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CBSE Class 11 · Chemistry

Some Basic Concepts of Chemistry

Official NCERT chapter from Chemistry Part I–II (book code kech1). ExamMaster notes are original teaching at CBSE Class 11 depth.

This lesson follows the official chapter “Some Basic Concepts of Chemistry” in Chemistry Part I–II. The words below are ExamMaster’s teaching, not a paste from the book. Use the NCERT chapter for the classroom sequence; use these notes to hold the idea without copying exercises or figures.

  • CBSE Class 11
  • Medium level
  • 10 concepts

1Importance of Chemistry

Chemistry here is the school study of substances and how they change — a job-list (measure, name, count bits), not a poster of “lab coats”. Importance is a use-column: a fuel-story, a water-purity story, a taught material — one honest use is enough.

A brand-ad is not the importance-test.

Figure. Chemistry is the study of what particles do when matter changes. A test — gas, precipitate, heat, or a calculated quantity — comes before the reaction name.

How it works

  1. Name a substance-change you can point toThe job.
  2. Name one taught useThe importance.
  3. Keep it a school sentenceNot a slogan.

2Nature of Matter

Matter is anything that has mass and takes space in the school test; it sits as solid, liquid, or gas as looks, or as mixture versus pure as a sort. Nature of matter is that sort, not a philosophy. One ice–water–steam triangle is enough.

A light-beam with no mass-story is not this matter-heading unless the lesson split it.

Figure. Matter splits first into mixture or pure substance. A mixture is the same throughout or visibly mixed; a pure substance is one element or one compound. This is the board classification tree, not a photograph of samples.

How it works

  1. Run mass-and-spaceThe object.
  2. Hang solid/liquid/gas or mix/pureThe nature.
  3. Refuse a mood as a stateA look-test.

3Properties of Matter and their Measurement

Properties are looks you can measure: mass on a balance, volume in a marked jar, a temperature. Extensive versus intensive if taught (mass versus density). A property without a unit is a mute adjective.

“Shiny” with no conduct-test is a weak property here.

Figure. Mass is extensive: two 50 g water samples join to a bar twice as wide, 100 g. Density is intensive: each sample is 1.0 g/cm³ and the joined sample is still a 1.0-wide bar, not a double. Width is the quantity in each row.

How it works

  1. Name the look and the toolThe measure.
  2. Write the number and the unitThe property.
  3. Keep intensive/extensive if the lesson used themThe sort.

4Uncertainty in Measurement

Uncertainty: a measured 12.4 cm is not infinitely sharp; significant figures and a taught plus-minus are the honesty. 12.40 cm claims a finer last digit than 12.4 cm. A calculator dump of 8 decimals is not more scientific.

Writing 2/3 as 0.666666667 from a 2-significant start is a fake fine.

Figure. The same length on two scales. The coarse rule is marked only to 0.1 cm, so the reading is 2.4 cm with the last digit uncertain. The finer rule adds a mid-tick, so 2.45 cm is the honest report. Extra digits you cannot see are not extra certainty.

How it works

  1. Name the instrument’s last honest digitThe uncertainty.
  2. Keep sig-figs through a multiply as taughtThe rule.
  3. Refuse extra pretty digitsHonest.

5Laws of Chemical Combinations

Laws of combination (school): mass is conserved in a closed change; a given compound keeps a definite mass ratio; gases in the taught volume-story combine in simple ratios. The laws are look-summaries, not a biography.

A kitchen splash “some H, some O” as the same water is a miss.

Figure. Law of definite proportions: every water sample is 1 g hydrogen to 8 g oxygen. Sample A is 2 g + 16 g; sample B is 1 g + 8 g. The stacked lengths keep the same 1:8 split, so the ratio is a property of the compound, not of the sample size.

How it works

  1. Name conserve-mass, definite ratio, or the gas-volume law as taughtThe law.
  2. Keep one compound as one ratioDefinite.
  3. Refuse a different mix as the same compoundHonest.

6Dalton’s Atomic Theory

Dalton’s school theory: bits (atoms) of an element are alike in that story; they combine in small whole numbers. It earned the laws a particle-picture. Later headings will split the “alike” — do not dump the nucleus here.

A marble-jar is a model, not the whole modern atom.

Figure. Dalton: atoms are tiles that combine in simple whole numbers. Two hydrogen tiles and one oxygen tile make one water unit — never a fraction of a tile. Squares stand for atoms because this vocabulary has no circle primitive.

How it works

  1. Name atoms as the tiny unitsThe picture.
  2. Name small whole-number combinesThe use.
  3. Keep later splits for the atom chapterThis class’s Dalton-size.

