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Some Basic Concepts of Chemistry
Chemistry · Class 11NCERT pp. 1–242 sets × 25 questionsFree
Key facts and formulas for Some Basic Concepts of Chemistry, three worked sample questions, and two full practice sets. Every question has a hint, a reason for every option and a step-by-step solution, with a calculator or animation where one helps. English and Hinglish.
Formula sheet for this chapter
| Idea | Rule or fact |
|---|
| Moles | n = mass / molar mass = N / (6.022 × 10²³) = V(STP) / 22.4 L |
| Mass percent | % element = (mass of element in 1 mol / molar mass) × 100 |
| Empirical to molecular | n = molar mass / empirical formula mass; molecular = (empirical)ₙ |
| Average atomic mass | Σ (isotope mass × fractional abundance) |
| Molarity and molality | M = mol / L of solution (changes with T); m = mol / kg of solvent (does not) |
| Mole fraction | x₁ = n₁ / (n₁ + n₂) |
| Dilution and mixing | M₁V₁ = M₂V₂; mixing: M = (M₁V₁ + M₂V₂) / (V₁ + V₂) |
| Limiting reagent | Divide moles by coefficient; the smallest value limits |
| Percentage yield and purity | % yield = actual / theoretical × 100; purity = pure mass / sample mass × 100 |
| Significant figures | Multiply or divide: least sig. figs; add or subtract: least decimal places |
| Vapour density | Molar mass = 2 × vapour density |
| Normality | N = M × n-factor; equivalent mass = atomic mass / charge |
Three sample questions
Q1. Which one of the following is a heterogeneous mixture?
- Air
- Salt mixed with sugar
- Sugar dissolved in water
- Glucose
Hint and solution
Hint: A heterogeneous mixture has visibly different parts. Ask which option is not uniform throughout.
- Heterogeneous = non-uniform composition — A heterogeneous mixture has components that can be seen separately and composition varies from place to place.
- Air, sugar solution: uniform — Air and sugar in water look the same everywhere, so both are homogeneous.
- Glucose = pure compound — Glucose is a single pure substance, so it is not a mixture.
- Salt + sugar = heterogeneous — Crystals of salt and sugar stay distinct. So the answer is salt mixed with sugar. Option B.
Answer: B. A salt and sugar blend has parts that differ from place to place. Air and sugar solution are uniform; glucose is a pure compound.
Q2. Consider the statements:
(i) Air is a homogeneous mixture.
(ii) The elements of a compound can be separated by filtration.
(iii) A liquid has a fixed volume.
Which are correct?
- (i) and (ii) only
- (ii) and (iii) only
- (i), (ii) and (iii)
- (i) and (iii) only
Hint and solution
Hint: Check each statement separately. Ask what kind of method is needed to split a compound.
- (i) Air = homogeneous mixture — Air is a uniform mixture of gases, so statement one is true.
- (ii) Compound: chemical methods only — Filtration is a physical separation of a mixture. Elements in a compound are chemically bonded, so statement two is false.
- (iii) Liquid: fixed volume — A liquid takes the shape of its container but keeps its volume, so statement three is true.
- Correct: (i) and (iii) — Only statements one and three are true. Option D.
Answer: D. Air is uniform, so (i) is true. Compounds need chemical methods, so (ii) is false. Liquids have fixed volume, so (iii) is true.
Q3. Which of the following is NOT a base unit in SI?
- newton
- kelvin
- mole
- candela
Hint and solution
Hint: SI has seven base units. Think which option is built from other units like mass, length and time.
- Seven SI base units — m, kg, s, A, K, mol and cd are the seven base units.
- Kelvin, mole, candela are base — These three are on the base list, so they are rejected.
- N = kg m s⁻² — Force is mass times acceleration, so newton is built from base units. It is derived.
- Answer: newton — So newton is not a base unit. Option A.
Answer: A. Newton is a derived unit (kg m s⁻²). Kelvin, mole and candela are base units.
How to use this chapter
Read the NCERT pages first, then attempt Set 1 without the hints. Note every wrong answer in a mistake list, read the reason for each option, and retry Set 2 two days later. A topic is solid when you can say why each wrong option is wrong.