Lesson 7: Acids and Bases

Question 1

Before the more broad definitions of acids and bases were developed, Arrhenius created the simplest definition of acids and bases. Explain what Arrhenius acids and bases are, and how they are related to water.

Answer 1

Arrhenius acids are compounds that will, in an aqueous solution, ionize to produce Hydrogen ions (H+). Arrhenius bases, likewise, will ionize to form hydroxide ions (OH-).

Question 2

Define a Bronsted-Lowry base vs. Bronsted-Lowry acid?

Define a Lewis base vs. Lewis acid?

Answer 2

A Bronsted-Lowry base is a proton acceptor, and a Bronsted-Lowry acid is a proton donator.

A Lewis base is an electron donator, and a Lewis acid is an electron acceptor.

Question 3

Which of the following compounds can behave as a Bronsted-Lowry acid but not a Lewis acid?

(A) I2
(B) H2O
(C) HF
(D) None of the above

Answer 3

(C) HF

The answer must have a Hydrogen to donate, and cannot be able to accept any electron pairs. This is only represented by HF.

Question 4

The acid ionization constant is also known as the:

(A) Acid association constant
(B) Acid dissociation constant
(C) Acid solubility constant
(D) Acid precipitation constant

Answer 4

(B) Acid dissociation constant

The acid ionization constant is also known as the acid dissociation constant (Ka).

Question 5

Write the acid dissociation constant expression for the following reaction: HA H+ + A-

Answer 5

Question 6

Which is a stronger base, H2O or Cl-, in the following reaction: H2O + HCl H3O+ + Cl-

Answer 6

H2O is a stronger base than Cl- because Cl- is the conjugate base of a strong acid, making it a very weak base.

Question 7

True or false? The larger the Ka value, the weaker the acid.

Answer 7

False. The larger the Ka value, the STRONGER the acid.

Question 8

What does it mean to say that water is amphoteric?

Answer 8

To say that water is amphoteric means that water can act as both an acid and a base.

Question 9

Two identical amphoteric compounds can react with each other to form an acid and a base. What is this process called?

(A) Ionization
(B) Propionation
(C) Single-displacement reaction
(D) Autoionization

Answer 9

(D) Autoionization

Autoionization is where two identical molecules react with one another to form an acid and base.

Question 10

What is the relationship between Ka and Kw?

Answer 10

Kw (the autoionization constant of water) is the Ka (acid dissociation constant) for water.

Question 11

What is the value of Kw?

Answer 11

1 ⋅ 10^-14

Question 12

The concentration of [H3O+] in a solution is 4.30 ⋅ 10^-4 M. What is the concentration of [OH-] in this solution?

(A) 4.30 ⋅ 10^-4 M
(B) 8.76 ⋅ 10^-5 M
(C) 1.22 ⋅ 10^-10 M
(D) 2.33 ⋅ 10^-11 M

Answer 12

(D) 2.33 ⋅ 10^-11 M

Kw = [H3O+][OH-]
1 ⋅ 10^-14 = (4.3 ⋅ 10^-4)[OH-]
(1 ⋅ 10^-14)/(4.3 ⋅ 10^-4) =
[OH-]
[OH-] = approx. .25 ⋅ 10^-10 -> 2.5 ⋅ 10^-11 (actual: 2.33 ⋅ 10^-11)

Question 13

What is the pH when you have a [OH-] concentration equal to 9.84⋅10^-8 M?

(A) 8.67
(B) 7.92
(C) 6.99
(D) 5.43

Answer 13

(C) 6.99

Kw = [H3O+][OH-]
1⋅10^-14 = [H3O+](9.84⋅10^-8)
(1⋅10^-14)/(9.84⋅10^-8) = [H3O+]
[H3O+] = approx. .1⋅10^-6 -\> 1⋅10^-7 (actual: 1.02⋅10^-7)
pH = -log( 1⋅10^-7)

Question 14

Name the 7 Strong Acids

Answer 14

Question 15

During digestion, the contents of the stomach are near a pH of 2, but in the duodenum the pH is raised to about 6. How many times more acidic (Higher [H+]) is the chyme in the stomach than the chyme in the duodenum?

