BUFFER SOLUTION
Solution SBI 2008 42
Buffer solution is a solution which is composed from weak acid and its salt or weak base and its salt.
Example:1. CH3COOH and CH3COONa
This buffer solution composed of weak acid (CH3COOH) and its salt (CH3COONa)
2. NH4OH and NH4Cl
This buffer solution composed of weak base (NH4OH) and its salt (NH4Cl)
BUFFER SOLUTION(cont’d)
Solution SBI 2008 43
In the first solution, if we add acid, then the acid will be neutralized by left over of acid (sisa asam)
CH3COO- + H+ CH3COOH
If we add base (OH-), it will be neutralized by the acid; CH3COOH + OH- CH3COO- + H2O
The same thing occur when we add acid or base to the buffer solution 2 ( NH4OH and NH4Cl)
To calculate pH of the two buffer solutions the formula used as the following:
BUFFER SOLUTION(cont’d)
Weak acid and its salt
[H+] = Ka .
[Acid]
[Salt]
- log [H+] = - log Ka - log
[Acid]
[Salt]
So pH will be equaled to:
pH = pKa + log [Acid]
[Salt]
Solution SBI 2008 44
Weak base and its salt
[OH-] = Kb .
[Base]
[Salt]
- log [OH-] = - log Kb - log
[Base]
[Salt]
So pOH will be equaled to:
pOH = pKb + log [Salt]
[Base]
and remember pH + pOH = pKw
BUFFER SOLUTION(cont’d)
Solution SBI 2008 45
[Acid] = the molar concentration of weak acid[Base] = the molar concentration of weak base[Salt] = the molar concentration of salt either
from weak acid or weak base
Example:1. 0.1 M acetic acid mixed with 0.01 M sodium
acetate. If Ka is 1 x 10-5, the pH of this solution will be:
BUFFER SOLUTION(cont’d)
Solution SBI 2008 46
Answer:This solution is composed of weak acid and its
salt, so the buffer formula applies to calculate pH of this solution.
[Acid] = 0.1 M; [Salt] = 0.01 MThe pH = pKa + log [Salt]/[Acid]
pH = - log 1 x 10-5 + log (0.01/0.1) = 5 – 1 = 4
BUFFER SOLUTION(cont’d)
Solution SBI 2008 47
2. 0.01 M ammonium hydroxide mixed with 0.1 M ammonium chloride. If Kb is 1 x 10-5, the pH of this solution will be:
Answer:This solution is composed of weak base
(NH4OH) and its salt (NH4Cl), so the buffer formula applies to calculate pH of this solution.
[Base] = 0.01 M; [Salt] = 0.1 MThe pOH = pKb + log [Salt]/[Base]
BUFFER SOLUTION(cont’d)
Solution SBI 2008 48
The pOH = pKb + log [Salt]/[Base] pOH = - log 1 x 10-5 + log (0.1/0.01)
= 5 +1 = 6
pH = pKw – pOH = 14 – 6 = 8
Question 1. 6 grams of acetic acid (Mr 60) and 9.8 grams
of potassium acetate (Mr 98) is mixed in 1 L solution, what the pH of the solution will be? (Ka 1 x 10-5)
BUFFER SOLUTION(cont’d)
Solution SBI 2008 49
Question 2. 35 grams of NH4OH (Mr 35) and 26.75 grams
of NH4Cl (Mr 53.5) is mixed in 0.5 L solution, what the pH of the solution will be? (Kb 1 x 10-5)
Question 3. If we react 12 g of CH3COOH (Mr 60) and 4 g
NaOH (Mr 40) in 1 L solution, what the pH of the solution (Ka 1 x 10-5)
BUFFER SOLUTION(cont’d)
Solution SBI 2008 50
Question 4. One RSBI student wants to make a buffer
solution of NH4OH (Mr 35) and NH4Cl (Mr 53.5) with the pH of 10 in 500 mL. If Kb is 1 x 10-5, what the mass ratio of base and salt must be added?
HYDROLYSIS
Solution SBI 2008 51
Hydrolysis is a reaction between a salt and water in which the anion (from the weak acid) or the cation (the weak base) of the salt react with water so the solution will be acidic or basic
The hydrolysis can only occur when: the number of moles reacts between acid and base are equal
The solution become basic or acidic depends on the salt that was hydrolyzed.
Hydrolysis(Cont’d)
Solution SBI 2008 52
1. The reaction between salt formed of strong acid and strong base with water
Example as in:NaCl Na+ + Cl-
Na+ is from NaOH Cl- is from HCl
As Na+ and Cl- are from strong base and acid, so NaCl is neutral, then the Na+ and Cl- ions will not be able to react with water.
Hydrolysis(Cont’d)
Solution SBI 2008 53
As a result, the hydrolysis in not occurred to the salt which are formed strong acid and strong base.
How about KCl or MgSO4?
Are they hydrolyzed?
They can not be hydrolyzed as well, as they are formed from strong acid and base.
