Strength of Acids and Bases
अम्लों एवं क्षारकों की प्रबलता
Class 10 Science – Chapter 2
CBSE Board + Foundation + Competitive Level
Complete Concept – पूर्ण अवधारणा
1. What is Strength of an Acid?
Strength of an acid refers to the extent to which an acid ionises
in aqueous solution.
अम्ल की प्रबलता से तात्पर्य है कि वह जल में कितनी मात्रा तक ionise होकर ions बनाता है।
यदि अम्ल लगभग पूरी तरह ionise हो जाए → Strong Acid
यदि अम्ल केवल आंशिक रूप से ionise हो → Weak Acid
अम्ल की प्रबलता से तात्पर्य है कि वह जल में कितनी मात्रा तक ionise होकर ions बनाता है।
यदि अम्ल लगभग पूरी तरह ionise हो जाए → Strong Acid
यदि अम्ल केवल आंशिक रूप से ionise हो → Weak Acid
2. Strong Acids – प्रबल अम्ल
Strong acids ionise almost completely in water.
प्रबल अम्ल जल में लगभग पूर्णतः आयनित हो जाते हैं।
प्रबल अम्ल जल में लगभग पूर्णतः आयनित हो जाते हैं।
HCl + H₂O → H₃O⁺ + Cl⁻
HNO₃ + H₂O → H₃O⁺ + NO₃⁻
Examples:
- Hydrochloric acid – HCl
- Nitric acid – HNO₃
- Sulphuric acid – H₂SO₄
3. Weak Acids – दुर्बल अम्ल
Weak acids ionise only partially in water.
दुर्बल अम्ल जल में केवल आंशिक रूप से आयनित होते हैं।
दुर्बल अम्ल जल में केवल आंशिक रूप से आयनित होते हैं।
CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻
Examples:
- Acetic acid – CH₃COOH
- Carbonic acid – H₂CO₃
- Formic acid – HCOOH
4. Strong and Weak Bases
Strong bases produce OH⁻ ions extensively in aqueous solution.
प्रबल क्षारक जल में OH⁻ ions की अधिक मात्रा उत्पन्न करते हैं।
प्रबल क्षारक जल में OH⁻ ions की अधिक मात्रा उत्पन्न करते हैं।
NaOH → Na⁺ + OH⁻
KOH → K⁺ + OH⁻
Examples of strong bases:
NaOH, KOH
Weak bases ionise only partially. Example: NH₄OH ⇌ NH₄⁺ + OH⁻
Weak bases ionise only partially. Example: NH₄OH ⇌ NH₄⁺ + OH⁻
5. Strength ≠ Concentration
बहुत महत्वपूर्ण परीक्षा बिंदु:
Strong acid का अर्थ यह नहीं है कि वह हमेशा concentrated होगा।
Strength = Degree of ionisation
Concentration = Amount of solute present per unit volume
Strength = Degree of ionisation
Concentration = Amount of solute present per unit volume
6. Relation with H₃O⁺ and OH⁻
| Condition | Ion concentration | Nature |
|---|---|---|
| High H₃O⁺ | H₃O⁺ ज्यादा | More acidic |
| Low H₃O⁺ | H₃O⁺ कम | Less acidic |
| High OH⁻ | OH⁻ ज्यादा | More basic |
| Low OH⁻ | OH⁻ कम | Less basic |
7. pH Scale
pH scale is used to measure how acidic or basic a solution is.
pH generally ranges from 0 to 14.
pH generally ranges from 0 to 14.
pH = −log[H₃O⁺]
At 25°C:
pH < 7 → Acidic
pH = 7 → Neutral
pH > 7 → Basic
pH < 7 → Acidic
pH = 7 → Neutral
pH > 7 → Basic
8. Important Concept – 10 Times Difference
The pH scale is logarithmic.