(A) 4
(B) 10,000
(C) 100
(D) 40

Answer 15

(B) 10,000

Because there is a pH difference of 4, the difference in [H+] is a factor of 10^4, which is equal to 10,000.

Question 16

Calculate the pH of a 4.56 ⋅ 10^-3 M solution of HClO4?

(A) 3.46
(B) 3.03
(C) 2.34
(D) 1.95

Answer 16

(C) 2.34

HClO4 is a strong acid, so treat its concentration as [H+]

[H3O+] = 4.56 ⋅ 10^-3

pH = -log(4.56 ⋅ 10^-3)
pH = approx. 2.5 (actual: 2.34)

Question 17

Strong bases are formed from which two groups of the periodic table mixed with OH?

Answer 17

Groups 1A and 2A

Question 18

What is pKw in terms of pH and pOH?

Answer 18

pKw (14) = pOH + pH

Question 19

Calculate the pH of a 7.45 ⋅ 10^-6 M solution of Ba(OH)2?

(A) 9.17
(B) 7.34
(C) 4.83
(D) 3.98

Answer 19

(A) 9.17

[OH-] = 7.45 ⋅ 10^-6 x 2
[OH-] = 14.9 ⋅ 10^-6 OH-] = 1.49 ⋅ 10^-5

pOH = -log([OH-])
pOH = -log(1.49 ⋅ 10^-5)
pOH = approx. 5 (actual: 4.83)

pKw = pH + pOH
14 = pH + 5
pH = 9 (actual: 9.17)

Question 20

Which of the following is the strongest base?

(A) F-
(B) CH3O-
(C) CH3COO-
(D) NO3-

Answer 20

(B) CH3O-

CH3OH is a very weak acid, so its conjugate base will be extremely strong.

Question 21

What is the pKa of acetic acid (Ka = 1.8 ⋅ 10^-5)?

(A) 5.54
(B) 5.13
(C) 4.74
(D) 4.13

Answer 21

(C) 4.74

pKa = -log(Ka)
pKa = -log(1.8 ⋅ 10^-5)
pKa = approx. 4.82 (actual 4.74)

Question 22

What is the pH of a 4.65 ⋅ 10^-4 M solution of acetic acid (Ka = 1.8 ⋅ 10^-5)?

(A) 7.86
(B) 6.02
(C) 4.89
(D) 4.08

Answer 22

(D) 4.08

Ka = ([CH3COO-][H3O+])/[CH3COOH]
1.8 ⋅ 10^-5 = x^2/4.65 ⋅ 10^-4 - x (but x is so little that we can disregard it), making our equation:
1.8 ⋅ 10^-5 = x^2/4.65 ⋅ 10^-4
(1.8 ⋅ 10^-5)(4.65 ⋅ 10^-4) = x^2
approx. 90 ⋅ 10^-10 = x^2
approx. 1 ⋅ 10^-4 = x
pH = -log(1 ⋅ 10^-4)
pH = approx. 4 (actual: 4.08)

Question 23

In terms of acids and bases, define a neutralization reaction.

Answer 23

A neutralization reaction is where an acid and a base react to form a salt. Oftentimes, water is also a product of these reactions.

Question 24

For a polyvalent or polyprotic acid or base, the same molecule can act as an acid or base multiple times. Using the example of H2SO4, explain the concept of gram equivalent weight.

Answer 24

H2SO4 is a diprotic acid, so half of a mole of H2SO4 can donate as many H+ as a mole of a normal acid (like HCl). The gram equivalent weight is how much of the polyprotic acid is needed to act like a 1N acid, and in this example, is equal to half of the molecular weight of H2SO4.