Grade 12 Weak base

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Poll

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Two Truths and a Lie

Lesson 15.6: Weak bases and base
ionization constant

Open Ended

Examples of bases: ........

Svante Arrhenius.

Johannes Brønsted
and Thomas Lowry

Open Ended

Arrhenius Base:

Open Ended

Bronsted-Lowry Base:

Open Ended

Think Pair Share

Mention the main differences between the two theories

Strong base:
● Will ionize completely in water
● All alkali metals (1A) hydroxides are strong bases:

(LiOH, NaOH, KOH, RbOH, and CsOH).

● Certain Alkaline earth metals (2A) hydroxides are

strong bases: Ca(OH)2, Sr(OH)2 Ba(OH)2

Calculate the PH of a 5.0 x 10-2 M NaOH solution

Multiple Choice

Calculate the pH of a 5.0 x 10-2 M NaOH solution

• 1.30

• 12.70

• 11.00
• 10.25

Calculate the PH of a 5.0 x 10-2 M NaOH solution

NaOH is strong base

NaOH 🡪 Na+ + OH-

Initial 5 x 10 -2 0 0

Final 0 5 x 10 -2 5 x 10 -2

POH= -log [OH-] = -log(5x10-2)= 1.3
PH + POH= 14
PH = 14 -1.30 =12.7

Calculate PH of 0.020 M Ba(OH)2

Multiple Choice

Calculate pH of 0.020 M Ba(OH)2 solution

• 10.50
• 14.00
• 1.40

• 12.60

Ba(OH)2 🡪 Ba2+ + 2OH-

Initial 0.02 0 0

Final 0 0.02 2x0.02

[OH-]= 2x 0.02= 0.04 M

pOH= -log[OH-]= -log 0.04 = 1.4

pH + pOH = 14 PH = 14 – pOH

pH = 14 – 1.4= 12.6

Calculate PH of 0.020 M Ba(OH)2

Weak bases: ionize to limited extend
At equilibrium, An aqueous solution contains; a non-ionized base, OH- ions and a conjugate acid.
• Ammonia is a weak base: NH3

Multiple Choice

A weak base,....

• Completely ionizes, e.g. KOH

• Partially ionizes, e.g. KOH

• Partially ionizes, e.g. Ammonia

• Completely ionizes, e.g. Ammonia

Multiple Choice

The higher the Kb value the stronger the base?

• No relation between kb and base strength

• False
• Higher Kb values indicate weaker bases
• True

STEPS TO SOLVE WEAK base PROBLEMS
1- Identify your substance (weak or strong base )
2- If it is weak base use an ICE BOX
3- Write equilibrium constant Kb in terms of X ( X
= [OH-] )
4- Solve for x the “easy” way by assuming that
[B]0-X=[B]0
5- Use the 5% rule to verify whether the
approximation is valid
6- Calculate [OH-], [H+] and PH Or Calculate [OH-],
pOH then pH

NH3 is a weak base

Calculate the pH for a 15.0 M solution of NH3 (Kb=1.8 x 10-5)

Major Species in NH3 solution are: NH3 and H2O

Minor species: NH4

+, OH- and H+

NH3 is a weak base 🡪 ice box

NH3 + H2O ⇄ NH4

+ + OH-

Initial 15 M --- 0 0
Change -X --- +X +X
Final 15-X X X

Base ionization constant Kb = [NH4

+] [OH-] = 1.8 X 10-5

[NH3]

Kb= X2 assume that [NH3]0 – X = [NH3] 0 15-X = 15
15-X

Calculate the PH for a 15.0 M solution of NH3 (Kb=1.8 x 10-5)

kb= X2 = X2 = 1.8 x 10 -5

15-X 15

X= 1.6 x10-2
Use the 5% rule to verify whether the approximation is
valid:



X . 100 = 1.6 x10-2 . 100 = 0.106

[NH3]0 15

Since the value is much less than 5% , the approximation is considered valid

X= [OH-]= 1.6 x 10-2 M

POH = - log[OH-]
pOH = - log (1.6 x 10-2 )= 1.8

PH + pOH = 14

PH= 14-1.8= 12.20

Or

Kw= [H+] [OH-]=10-14

[H+] = 10-14 / (1.6x10-2)

[H+] = 6.3 x 10-13 M

PH= -log [H+]
= -(log 6.3 x 10-13)

pH= 12.20

Short-cut:

X= [OH-]= ⎷Kb x [base]0

pH= 14 + log ⎷Kb x [base]0

Multiple Choice

Calculate the pH for a 1.50 M solution of NH3 (Kb=1.8 x 10-5)

• 9.25
• 10.00
• 11.54
• 12.30

Multiple Choice

What is the pH of 0.40M NH3 solution?

• 7.00
• 9.25
• 11.43

• 10.50

Solve E-book page;
719 #56, 57

Multiple Choice

the pH of 0.30 M solution of weak base is 10.66, what is the k_b of the base?

• 1.25 x 10^-5

• 3.40 x 10^-8

• 8.00 x 10^-6

• 6.95 x 10^-7

Multiple Choice

What is the original molarity of a solution of ammonia whose pH is 11.22? Kb of ammonia is 1.8 X 10_^-5

• 0.075 M
• 0.153 M
• 0.200 M
• 0.100 M

​Step 1: Calculate pOH
pOH = 14 − pH
= 14 − 11.22 = 2.78

​Step 2: Calculate [ 𝑂 𝐻 − ]
[ 𝑂 𝐻 − ] = 1 0 ^{− pOH}
[ 𝑂 𝐻 − ] = 1 0 ^{− 2.78} ≈ 1.66 × 1 0 ^{− 3}

​Step 3 : find [NH3]₀ use: [OH-]= ⎷Kb x [base]0
[NH3​]≈0.153M

​K b ​ for ammonia=1.8×10 ⁻⁵

Multiple Choice

In 0.080 M ammonia solution, what is percent ionization? Kb​=1.8×10⁻⁵

• 1.5%

• 5.0%
• 0.2%
• 1.0%

​Step 1: find [OH^-]

[OH^-]= √Kb⋅[NH3]
[OH^-]= 1.2×10⁻³ M

​Step 2: Calculate percent ionization

Percent ionization = [OH^-] / [NH₃] ​×100

Percent ionization = (1.2×10^-3/ 0.080) ×100

%Percent ionization ≈ 1.5%

​K b ​ for ammonia=1.8×10 ⁻⁵

Relation between Ka and Kb

• The stronger an acid, the weaker its conjugate base,

and conversely,

The stronger a base the weaker its conjugate acid

• Ka x Kb = Kw = 10^-14

• Ka and Kb are inversely proportional.

• We can conclude that the higher the ka of a given acid, the lower the Kb of its conjugate base.

Consider this example about 2
weak acids:

CH3COOH Ka1= 1.8x10-5

HCN Ka2= 6.2 x10-10

which conjugate base is stronger (CN⁻) or (CH₃COO⁻)?

Multiple Choice

which is stronger conjugate base, (CN⁻) or (CH₃COO⁻)?

• CN⁻
• HCOO⁻
• HCOOH

• HCN

A household cleaner is labeled as having a pH of 10.

Given that the cleaner contains ammonia (Kb = 1.77 x 10^-5) as the
active ingredient.

Explain how does the pH of the cleaner relate to the effectiveness of the cleaner in removing grease and grime?

The cleaner’s pH of 10 indicates it is alkaline, which is effective in breaking down grease and grime. The pH of 10 suggests the cleaner is strong enough to dissolve grease without being overly harsh.