इसका अर्थ है कि pH में 1 unit का परिवर्तन H₃O⁺ concentration में
10 times change दर्शाता है।
For example: pH 3 solution has 10 times more H₃O⁺ than pH 4 solution.
pH 2 solution has 100 times more H₃O⁺ than pH 4 solution.
For example: pH 3 solution has 10 times more H₃O⁺ than pH 4 solution.
pH 2 solution has 100 times more H₃O⁺ than pH 4 solution.
Strong & Weak Acids/Bases
Strong Acid vs Weak Acid
| Strong Acid / प्रबल अम्ल | Weak Acid / दुर्बल अम्ल |
|---|---|
| Almost completely ionises | Partially ionises |
| More H₃O⁺ for comparable concentration | Less H₃O⁺ for comparable concentration |
| HCl, HNO₃ | CH₃COOH, H₂CO₃ |
| Usually lower pH at equal concentration | Usually higher pH at equal concentration |
Strong Base vs Weak Base
| Strong Base | Weak Base |
|---|---|
| Almost complete dissociation/ionisation | Partial ionisation |
| Produces relatively more OH⁻ | Produces relatively less OH⁻ |
| NaOH, KOH | NH₄OH |
Competitive Concept:
At the same molar concentration, a strong acid generally has a lower
pH than a weak acid because it produces a greater concentration of
H₃O⁺ ions.
pH & Strength – pH एवं प्रबलता
pH and H₃O⁺ Concentration
pH = −log[H₃O⁺]
यदि H₃O⁺ concentration बढ़ती है तो pH घटता है।
If H₃O⁺ concentration increases → pH decreases.
If H₃O⁺ concentration decreases → pH increases.
If H₃O⁺ concentration increases → pH decreases.
If H₃O⁺ concentration decreases → pH increases.
Solved Example 1
Question:
Which solution is more acidic: pH 2 or pH 5?
कौन-सा विलयन अधिक अम्लीय है?
कौन-सा विलयन अधिक अम्लीय है?
pH 2 is lower than pH 5.
Lower pH means greater H₃O⁺ concentration.
Difference = 5 − 2 = 3 pH units.
Therefore H₃O⁺ ratio = 10³ = 1000.
Answer: pH 2 solution is 1000 times more concentrated in H₃O⁺ ions than pH 5 solution.
Lower pH means greater H₃O⁺ concentration.
Difference = 5 − 2 = 3 pH units.
Therefore H₃O⁺ ratio = 10³ = 1000.
Answer: pH 2 solution is 1000 times more concentrated in H₃O⁺ ions than pH 5 solution.
Solved Example 2
Question:
If [H₃O⁺] = 10⁻³ mol/L, calculate pH.
Formula:
pH = −log[H₃O⁺]
= −log(10⁻³)
= 3
Answer: pH = 3
pH = −log[H₃O⁺]
= −log(10⁻³)
= 3
Answer: pH = 3
Solved Example 3
Question:
If pH = 4, find [H₃O⁺].
pH = −log[H₃O⁺]
Therefore: [H₃O⁺] = 10⁻⁴ mol/L
Answer = 10⁻⁴ mol/L
Therefore: [H₃O⁺] = 10⁻⁴ mol/L
Answer = 10⁻⁴ mol/L
Solved Numericals – Numerical Questions
Numerical 1:
The H₃O⁺ concentration of a solution is 10⁻² mol/L. Calculate pH.
pH = −log[H₃O⁺]
= −log(10⁻²)
= 2
Answer: pH = 2
= −log(10⁻²)
= 2
Answer: pH = 2
Numerical 2:
The pH of a solution is 5. Find its H₃O⁺ concentration.
[H₃O⁺] = 10⁻pH
= 10⁻⁵ mol/L
Answer: 10⁻⁵ mol/L
= 10⁻⁵ mol/L
Answer: 10⁻⁵ mol/L
Numerical 3:
A solution has pH 3 and another has pH 6. How many times is the first
solution more acidic?
