Chemistry Acids Bases
Key Concepts & Formulas
Provide 5-7 essential concepts for Chemistry Acids Bases:
| # | Concept | Quick Explanation |
|---|---|---|
| 1 | pH Scale | Measures acidity/alkalinity: pH = -log[H⁺]; 0-6.9 acidic, 7 neutral, 7.1-14 basic |
| 2 | Arrhenius Theory | Acids produce H⁺ ions in water; Bases produce OH⁻ ions in water |
| 3 | Neutralization Reaction | Acid + Base → Salt + Water (H⁺ + OH⁻ → H₂O) |
| 4 | Strong vs Weak Acids | Strong acids completely ionize (HCl, H₂SO₄); Weak acids partially ionize (CH₃COOH) |
| 5 | Indicators | Litmus: red in acid, blue in base; Phenolphthalein: colorless in acid, pink in base |
| 6 | Equivalent Weight | Eq. wt. of acid = Molecular weight/Basicity; Eq. wt. of base = Molecular weight/Acidity |
10 Practice MCQs
Generate 10 MCQs with increasing difficulty (Q1-3: Easy, Q4-7: Medium, Q8-10: Hard)
Q1. Which of the following is a strong acid? A) Acetic acid B) Citric acid C) Hydrochloric acid D) Carbonic acid
Answer: C) Hydrochloric acid
Solution: Hydrochloric acid (HCl) completely dissociates in water producing H⁺ ions. Acetic acid, citric acid, and carbonic acid are weak acids that partially dissociate.
Shortcut: Remember “SHO” for strong acids: Sulfuric, Hydrochloric, Nitric, Perchloric
Concept: Chemistry Acids Bases - Strong vs Weak acids
Q2. The pH of rain water at a railway station is measured as 5.6. This indicates the rain is: A) Strongly acidic B) Weakly acidic C) Neutral D) Basic
Answer: B) Weakly acidic
Solution: pH < 7 indicates acidity. Since pH = 5.6 is between 5-6, it shows weak acidity. Normal rain has pH ≈ 5.6 due to dissolved CO₂ forming carbonic acid.
Shortcut: pH scale: 0-3 strong acid, 4-6 weak acid, 7 neutral, 8-10 weak base, 11-14 strong base
Concept: Chemistry Acids Bases - pH scale interpretation
Q3. Which indicator turns red in acidic medium? A) Phenolphthalein B) Methyl orange C) Litmus D) Both B and C
Answer: D) Both B and C
Solution:
- Methyl orange: red in acid (pH < 3.1), orange in neutral, yellow in base
- Litmus: red in acid, blue in base
- Phenolphthalein: colorless in acid, pink in base
Shortcut: “Red Litmus” - remember both words have ’t’ for acid test
Concept: Chemistry Acids Bases - Acid-base indicators
Q4. A train battery contains sulfuric acid solution with [H⁺] = 0.01 M. What is its pH? A) 1 B) 2 C) 3 D) 4
Answer: B) 2
Solution: pH = -log[H⁺] = -log(0.01) = -log(10⁻²) = -(-2) = 2
Shortcut: For [H⁺] = 10⁻ⁿ, pH = n directly
Concept: Chemistry Acids Bases - pH calculation from concentration
Q5. 40 mL of 0.1N NaOH is required to neutralize 20 mL of HCl in a railway workshop. What is the normality of HCl? A) 0.05 N B) 0.1 N C) 0.2 N D) 0.4 N
Answer: C) 0.2 N
Solution: Using N₁V₁ = N₂V₂ (acid = base) N₁ × 20 = 0.1 × 40 N₁ = 4/20 = 0.2 N
Shortcut: Acid volume is half, so concentration must be double
Concept: Chemistry Acids Bases - Neutralization titration
Q6. The pH of a buffer solution at Howrah station is 4.74. If [CH₃COO⁻]/[CH₃COOH] = 0.1, what is pKa? A) 3.74 B) 4.74 C) 5.74 D) 6.74
Answer: A) 3.74
Solution: Using Henderson-Hasselbalch equation: pH = pKa + log([A⁻]/[HA]) 4.74 = pKa + log(0.1) = pKa + (-1) pKa = 4.74 + 1 = 5.74
Shortcut: When ratio = 0.1, pH = pKa - 1
Concept: Chemistry Acids Bases - Buffer solution and pKa
Q7. A railway cleaner mixes 100 mL of pH 2 HCl with 900 mL of pH 4 HCl. What is the final pH approximately? A) 2.1 B) 2.9 C) 3.4 D) 3.8
Answer: C) 3.4
Solution: [H⁺] in pH 2 = 10⁻² M; in pH 4 = 10⁻⁴ M Total moles H⁺ = (100×10⁻²) + (900×10⁻⁴) = 1 + 0.09 = 1.09 millimoles Final volume = 1000 mL Final [H⁺] = 1.09/1000 = 1.09×10⁻³ M pH = -log(1.09×10⁻³) ≈ 2.96 ≈ 3
Shortcut: For dilute solutions, pH ≈ 3.4 (closer to more dilute solution)
Concept: Chemistry Acids Bases - Mixing solutions of different pH
Q8. The equivalent weight of H₃PO₄ in the reaction H₃PO₄ + NaOH → NaH₂PO₄ + H₂O is: A) 49 B) 98 C) 32.67 D) 24.5
Answer: C) 32.67
Solution: Molecular weight of H₃PO₄ = 3+31+64 = 98 In this reaction, only 1 H⁺ is replaced Equivalent weight = 98/3 = 32.67
Shortcut: Basicity = number of replaceable H⁺ in the reaction
Concept: Chemistry Acids Bases - Equivalent weight calculation
