a = K_w/K_b = [NH₃][H⁺] / [NH₄⁺] = x²/(M - x)
• Because x is very small, K_w/K_b = x²/M → solve for x.
• pH = -log[H⁺] = -log(x).

Question 17

K_a

Answer 17

H-Acid ↔ H+ + Acid-

Question 18

K_b

Answer 18

Base + H2O ↔ H-Base+ + OH-

Question 19

K_a x K_b

Answer 19

K_a x K_b = K_w = 10^-14

Question 20

pKa related to acid strength

Answer 20

pK_a = -log(K_a). Lower pK_a values (higher K_a values) means stronger acid.

Question 21

pK_b

Answer 21

pK_b = -log(K_b)

Question 22

pK_a + pK_b = ?

Answer 22

14

Question 23

Definition of buffer

Answer 23

Solutions that resist changes to pH

Question 24

Buffer System Concept

Answer 24

• The concept is that acidic species of the buffer system will donate protons to resist increases in pH, while the basic species of the buffer system will accept protons to resist decreases in pH.
• Buffer systems formed by weak acids have maximum buffering capacity at the pH = pK_a of the acid.
• When [acid] = [conjugate base], the system is buffered at pH = pK_a of the acid.
• Buffer systems formed by weak bases have maximum buffering capacity at the pH = 14 - pK_b of the base.
• When [base] = [conjugate acid], the system is buffered at pH = 14 - pK_b of the base.

Question 25

Influence of buffers on titration curves

Answer 25

Buffers make the titration curve “flat” at the region where buffering occurs.

Question 26

Indicators

Answer 26

• H-In ↔ H⁺ + In^-
• K_a = [H⁺][In^-] / [H-In]
• Indicators behave just like weak acids/bases.
• The indicator is present in such a small amount that it doesn’t affect the solution’s pH.
• When the solution has a low pH (high [H⁺]), the indicator is mostly in the H-In form, which is of one color.
• When the solution has a high pH (low [H⁺]), the indicator is mostly in the In^- form, which is of another color.

Question 27

Neutralization

Answer 27

Acid + Base = Salt + Water.

Question 28

Adding NaOH to Acetic Acid titration curve

Question 29

Adding HCl to NH₃titration curve

Question 30

Adding NaOH to H₂CO₃

Question 31

For a titration curve of H₂CO₃, what is the dominant species at each equivalence point?

Answer 31

• pK_a1: [H₂CO₃] = [HCO₃^-]
• Equivalence point 1: almost everything is HCO₃^-
• pK_a2: [HCO₃^-] = [CO₃^2-]
• Equivalence point 2: almost everything is CO₃^2-