Difference in pH:
6 − 3 = 3
H₃O⁺ ratio = 10³
= 1000
Answer: First solution is 1000 times more acidic in terms of H₃O⁺ concentration.
H₃O⁺ ratio = 10³
= 1000
Answer: First solution is 1000 times more acidic in terms of H₃O⁺ concentration.
Numerical 4:
Compare the H₃O⁺ concentration of pH 2 and pH 7 solutions.
Difference:
7 − 2 = 5
Ratio = 10⁵
= 100000
Answer: pH 2 solution has 100,000 times greater H₃O⁺ concentration than pH 7 solution.
Ratio = 10⁵
= 100000
Answer: pH 2 solution has 100,000 times greater H₃O⁺ concentration than pH 7 solution.
Numerical 5:
A solution has [H₃O⁺] = 10⁻⁷ mol/L. Find its pH and nature.
pH = −log(10⁻⁷)
pH = 7
Therefore solution is neutral at 25°C.
Answer: pH = 7, Neutral.
pH = 7
Therefore solution is neutral at 25°C.
Answer: pH = 7, Neutral.
Numerical 6:
A solution has [H₃O⁺] = 10⁻¹⁰ mol/L. Find pH and nature.
pH = −log(10⁻¹⁰)
pH = 10
Since pH > 7:
Solution is basic.
pH = 10
Since pH > 7:
Solution is basic.
Quick Formula:
pH = −log[H₃O⁺]
[H₃O⁺] = 10⁻pH
pH difference = n
H₃O⁺ ratio = 10ⁿ
pH = −log[H₃O⁺]
[H₃O⁺] = 10⁻pH
pH difference = n
H₃O⁺ ratio = 10ⁿ
30 MCQ – With Answers & Explanation
Q1. What determines the strength of an acid?
अम्ल की प्रबलता किस पर निर्भर करती है?
अम्ल की प्रबलता किस पर निर्भर करती है?
A. Colour
B. Degree of ionisation
C. Density only
D. Temperature only
Answer: B. Degree of ionisation
Strong acids ionise more extensively in water.
Strong acids ionise more extensively in water.
Q2. Which is a strong acid?
A. CH₃COOH
B. H₂CO₃
C. HCl
D. HCOOH
Answer: C. HCl
Q3. Which is a weak acid?
A. HCl
B. HNO₃
C. CH₃COOH
D. H₂SO₄
Answer: C. CH₃COOH
Q4. A strong acid ionises:
A. Partially
B. Almost completely
C. Never
D. Only in solid state
Answer: B
Q5. A weak acid ionises:
A. Completely
B. Partially
C. Never
D. Explosively
Answer: B
Q6. Which ion is responsible for acidic nature?
A. OH⁻
B. H₃O⁺
C. Na⁺
D. Cl⁻
Answer: B. H₃O⁺
Q7. Which ion is responsible for basic nature?
A. H₃O⁺
B. H⁺
C. OH⁻
D. Cl⁻
Answer: C. OH⁻
Q8. Which has lower pH at equal concentration?
A. Strong acid
B. Weak acid
C. Neutral solution
D. Pure water
Answer: A. Strong acid
Because it produces more H₃O⁺ ions.
Because it produces more H₃O⁺ ions.
Q9. pH is defined as:
A. log[H₃O⁺]
B. −log[H₃O⁺]
C. log[OH⁻]
D. −log[Na⁺]
Answer: B
Q10. pH of a neutral solution at 25°C is:
A. 0
B. 5
C. 7
D. 14
Answer: C. 7
Q11. A solution of pH 2 is:
A. Acidic
B. Basic
C. Neutral
D. Salt only
Answer: A. Acidic
Q12. A solution of pH 12 is:
A. Acidic
B. Basic
C. Neutral
D. Non-aqueous
Answer: B. Basic
Q13. If pH decreases by 1 unit, H₃O⁺ concentration changes by:
A. 2 times
B. 5 times
C. 10 times
D. 100 times
Answer: C. 10 times
Q14. pH 3 compared with pH 5 has how much more H₃O⁺?
