Chemistry Basics
Key Concepts & Formulas
| # | Concept | Quick Explanation |
|---|---|---|
| 1 | Atomic Structure | Atom = Protons (+) + Neutrons (0) + Electrons (-); Atomic No. = Protons; Mass No. = Protons + Neutrons |
| 2 | Valency | Combining capacity of elements; Group 1 = 1+, Group 2 = 2+, Group 17 = 1-, Group 18 = 0 |
| 3 | Chemical Formula | Compound written with criss-cross method: Al³⁺ + O²⁻ → Al₂O₃ (swap valencies) |
| 4 | Mole Concept | 1 mole = 6.022×10²³ particles = Gram molecular mass; No. of moles = Given mass/Molar mass |
| 5 | pH Scale | pH = -log[H⁺]; 0-7 = Acidic, 7 = Neutral, 7-14 = Basic; pH + pOH = 14 at 25°C |
| 6 | Balancing Equations | Mass conserved: Reactants atoms = Products atoms; use hit-trial or algebraic method |
| 7 | Oxidation-Reduction | Oxidation = Loss of electrons (increase in oxidation state); Reduction = Gain of electrons (decrease in oxidation state) |
10 Practice MCQs
Q1. What is the atomic number of Carbon? A) 6 B) 12 C) 14 D) 8
Answer: A) 6
Solution: Carbon has 6 protons in its nucleus. Atomic number = Number of protons = 6
Shortcut: Remember: “Carbon-12” is mass number, atomic number is always smaller
Concept: Chemistry Basics - Atomic Structure
Q2. The chemical formula of sodium chloride is: A) NaCl B) Na₂Cl C) NaCl₂ D) Na₂Cl₂
Answer: A) NaCl
Solution: Sodium (Na) = 1+ valency, Chlorine (Cl) = 1- valency Using criss-cross: Na¹⁺ + Cl¹⁻ → NaCl
Shortcut: Common salt formula = NaCl (1:1 ratio)
Concept: Chemistry Basics - Chemical Formula
Q3. Which of the following is a noble gas? A) Oxygen B) Nitrogen C) Argon D) Hydrogen
Answer: C) Argon
Solution: Argon (Ar) is in Group 18 with complete octet, making it inert/noble gas
Shortcut: Group 18 elements = He, Ne, Ar, Kr, Xe, Rn = Noble gases
Concept: Chemistry Basics - Periodic Table
Q4. How many moles are present in 44g of CO₂? [C=12, O=16] A) 0.5 B) 1 C) 2 D) 1.5
Answer: B) 1
Solution: Molar mass of CO₂ = 12 + (2×16) = 12 + 32 = 44g/mol Number of moles = Given mass/Molar mass = 44g/44g/mol = 1 mole
Shortcut: 44g CO₂ = 1 mole (memorize common values)
Concept: Chemistry Basics - Mole Concept
Q5. The pH of a solution is 3. What is its pOH value? A) 3 B) 7 C) 11 D) 14
Answer: C) 11
Solution: pH + pOH = 14 Given pH = 3, therefore pOH = 14 - 3 = 11
Shortcut: pH + pOH = 14 (always at 25°C)
Concept: Chemistry Basics - pH Scale
Q6. Balance the equation: Fe + H₂O → Fe₃O₄ + H₂ A) 3,4 → 1,4 B) 2,3 → 1,3 C) 1,2 → 1,2 D) 4,3 → 1,4
Answer: A) 3,4 → 1,4
Solution: Balanced equation: 3Fe + 4H₂O → Fe₃O₄ + 4H₂ Fe: 3 = 3, O: 4 = 4, H: 8 = 8
Shortcut: Start with most complex compound (Fe₃O₄), balance Fe first
Concept: Chemistry Basics - Balancing Equations
Q7. What is the oxidation state of Mn in KMnO₄? A) +2 B) +5 C) +7 D) +4
Answer: C) +7
Solution: K = +1, O = -2 (each) Let Mn = x +1 + x + 4(-2) = 0 +1 + x - 8 = 0 x = +7
Shortcut: KMnO₄ = Permanganate, Mn always +7 here
Concept: Chemistry Basics - Oxidation State
Q8. A train boiler uses water treatment chemicals. If 0.5 mole CaCl₂ is added to 500L water, what is the concentration in mol/L? A) 0.001 B) 0.01 C) 0.1 D) 1
Answer: A) 0.001
Solution: Concentration = Moles/Volume in Litres = 0.5 mole/500L = 0.001 mol/L = 1×10⁻³ mol/L
Shortcut: 500L = 0.5×1000L, so 0.5/500 = 1/1000 = 0.001
Concept: Chemistry Basics - Concentration Calculation
Q9. Railway tracks expand in heat. Which gas law explains this for air trapped in rail joints? A) Boyle’s B) Charles’s C) Avogadro’s D) Graham’s
Answer: B) Charles’s
Solution: Charles’s Law: V ∝ T at constant pressure (V₁/T₁ = V₂/T₂) As temperature increases, volume of trapped air expands
Shortcut: Charles = Temperature-Volume relationship
Concept: Chemistry Basics - Gas Laws
Q10. Railway diesel contains 84% C, 16% H by mass. What is the empirical formula? [C=12, H=1] A) CH₂ B) C₇H₁₆ C) CH₃ D) C₆H₁₄