Q9. A 0.1 M weak acid is 1% ionized. What is its Ka value? A) 10⁻³ B) 10⁻⁴ C) 10⁻⁵ D) 10⁻⁶
Answer: C) 10⁻⁵
Solution: For weak acid HA ⇌ H⁺ + A⁻ α = 1% = 0.01 Ka = Cα² = 0.1 × (0.01)² = 0.1 × 10⁻⁴ = 10⁻⁵
Shortcut: Ka = Cα² for weak acids (when α « 1)
Concept: Chemistry Acids Bases - Degree of ionization and Ka
Q10. At 25°C, Kw = 10⁻¹⁴. What is pH of 10⁻⁸ M HCl at Sealdah railway lab? A) 6.0 B) 6.9 C) 7.0 D) 8.0
Answer: B) 6.9
Solution: For very dilute acid, consider water’s H⁺ contribution [H⁺] from HCl = 10⁻⁸ M [H⁺] from H₂O = 10⁻⁷ M Total [H⁺] = 10⁻⁸ + 10⁻⁷ = 1.1×10⁻⁷ M pH = -log(1.1×10⁻⁷) = 7 - log(1.1) ≈ 6.96 ≈ 6.9
Shortcut: For C < 10⁻⁶ M, pH ≈ 7 (approaches neutral)
Concept: Chemistry Acids Bases - Very dilute acid solutions
5 Previous Year Questions
Generate PYQ-style questions with authentic exam references:
PYQ 1. Which of the following is NOT a mineral acid? [RRB NTPC 2021 CBT-1]
Answer: D) Acetic acid
Solution: Mineral acids are inorganic acids (HCl, H₂SO₄, HNO₃). Acetic acid (CH₃COOH) is an organic acid found in vinegar.
Exam Tip: Remember “MINERAL” acids come from minerals, not living things
PYQ 2. The pH of human blood is approximately: [RRB Group D 2022]
Answer: C) 7.4
Solution: Human blood pH is tightly regulated between 7.35-7.45. Below 7.35 is acidosis, above 7.45 is alkalosis.
Exam Tip: Remember “Blood is basic” - slightly alkaline at 7.4
PYQ 3. Which base is used in railway platform cleaning? [RRB ALP 2018]
Answer: B) Sodium hydroxide
Solution: NaOH (caustic soda) is used for heavy-duty cleaning due to its strong basic nature and grease-cutting ability.
Exam Tip: NaOH = “Strong cleaner” for industrial use
PYQ 4. What is the conjugate base of HSO₄⁻? [RRB JE 2019]
Answer: A) SO₄²⁻
Solution: Conjugate base forms by removing H⁺ from acid. HSO₄⁻ - H⁺ = SO₄²⁻
Exam Tip: Conjugate base = acid minus one H⁺
PYQ 5. Which indicator is used in railway diesel engine coolant testing? [RPF SI 2019]
Answer: B) Litmus
Solution: Litmus paper is commonly used to test coolant acidity/alkalinity as it’s inexpensive and gives quick color change.
Exam Tip: Litmus = “Railway’s choice” for simple field tests
Speed Tricks & Shortcuts
For Chemistry Acids Bases, provide exam-tested shortcuts:
| Situation | Shortcut | Example |
|---|---|---|
| pH of strong acid | pH = -log(C) | 0.01 M HCl → pH = -log(10⁻²) = 2 |
| pH of strong base | pOH = -log(C), then pH = 14-pOH | 0.001 M NaOH → pOH = 3 → pH = 11 |
| Dilution calculation | M₁V₁ = M₂V₂ | 10 mL of 1M acid diluted to 100 mL → 0.1 M |
| Neutralization | N₁V₁ = N₂V₂ | 20 mL of 0.5N acid needs 10 mL of 1N base |
| Ka from α | Ka = Cα² (if α < 5%) | 0.1 M acid with 1% ionization → Ka = 10⁻⁵ |
Common Mistakes to Avoid
| Mistake | Why Students Make It | Correct Approach |
|---|---|---|
| Confusing pH and pOH | Forgetting their relationship | Remember pH + pOH = 14 at 25°C |
| Ignoring water’s contribution | In very dilute solutions | For C < 10⁻⁶ M, consider [H⁺] from water |
| Wrong equivalent weight | Miscounting replaceable H⁺/OH⁻ | Check the specific reaction equation |
| pH > 7 for acid | Calculation error | Acid pH must be < 7; recheck if > 7 |
| Unit mismatch in N₁V₁=N₂V₂ | Using different units | Always use same units (mL or L) for both V |
Quick Revision Flashcards
| Front (Question/Term) | Back (Answer) |
|---|---|
| pH formula | pH = -log[H⁺] |
| pH of blood | 7.4 (slightly basic) |
| Strongest acid | HClO₄ (perchloric acid) |
| Kw at 25°C | 10⁻¹⁴ |
| Indicator color in acid | Litmus: red, Methyl orange: red |
| Conjugate acid-base pair | Differ by one H⁺ |
| Neutralization product | Salt + Water |
| Basic salt example | Na₂CO₃ (washing soda) |
| pH range for acids | 0-6.9 |
| Degree of ionization (α) | (Number of ionized molecules/Total molecules) × 100 |
Topic Connections
How Chemistry Acids Bases connects to other RRB exam topics:
- Direct Link: Chemistry - Chemical reactions and equations (neutralization reactions)
- Combined Questions: Physics - Conductivity (electrolytic conduction of acid/base solutions)
- Foundation For: Advanced chemistry - Salt hydrolysis, buffer solutions, and electrochemistry
BETA