A. 2 times
B. 10 times
C. 100 times
D. 1000 times
Answer: C. 100 times
10² = 100.
10² = 100.
Q15. Which is a strong base?
A. NH₄OH
B. NaOH
C. CH₃COOH
D. HCl
Answer: B. NaOH
Q16. Which is generally considered a weak base?
A. NaOH
B. KOH
C. NH₄OH
D. Ba(OH)₂
Answer: C. NH₄OH
Q17. Strong acid + water produces a relatively high concentration of:
A. H₃O⁺
B. OH⁻ only
C. Na⁺
D. CO₃²⁻
Answer: A
Q18. Strength and concentration are:
A. Exactly same
B. Different concepts
C. Always proportional
D. Identical terms
Answer: B
Q19. A concentrated weak acid can be:
A. Impossible
B. Possible
C. Always strong
D. Always neutral
Answer: B. Possible
Q20. If [H₃O⁺] = 10⁻⁴ M, pH is:
A. 2
B. 4
C. 6
D. 10
Answer: B. 4
Q21. If pH = 6, [H₃O⁺] equals:
A. 10⁻⁶ M
B. 10⁶ M
C. 6 M
D. 10⁻³ M
Answer: A. 10⁻⁶ M
Q22. Which solution has maximum H₃O⁺ concentration?
A. pH 2
B. pH 5
C. pH 7
D. pH 9
Answer: A. pH 2
Q23. Which solution has maximum basicity?
A. pH 2
B. pH 5
C. pH 9
D. pH 13
Answer: D. pH 13
Q24. Which equation represents weak acid ionisation?
A. CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻
B. NaOH → Na⁺ + OH⁻
C. NaCl → Na⁺ + Cl⁻
D. CaCO₃ → Ca²⁺ + CO₃²⁻
Answer: A
Q25. Which acid is present in vinegar?
A. HCl
B. CH₃COOH
C. H₂SO₄
D. HNO₃
Answer: B. CH₃COOH
Q26. At same concentration, which generally has lower pH?
A. Strong acid
B. Weak acid
C. Neutral solution
D. Water
Answer: A
Q27. pH scale is:
A. Linear
B. Logarithmic
C. Random
D. Constant
Answer: B. Logarithmic
Q28. If pH changes from 3 to 6, H₃O⁺ concentration:
A. Increases 1000 times
B. Decreases 1000 times
C. Remains same
D. Doubles
Answer: B. Decreases 1000 times
Q29. Which statement is correct?
A. Strong acid always means concentrated acid
B. Weak acid never produces H₃O⁺
C. Strong acid ionises more extensively
D. pH has no relation with H₃O⁺
Answer: C
Q30. Which is the correct relation?
A. More H₃O⁺ → higher pH
B. More H₃O⁺ → lower pH
C. More H₃O⁺ → no change
D. H₃O⁺ does not affect pH
Answer: B
30 Questions – 1 to 6 Marks | Full Solutions
Pattern: 30 questions with complete solutions.
CBSE Board + Foundation + Competitive Level.
CBSE Board + Foundation + Competitive Level.
1 Mark Questions
1 Mark
Q1. What is meant by strength of an acid?
अम्ल की प्रबलता से क्या तात्पर्य है?
अम्ल की प्रबलता से क्या तात्पर्य है?
Strength is the extent to which an acid ionises in aqueous solution.
Answer: जल में अम्ल के आयनीकरण की मात्रा को अम्ल की प्रबलता कहते हैं।
Answer: जल में अम्ल के आयनीकरण की मात्रा को अम्ल की प्रबलता कहते हैं।
1 Mark
Q2. Give one example of a strong acid.