Answer: A) CH₂
Solution: Assume 100g sample: C: 84g ÷ 12 = 7 moles H: 16g ÷ 1 = 16 moles Ratio: C:H = 7:16 ≈ 1:2.3 ≈ 1:2 Simplest ratio = CH₂
Shortcut: Divide by atomic masses, find simplest whole number ratio
Concept: Chemistry Basics - Empirical Formula
5 Previous Year Questions
PYQ 1. What is the mass of 2 moles of water? [RRB NTPC 2021 CBT-1]
Answer: 36g
Solution: Molar mass of H₂O = 2(1) + 16 = 18g/mol Mass = Moles × Molar mass = 2 × 18 = 36g
Exam Tip: Water molar mass = 18g/mol (memorize this)
PYQ 2. Which element has the electronic configuration 2,8,7? [RRB Group D 2022]
Answer: Chlorine (Cl)
Solution: Total electrons = 2+8+7 = 17 Atomic number = 17 = Chlorine Group 17, Period 3
Exam Tip: Electronic configuration sum = Atomic number
PYQ 3. Calculate the number of molecules in 11g of CO₂. [RRB ALP 2018]
Answer: 1.505×10²³ molecules
Solution: Molar mass CO₂ = 44g/mol Moles = 11g/44g/mol = 0.25 mol Molecules = 0.25 × 6.022×10²³ = 1.505×10²³
Exam Tip: 1/4 mole = 1/4 of Avogadro’s number
PYQ 4. The pH of railway station rainwater is measured as 4.5. Is it acidic or basic? [RRB JE 2019]
Answer: Acidic
Solution: pH = 4.5 < 7, therefore acidic Normal rainwater pH ≈ 5.6 due to dissolved CO₂ pH 4.5 indicates acid rain
Exam Tip: pH < 7 = Acidic (remember: Acid = Low pH)
PYQ 5. Balance: Ca(OH)₂ + HCl → CaCl₂ + H₂O [RPF SI 2019]
Answer: Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O
Solution: Ca: 1=1, O: 2=2, H: 4=4, Cl: 2=2 Balance OH and H separately
Exam Tip: Count polyatomic ions as units first
Speed Tricks & Shortcuts
| Situation | Shortcut | Example |
|---|---|---|
| Finding valency | Group No. = Valency (for main groups) | Na in Group 1 = 1+, Mg in Group 2 = 2+ |
| pH calculation | pH = -log[H⁺], but [H⁺] = 10⁻ᵖᴴ | pH 3 → [H⁺] = 10⁻³ mol/L |
| Mole conversions | 22.4L gas at STP = 1 mole | 11.2L CO₂ at STP = 0.5 mole |
| Oxidation state | O = -2 (always), H = +1 (usually) | In H₂SO₄: 2(+1) + x + 4(-2) = 0 → x = +6 |
| Empirical formula | % → g → mol → ratio | 40% C, 60% H → 40/12 : 60/1 → 3.33:60 → 1:18 → CH₁₈ |
Common Mistakes to Avoid
| Mistake | Why Students Make It | Correct Approach |
|---|---|---|
| Confusing atomic & mass number | Similar terms, both have “number” | Atomic = protons only, Mass = protons + neutrons |
| Wrong formula writing | Forgetting criss-cross rule | Al³⁺O²⁻ → Al₂O₃ (swap valencies, simplify) |
| pH vs pOH confusion | Both add to 14, which is which? | pH measures acidity, pOH measures basicity |
| Gas law conditions | Using wrong temperature units | Always convert °C to K: K = °C + 273 |
| Balancing polyatomic ions | Breaking them apart incorrectly | Keep OH⁻, SO₄²⁻, NO₃⁻ together if unchanged |
Quick Revision Flashcards
| Front (Question/Term) | Back (Answer) |
|---|---|
| Avogadro’s Number | 6.022×10²³ particles/mole |
| pH of blood | 7.4 (slightly basic) |
| Formula of rust | Fe₂O₃·xH₂O |
| Strongest acid among common acids | HCl (Hydrochloric acid) |
| Valency of Iron in Fe₂O₃ | +3 |
| Molar volume at STP | 22.4 L/mol |
| Chemical name of NaHCO₃ | Sodium bicarbonate |
| Oxidation state of O in H₂O₂ | -1 (peroxide) |
| pH range of acid rain | Below 5.6 |
| Common name of CaO | Quick lime |
Topic Connections
Direct Link:
- Physics: Gas laws (Boyle’s, Charles’s) apply to steam engines
- Biology: pH important for digestion, blood
- Environment: Acid rain affects railway infrastructure
Combined Questions:
- Chemistry + Physics: Thermodynamics in engines
- Chemistry + Biology: Enzyme pH optimization
- Chemistry + Environment: Pollution control in diesel engines
Foundation For:
- Advanced Chemistry: Organic reactions, electrochemistry
- Material Science: Railway track alloys, corrosion prevention
- Industrial Chemistry: Fuel combustion, lubricants
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