7Atomic and Molecular Masses

Atomic mass is a relative school-mass (C-12 scale as taught); molecular mass is the sum for a formula. H=1, C=12, O=16 → H2O is 18 u. A mass is a sum, not a weigh of one molecule on a kitchen scale.

Using 18 as oxygen because water is 18 drops an H and swaps the heading.

Figure. Atomic mass is counted on the carbon-12 scale: C is exactly 12 u. School values H = 1 u and O = 16 u then make H₂O = 18 u by addition, 1+1+16. The bars are those numbers, so H₂O is as long as H+H+O.

How it works

  1. Write the formulaThe given.
  2. Sum atomic masses × countsThe molecular mass.
  3. Keep u or the taught scaleRelative.

8Mole Concept and Molar Masses

A mole is the school pack of 6.022×10²³ bits (as taught) and the molar mass in grams is the same number as the u-mass. 18 g of H2O is 1 mol; 36 g is 2 mol. Mole is a count-word, not a small animal.

Calling 18 g “18 moles” is a unit-swap.

Figure. A mole is an amount of specified particles, not a weight. 36 g of water at M = 18 g/mol is 36/18 = 2.00 mol, then 2.00 × 6.022×10²³ = 1.204×10²⁴ molecules.

How it works

  1. Name the grams and the molar massThe given.
  2. n = m/MThe mole.
  3. Keep N = n × N_A if a bit-count was askedThe pack.

36 g water

How many moles in 36 g of H2O (M=18 g/mol)?

  • n=m/M36/18
  • n2 mol
  • Bits if asked2 × 6.022×10²³

Pro tip. Molar mass in g/mol; 18 g is one pack.

9Percentage Composition

Percentage composition is the mass-share of an element in the formula: (count × atomic mass / molar mass) × 100. In H2O, H is 2/18 ≈ 11%. A percent is a share, not a mole-fraction unless asked.

Using 2/3 as the H percent because two of three letters are H is a miss.

Figure. Percentage composition is each element’s mass over the formula mass. For H₂O on the school scale, 2/18 = 11.1% hydrogen and 16/18 = 88.9% oxygen. The stack is 2 u against 16 u, so the percentages are those lengths of the same 18 u bar.

How it works

  1. Write the formula and the molar massThe given.
  2. Form the element’s mass-shareThe percent.
  3. Keep it a mass-percent unless the lesson asked molesHonest.

10Stoichiometry and Stoichiometric Calculations

Stoichiometry: the balanced equation’s coefficients are mole-ratios. 2 H2 + O2 → 2 H2O means 2 mol H2 need 1 mol O2. A leftover reagent is the limiting-story if taught. You may not invent a coefficient to make grams pretty.

Treating coefficients as gram-ratios is a miss.

Figure. Coefficients are mole ratios. For 2 H₂ + O₂ → 2 H₂O, 4 mol H₂ needs 2 mol O₂ and makes 4 mol H₂O. If 3 mol O₂ were supplied, 1 mol O₂ is leftover. Atoms are rearranged, not deleted.

How it works

  1. Balance, then read coefficients as molesThe ratio.
  2. Convert grams to moles, then use the ratioThe calculation.
  3. Name the limiting if two amounts were givenHonest leftover.
36 g of H2O (18 g/mol) is
  1. 2 mol
  2. 18 mol
  3. 36 mol

n=m/M.

Notes

  • Mapped to the official NCERT chapter “Some Basic Concepts of Chemistry”. Original teaching only — no textbook sentences.
  • Science here is Physics, Chemistry and Biology ideas at this class, never a language or social-science chapter.

Formulas

  • n=m/M
  • N=n N_A
  • mass % = (count×A/M)×100

Recap

Hold these pegs from the official chapter “Some Basic Concepts of Chemistry”. The wording is ExamMaster’s teaching, not a textbook recap.

Importance of Chemistry
Chemistry here is the school study of substances and how they change — a job-list (measure, name, count bits), not a poster of “lab coats”.
Nature of Matter
Matter is anything that has mass and takes space in the school test; it sits as solid, liquid, or gas as looks, or as mixture versus pure as a sort.
Properties of Matter and their Measurement
Properties are looks you can measure: mass on a balance, volume in a marked jar, a temperature.
Uncertainty in Measurement
Uncertainty: a measured 12.4 cm is not infinitely sharp; significant figures and a taught plus-minus are the honesty.
Laws of Chemical Combinations
Laws of combination (school): mass is conserved in a closed change; a given compound keeps a definite mass ratio; gases in the taught volume-story combine in simple ratios.
Dalton’s Atomic Theory
Dalton’s school theory: bits (atoms) of an element are alike in that story; they combine in small whole numbers.

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