एक प्रबल अम्ल का उदाहरण दीजिए।
एक प्रबल अम्ल का उदाहरण दीजिए।
Answer: HCl – Hydrochloric acid.
1 Mark
Q3. Give one example of a weak acid.
एक दुर्बल अम्ल का उदाहरण दीजिए।
एक दुर्बल अम्ल का उदाहरण दीजिए।
Answer: CH₃COOH – Acetic acid.
1 Mark
Q4. Write the formula for pH.
pH = −log[H₃O⁺]
1 Mark
Q5. What is the pH of a neutral solution at 25°C?
Answer: 7
2 Marks Questions
2 Marks
Q6. Differentiate between strong and weak acids.
प्रबल एवं दुर्बल अम्ल में अंतर लिखिए।
प्रबल एवं दुर्बल अम्ल में अंतर लिखिए।
Strong acid: Almost completely ionises in water.
Example: HCl.
Weak acid: Partially ionises in water.
Example: CH₃COOH.
Example: HCl.
Weak acid: Partially ionises in water.
Example: CH₃COOH.
2 Marks
Q7. Why is CH₃COOH a weak acid?
CH₃COOH दुर्बल अम्ल क्यों है?
CH₃COOH दुर्बल अम्ल क्यों है?
CH₃COOH does not completely ionise in water. It establishes an
equilibrium:
CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻
Hence it is a weak acid.
CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻
Hence it is a weak acid.
2 Marks
Q8. What is the relation between pH and H₃O⁺ concentration?
pH = −log[H₃O⁺]
Therefore, higher H₃O⁺ concentration means lower pH.
Therefore, higher H₃O⁺ concentration means lower pH.
2 Marks
Q9. Why is pH 3 more acidic than pH 5?
Difference = 5 − 3 = 2.
H₃O⁺ concentration ratio = 10² = 100.
Therefore pH 3 solution has 100 times more H₃O⁺ ions.
H₃O⁺ concentration ratio = 10² = 100.
Therefore pH 3 solution has 100 times more H₃O⁺ ions.
2 Marks
Q10. Differentiate between strength and concentration.
Strength: Extent of ionisation of an acid/base.
Concentration: Amount of solute present per unit volume of solution.
They are different concepts.
Concentration: Amount of solute present per unit volume of solution.
They are different concepts.
3 Marks Questions
3 Marks
Q11. Explain strong acid with an equation.
A strong acid ionises almost completely in water.
Example:
HCl + H₂O → H₃O⁺ + Cl⁻
Thus, a relatively high concentration of H₃O⁺ ions is produced.
Example:
HCl + H₂O → H₃O⁺ + Cl⁻
Thus, a relatively high concentration of H₃O⁺ ions is produced.
3 Marks
Q12. Explain weak acid with an equation.
A weak acid ionises partially in water.
Example:
CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻
The equilibrium symbol indicates incomplete ionisation.
Example:
CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻
The equilibrium symbol indicates incomplete ionisation.
3 Marks
Q13. Calculate pH if [H₃O⁺] = 10⁻⁵ M.
pH = −log[H₃O⁺]
= −log(10⁻⁵)
= 5
Answer: pH = 5
= −log(10⁻⁵)
= 5
Answer: pH = 5
3 Marks
Q14. Find [H₃O⁺] if pH = 3.
[H₃O⁺] = 10⁻pH
= 10⁻³ M
Answer: 10⁻³ M
= 10⁻³ M
Answer: 10⁻³ M
3 Marks
Q15. Why does pH decrease when H₃O⁺ concentration increases?
pH is defined as:
pH = −log[H₃O⁺]
Therefore, as H₃O⁺ concentration increases, the logarithmic value increases and because of the negative sign, pH decreases.
Hence solution becomes more acidic.
pH = −log[H₃O⁺]
Therefore, as H₃O⁺ concentration increases, the logarithmic value increases and because of the negative sign, pH decreases.