Hydrolysis(Cont’d)
Solution SBI 2008 54
2. The reaction between salt formed of strong acid and weak base with water
Example as in: NH4Cl
In water, NH4Cl will be ionized as follows:
NH4Cl <==> NH4+ + Cl- (1)
NH4+ + H2O <==> NH4OH + H+ (2)
The NH4OH formed is a based, so the equilibrium constant of the last reaction (in equation 2) will be
Hydrolysis(Cont’d)
K = [H2O]
K = [NH4OH
+] [H
+]
[NH4+] [H2O]
[NH4OH+] [H
+]
[NH4+]
Solution SBI 2008 55
This constant value is given a symbol of Kh. This value is dependent on the salt and temperature.
The value of K. H2O is also a
constant value, as the concen-
tration of H2O as the solvent
is constant.
Kh = [NH4OH
+] [H
+]
[NH4+]
(3)
(4)
(5)
Hydrolysis(Cont’d)
Solution SBI 2008 56
As NH4OH is a weak base which is difficult to ionize, the equilibrium in Equation 2 will be shifted to the right, so the water ionization will increase, so the amount of H+ formed will increase too, so the solution will be acidic.
The Cl- ion will remain constant, as it will not react with water. Why?
Is there any relation between Kb, Kh and Kw?
In the last equation (5) let’s say, we multiply with [OH-]/ [OH-], so we have
Hydrolysis(Cont’d)
Solution SBI 2008 57
Kh = Kb
Kwor
Kh = [NH4OH]
[NH4+] [OH
-]
[H+][OH
-]
Remember: Kb = [NH4OH]
[NH4+] [OH
-]
and Kw = [H+] [OH
-] ; so
Kh = 1Kb
Kw
Hydrolysis(Cont’d)
Solution SBI 2008 58
If we insert Kh = Kw/Kb into Equation (5) then we get:
The concentration of NH4OH and H+ based on the reaction 2 is equal. By assuming that the amount of NH4
+ which reacted with water is very small compared to the initial concentration of NH4
+ (from NH4Cl), so Equation 6 can be changed into following equation to calculate pH of hydrolysis:
(6) [NH4
+]
[NH4OH] [H+]
Kb
Kw=
Hydrolysis(Cont’d)
Solution SBI 2008 59
If the concentration of [NH4+] = [Salt] in molar
concentration then:
[NH4+] Kb
Kw=
2 [H
+]
[H+] =
2 Kw
Kb. [NH4
+] ; So
Kw
Kb. [NH4
+][H
+] =
Hydrolysis(Cont’d)
Solution SBI 2008 60
Kw
Kb. [Salt][H
+] =
and
pH = 1/2 pKw - 1/2 pKb - 1/2 log [salt]
In this reaction the pH will be acidic. Why?
3.The reaction between salt formed of strong base and weak acid with water
For example is CH3COONa, a salt formed from NaOH and CH3COOH
Hydrolysis(Cont’d)
Solution SBI 2008 61
The process like in the salt of strong acid and weak base occurred.
The solution will be…….The value of Kh = Kw/Ka, so:
Kw
Ka. [Salt][OH
-] =
and
pOH = 1/2 pKw - 1/2 pKa - 1/2 log [salt]
pH = 1/2 pKw + 1/2 pKa + 1/2 log [salt]
or if we calculate pH straight way, we can use:
Hydrolysis(Cont’d)
Solution SBI 2008 62
In the hydrolysis no. 2 and 3, we call as partial hydrolysis as only one ion is hydrolyzed.
4.The reaction between salt formed of weak base and weak acid with water
In this hydrolysis as the salt are from an anion of weak acid and a cation of weak base, so all the anions will react with water. This is called as total hydrolysis.
The value of Kh = Kw/(Ka x Kb), so the value of pH will only depend on the value of Ka and Kb
Hydrolysis(Cont’d)
Solution SBI 2008 63
Example 1.Calculate pH of NH4Cl solution 0.1M (Kb NH4OH 1x 10-5)
Answer:
NH4Cl is a salt formed from ………
pH = ½ pKw – ½ pKb – ½ Log [Salt]= (½ x 14) – (½ x 5) – (½x-1) = 7 – 2.5 +0.5 = 4.5
Hydrolysis(Cont’d)
Solution SBI 2008 64
Example 2.Calculate pH of CH3COONa solution 0.1M (Ka
CH3COOH 1x 10-5)
Answer:
CH3COONa is a salt formed from ………
pH = ½ pKw + ½ pKa + ½ Log [Salt]= (½ x 14) + (½ x 5)+ (½x-1) = 7 + 2.5 -0.5 = 9
Hydrolysis(Cont’d)
Solution SBI 2008 64
Question Calculate pH of :5. NaOCl solution 0.5M (Ka HOCl 3x 10-8)6. 26.75 g NH4Cl (Mr 53.5) in 5 L solution (Kb NH4OH 1 x
10-5)7. 1.764 g NaCN (Mr 49) in 1.5 L solution (Ka HCN 5x10-10)8. 1.2 g CH3COOH (Mr 60; Ka 1 x 10-5) is reacted with 0.08
g NaOH (Mr 40)9. 125 mL NH4OH 0.2 M is reacted with 125 mL HCl 0.2 M
Kb NH4OH 1 x 10-5)
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