How to calculate pH of weak acid and weak base and explain common ion effect
With the addition of soluble compounds in the solution having a common ion with precipitate, the ionic precipitate solubility decreases. This is the concept of the common ion effect. In other terms, we refer to the shift in equilibrium due to the introduction of common ions in a solution is the common ion effect.
This effect is seen with the ionization of weak acid and weak base, which is also reversible. The reactions involve the formation of the precipitate, with the addition of chemicals having the same ions, is common with residue. You’ll understand ph change by common ion in case of weak acids and weak bases experiment with examples.
The pH of Weak Acid and Weak Base
The weak acid and weak base study is experimental-based observation and an important concept that deals with forming a salt precipitate. We will understand this concept in detail with examples:
Let us take one weak acid and one weak base as HA and BOH, respectively.
HA is H+ and A-
BOH is B+ and OH-
CH3COOH + H2O ⇋ CH3COO- + H3O+
CH3COONa → CH3COO- + Na+
In the first reaction above, H+ ions concentration increases. Thus reaction takes a reverse change and shifts equilibrium to the left-hand side. The second reaction states the shift in equilibrium.
CH3COOH is a weak acetic acid, and NH4OH is Ammonium hydroxide, a weak base.
The reaction depicts the shift is equilibrium with a decrease in H3O+ ions, and the pH of the entire solution increases.
Weak bases get partially dissociated. Thus ammonium hydroxide dissociates partially, and when we add Ammonium chloride to it, the concentration of NH4+ ions increases. The shift in equilibrium is noted in the below reaction:
NH4OH ⇋ NH4+ + OH-
NH4Cl → NH4+ + Cl-
Let us learn more about weak acid and weak base reaction.
Experiment on the pH of Weak Acid and Weak Base
The Common Ion effect deals with forming residue with a decrease in the concentration of ions. As studied under Le Chatelier's principle, a reaction’s equilibrium does not change unless you apply any external force. This external force can be in the form of tempurature, pressure, or the addition of some ions in the compound.
If a force is applied, it will help is re-establishing equilibrium. In all, the effect of solubility of solute decreases.
Aim:
To study pH change by common ion in case of weak acids and weak bases experiment.
Things Required:
You’ll require pH paper, test tubes, glass rod, Test tube stand, sodium acetate, acetic acid, ammonium chloride, and ammonium hydroxide for the experiment.
The Change in pH Level of a Solution for Weak Acid with Common Ion Effect:
The first step is to note the pH value of 10 ml acetic acid in a test tube.
To get the exact pH value, compare the shade with the standard pH chart.
With the help of a glass rod, mix 1g of sodium acetate with acetic acid in a test tube.
Now note down the final pH value considering the standard pH chart as a reference.
Now again, add 1g of sodium acetate to the solution and note down the solution’s pH value.
Repeat the experiment and keep noting your observations for the same.
Your Observations
The noticed changes show the solution’s pH value increases with the addition of common ions to weak acid.
The Change in pH Level of a Solution for Weak Base with Common Ion
In a test tube, add 10 ml of ammonium hydroxide, note down its pH value using pH paper, and consider the standard pH chart as a reference.
Now keep adding 1.5g of ammonium chloride in the test tube using a glass rod and keep mixing.
Take pH paper and note down pH value considering the pH chart as a reference.
Again add 1.5g of ammonium chloride to the test tube and mix well.
Note down the pH value of the final solution noting the pH chart as a reference.
Repeat the experiment and note down the final pH value in the observation table.
Your Observations
Change in pH of a weak base with the addition of conjugate acid is noted in the table below:
Experimental Precautions to be Taken:
Be careful while noting your observations after matching them with the standard pH chart.
Keep noting your observations simultaneously.
FAQs on Study of pH for Weak Acids and Weak Bases with Common Ion Effect
1. What is the pH of a weak acid?
The pH of a weak acid is determined by its acid dissociation constant (Ka) and initial concentration, and it partially ionizes in water. For a weak acid HA:
HA(aq) ⇌ H+(aq) + A-(aq)
The pH can be calculated using:
[H+] ≈ √(Ka × C)
where C is the initial concentration of the acid.
Finally:
pH = −log[H+]
Because weak acids ionize only partially, their pH is higher than that of a strong acid of the same concentration.
2. How do you calculate the pH of a weak base?
The pH of a weak base is calculated using its base dissociation constant (Kb) and initial concentration. For a weak base B:
B(aq) + H2O(l) ⇌ BH+(aq) + OH-(aq)
Use:
[OH-] ≈ √(Kb × C)
Then:
- pOH = −log[OH-]
- pH = 14 − pOH (at 25°C)
3. What is the common ion effect in chemistry?
The common ion effect is the suppression of the ionization of a weak electrolyte when a strong electrolyte containing a common ion is added. It is explained by Le Chatelier’s principle.
For example, acetic acid ionization:
CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq)
Adding sodium acetate (CH3COONa) increases CH3COO- concentration, shifting equilibrium to the left and decreasing ionization.
4. How does the common ion effect affect the pH of a weak acid?
The common ion effect decreases the ionization of a weak acid, resulting in a lower concentration of H+ and a slightly higher pH than expected without equilibrium shift.
For example:
CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq)
Adding CH3COONa increases CH3COO-, shifting equilibrium left and reducing further dissociation of acetic acid. This principle is important in buffer solutions.
5. How do you calculate the pH of a weak acid in the presence of a common ion?
The pH of a weak acid with a common ion is calculated using the Henderson–Hasselbalch equation.
pH = pKa + log([A-]/[HA])
Where:
- [A-] = concentration of conjugate base (common ion)
- [HA] = concentration of weak acid
- pKa = −logKa
6. What is the formula for pH of a weak base with a common ion?
The pH of a weak base with a common ion is calculated using the Henderson equation in terms of pOH:
pOH = pKb + log([BH+]/[B])
Then:
pH = 14 − pOH
Where:
- [BH+] = concentration of conjugate acid (common ion)
- [B] = concentration of weak base
- pKb = −logKb
7. Why does the common ion effect decrease the ionization of weak electrolytes?
The common ion effect decreases ionization because the added ion shifts the equilibrium backward according to Le Chatelier’s principle.
For a weak acid:
HA(aq) ⇌ H+(aq) + A-(aq)
Adding A- increases its concentration, so the equilibrium shifts left to reduce stress, decreasing further dissociation of HA.
8. What is the difference between a strong acid and a weak acid in terms of pH?
The key difference is that a strong acid completely ionizes in water, while a weak acid partially ionizes, resulting in a higher pH for the weak acid at the same concentration.
Example:
- Strong acid: HCl(aq) → H+(aq) + Cl-(aq)
- Weak acid: CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq)
9. Can you give an example of a weak acid and weak base buffer system?
A common example of a weak acid buffer is acetic acid and sodium acetate, and a weak base buffer is ammonia and ammonium chloride.
Acidic buffer equilibrium:
CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq)
Basic buffer equilibrium:
NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq)
These systems resist changes in pH due to the common ion effect.
10. What are common mistakes when calculating the pH of weak acids and bases?
Common mistakes when calculating the pH of weak acids and weak bases include incorrect equilibrium setup and ignoring common ion contributions.
Frequent errors:
- Using strong acid formulas instead of Ka or Kb expressions
- Forgetting to convert between pOH and pH
- Ignoring the presence of a common ion in buffer calculations
- Not checking whether approximation (√K×C) is valid