Hence solution becomes more acidic.
4 Marks Questions
4 Marks
Q16. Explain strong and weak acids with suitable examples.
Strong Acids:
These ionise almost completely in water.
Examples: HCl, HNO₃.
HCl + H₂O → H₃O⁺ + Cl⁻
Weak Acids: These ionise only partially.
Example: CH₃COOH.
CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻
Examples: HCl, HNO₃.
HCl + H₂O → H₃O⁺ + Cl⁻
Weak Acids: These ionise only partially.
Example: CH₃COOH.
CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻
4 Marks
Q17. Explain strong and weak bases.
Strong bases ionise/dissociate extensively to produce OH⁻ ions.
NaOH → Na⁺ + OH⁻
KOH → K⁺ + OH⁻
A weak base ionises partially.
NH₄OH ⇌ NH₄⁺ + OH⁻
NaOH → Na⁺ + OH⁻
KOH → K⁺ + OH⁻
A weak base ionises partially.
NH₄OH ⇌ NH₄⁺ + OH⁻
4 Marks
Q18. Calculate and compare the pH of solutions having
[H₃O⁺] = 10⁻² M and 10⁻⁵ M.
For first solution:
pH = −log(10⁻²) = 2
For second solution:
pH = −log(10⁻⁵) = 5
Therefore pH 2 solution is more acidic.
Difference = 3
H₃O⁺ ratio = 10³ = 1000
Answer: First solution has 1000 times greater H₃O⁺ concentration.
pH = −log(10⁻²) = 2
For second solution:
pH = −log(10⁻⁵) = 5
Therefore pH 2 solution is more acidic.
Difference = 3
H₃O⁺ ratio = 10³ = 1000
Answer: First solution has 1000 times greater H₃O⁺ concentration.
4 Marks
Q19. Explain why strong acid is not necessarily concentrated.
Strength and concentration are different concepts.
A strong acid ionises almost completely.
Concentration indicates the amount of acid present per unit volume.
Therefore a strong acid may be dilute, and a weak acid may be concentrated.
Conclusion: Strength depends on ionisation, whereas concentration depends on quantity per unit volume.
A strong acid ionises almost completely.
Concentration indicates the amount of acid present per unit volume.
Therefore a strong acid may be dilute, and a weak acid may be concentrated.
Conclusion: Strength depends on ionisation, whereas concentration depends on quantity per unit volume.
4 Marks
Q20. Explain the logarithmic nature of pH scale with an example.
pH scale is logarithmic.
A difference of one pH unit represents a tenfold difference in H₃O⁺ concentration.
For pH 3 and pH 5:
Difference = 2
Ratio = 10² = 100
Thus pH 3 has 100 times more H₃O⁺ than pH 5.
A difference of one pH unit represents a tenfold difference in H₃O⁺ concentration.
For pH 3 and pH 5:
Difference = 2
Ratio = 10² = 100
Thus pH 3 has 100 times more H₃O⁺ than pH 5.
5 Marks Questions
5 Marks
Q21. Explain the factors determining the strength of acids and bases.
The strength of an acid or base is related mainly to its extent of
ionisation in aqueous solution.
1. Strong Acid: Almost complete ionisation.
HCl + H₂O → H₃O⁺ + Cl⁻
2. Weak Acid: Partial ionisation.
CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻
3. Strong Base: Produces OH⁻ extensively.
NaOH → Na⁺ + OH⁻
4. Weak Base: Produces OH⁻ partially.
NH₄OH ⇌ NH₄⁺ + OH⁻
5. Conclusion: Greater ionisation means greater strength.
1. Strong Acid: Almost complete ionisation.
HCl + H₂O → H₃O⁺ + Cl⁻
2. Weak Acid: Partial ionisation.
CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻
3. Strong Base: Produces OH⁻ extensively.
NaOH → Na⁺ + OH⁻
4. Weak Base: Produces OH⁻ partially.
NH₄OH ⇌ NH₄⁺ + OH⁻
5. Conclusion: Greater ionisation means greater strength.
5 Marks
Q22. Explain the relationship between pH and acidity with examples.
pH is given by:
pH = −log[H₃O⁺]
Therefore:
More H₃O⁺ → Lower pH → More acidic
Less H₃O⁺ → Higher pH → Less acidic
Example: pH 2 and pH 5.
Difference = 3
Ratio = 10³ = 1000
Therefore pH 2 has 1000 times more H₃O⁺ concentration than pH 5.
pH = −log[H₃O⁺]
Therefore:
More H₃O⁺ → Lower pH → More acidic
Less H₃O⁺ → Higher pH → Less acidic
Example: pH 2 and pH 5.
Difference = 3
Ratio = 10³ = 1000
Therefore pH 2 has 1000 times more H₃O⁺ concentration than pH 5.
5 Marks
Q23. Explain the difference between strong acid, weak acid,
concentrated acid and dilute acid.
Strong Acid:
Almost complete ionisation.
Example: HCl.
Weak Acid: Partial ionisation. Example: CH₃COOH.
Concentrated Acid: Large amount of acid per unit volume.
Dilute Acid: Small amount of acid per unit volume.
Important: Strength and concentration are independent concepts.
Weak Acid: Partial ionisation. Example: CH₃COOH.
Concentrated Acid: Large amount of acid per unit volume.
Dilute Acid: Small amount of acid per unit volume.
Important: Strength and concentration are independent concepts.
5 Marks
Q24. Solve: A solution has pH 2. Another solution has pH 6.
Compare their H₃O⁺ concentrations.
Difference in pH:
6 − 2 = 4
H₃O⁺ concentration ratio:
10⁴
= 10,000
Answer: The pH 2 solution has 10,000 times greater H₃O⁺ concentration than the pH 6 solution.
6 − 2 = 4
H₃O⁺ concentration ratio:
10⁴
= 10,000
Answer: The pH 2 solution has 10,000 times greater H₃O⁺ concentration than the pH 6 solution.
5 Marks
Q25. Explain why aqueous solutions of strong acids and bases conduct
electricity.
Strong acids and bases ionise/dissociate extensively in water.
For example:
HCl + H₂O → H₃O⁺ + Cl⁻
NaOH → Na⁺ + OH⁻
These ions are charged particles and can move through the solution.
The movement of ions allows the aqueous solution to conduct electric current.
For example:
HCl + H₂O → H₃O⁺ + Cl⁻
NaOH → Na⁺ + OH⁻
These ions are charged particles and can move through the solution.
The movement of ions allows the aqueous solution to conduct electric current.
6 Marks Questions
6 Marks
Q26. Explain the complete concept of strength of acids and bases.
Acid Strength:
The strength of an acid depends on its extent of ionisation in water.
Strong Acid: Almost completely ionises.
HCl + H₂O → H₃O⁺ + Cl⁻
Weak Acid: Partially ionises.
CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻
Strong Base: Produces OH⁻ extensively.
NaOH → Na⁺ + OH⁻
Weak Base: Partially ionises.
NH₄OH ⇌ NH₄⁺ + OH⁻
pH: pH = −log[H₃O⁺]
Higher H₃O⁺ concentration means lower pH and stronger acidic character.
Thus strength is closely related to ionisation and ion concentration.
Strong Acid: Almost completely ionises.
HCl + H₂O → H₃O⁺ + Cl⁻
Weak Acid: Partially ionises.
CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻
Strong Base: Produces OH⁻ extensively.
NaOH → Na⁺ + OH⁻
Weak Base: Partially ionises.
NH₄OH ⇌ NH₄⁺ + OH⁻
pH: pH = −log[H₃O⁺]
Higher H₃O⁺ concentration means lower pH and stronger acidic character.
Thus strength is closely related to ionisation and ion concentration.
6 Marks
Q27. Explain strong and weak acids with comparison and equations.
Strong acid:
HCl + H₂O → H₃O⁺ + Cl⁻
Weak acid:
CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻
The single arrow represents essentially complete ionisation, whereas the equilibrium arrow indicates partial ionisation.
| Strong Acid | Weak Acid |
|---|---|
| Almost complete ionisation | Partial ionisation |
| More H₃O⁺ at comparable concentration | Less H₃O⁺ |
| Lower pH generally | Higher pH generally |
| HCl, HNO₃ | CH₃COOH |
Strong acid:
HCl + H₂O → H₃O⁺ + Cl⁻
Weak acid:
CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻
The single arrow represents essentially complete ionisation, whereas the equilibrium arrow indicates partial ionisation.
6 Marks
Q28. Explain pH scale and solve a numerical based on it.
pH is a measure of the acidic or basic nature of a solution.
Formula:
pH = −log[H₃O⁺]
At 25°C:
pH < 7 → Acidic
pH = 7 → Neutral
pH > 7 → Basic
Numerical: If [H₃O⁺] = 10⁻⁴ M, find pH.
pH = −log(10⁻⁴)
pH = 4
Therefore the solution is acidic.
Answer: pH = 4 and solution is acidic.
Formula:
pH = −log[H₃O⁺]
At 25°C:
pH < 7 → Acidic
pH = 7 → Neutral
pH > 7 → Basic
Numerical: If [H₃O⁺] = 10⁻⁴ M, find pH.
pH = −log(10⁻⁴)
pH = 4
Therefore the solution is acidic.
Answer: pH = 4 and solution is acidic.
6 Marks
Q29. A solution A has pH 2 and solution B has pH 5.
Calculate how many times solution A is more acidic than B.
Given:
pH(A) = 2
pH(B) = 5
Difference:
5 − 2 = 3
For every one pH unit, H₃O⁺ concentration changes by a factor of 10.
Therefore:
Ratio = 10³
= 1000
Answer: Solution A has 1000 times greater H₃O⁺ concentration than solution B. Hence A is much more acidic.
pH(A) = 2
pH(B) = 5
Difference:
5 − 2 = 3
For every one pH unit, H₃O⁺ concentration changes by a factor of 10.
Therefore:
Ratio = 10³
= 1000
Answer: Solution A has 1000 times greater H₃O⁺ concentration than solution B. Hence A is much more acidic.
6 Marks
Q30. Explain why strength and concentration of an acid are different
concepts. Give suitable examples and justify your answer.
Strength:
Strength tells us how extensively an acid ionises in water.
HCl is a strong acid because it ionises almost completely:
HCl + H₂O → H₃O⁺ + Cl⁻
Weak Acid: Acetic acid ionises only partially:
CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻
Concentration: Concentration tells us how much solute is present per unit volume.
Therefore:
A dilute HCl solution is still a strong acid in terms of nature, although its amount per unit volume is small.
Similarly, concentrated acetic acid can still be a weak acid because its degree of ionisation is limited.
Final Conclusion: Strength = extent of ionisation.
Concentration = amount of solute per unit volume.
Therefore, strong/weak and concentrated/dilute are not interchangeable terms.
HCl is a strong acid because it ionises almost completely:
HCl + H₂O → H₃O⁺ + Cl⁻
Weak Acid: Acetic acid ionises only partially:
CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻
Concentration: Concentration tells us how much solute is present per unit volume.
Therefore:
A dilute HCl solution is still a strong acid in terms of nature, although its amount per unit volume is small.
Similarly, concentrated acetic acid can still be a weak acid because its degree of ionisation is limited.
Final Conclusion: Strength = extent of ionisation.
Concentration = amount of solute per unit volume.
Therefore, strong/weak and concentrated/dilute are not interchangeable terms.
