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Chemistry · Class 11 Science

Ch 1Some Basic Concepts of Chemistry — Class 11 Chemistry, concept-first.

Matter around us — a stick of chalk, the air we breathe, the water we drink — is built from a small set of fundamental building blocks. Understanding those building blocks, and the precise language chemists use for them, is the first step in the study of chemistry.

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Key concepts

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Laws of Chemical Combination & Dalton's Atomic Theory

This concept owns the qualitative and classic-numerical foundation of how elements combine, before any mole arithmetic. IN SCOPE: the nature of matter and the ideas atom, molecule, element and compound; Dalton's atomic t…

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Chapter contents

The NCERT structure, section by section. Open a section to see its questions, then read the concept-first solution.

1.1

Elements, Atoms and Molecules

Matter around us — a stick of chalk, the air we breathe, the water we drink — is built from a small set of fundamental building blocks.

1.2

Laws of Chemical Combination and Dalton's Atomic Theory

By the end of the eighteenth century, careful weighing experiments on chemical reactions had revealed several striking regularities.

1.3

Atomic and Molecular Masses

Individual atoms are too small and too light to weigh on any balance, so chemists never state atomic mass in grams directly. Instead, atomic masses are expressed on a relative scale.

1.4

Mole Concept and Molar Mass

Chemical reactions happen atom-to-atom and molecule-to-molecule, but no laboratory balance can weigh a single atom.

1.5

Percentage Composition, Empirical and Molecular Formula

Percentage composition. Once the molar mass of a compound and the contribution of each element to that molar mass are known, it is straightforward to calculate what percentage of the compound's total…

1.6

Chemical Reactions and Stoichiometry

A balanced chemical equation is far more than a description of what reacts with what — its coefficients are a precise numerical recipe, stating the exact ratio in which moles of reactants combine and…

1.7

Concentration Terms of Solutions

Most chemical reactions studied in the laboratory take place between substances dissolved in a solvent, not between pure solids or gases — so chemists need precise, standardised ways of stating how mu…

1.8

Summary

This chapter has built, step by step, the quantitative toolkit that underlies the whole of chemistry:

Sample & Board Papers

Sample papers and previous-year board questions for this subject.

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  1. Q1The empirical formula of an organic compound is CH₂O and its molecular weight is 180. What is the molecular formula of the compound? (H = 1,…Preview

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  1. Example 1When $5.6\ \text{g}$ of calcium carbonate ($\text{CaCO}_3$) is heated strongly, it decomposes completely as $\text{CaCO}_3 \rightarrow \text…Free
  2. Example 2Nitrogen and hydrogen combine to form ammonia according to $\text{N}_2(g) + 3\text{H}_2(g) \rightarrow 2\text{NH}_3(g)$. If $10\ \text{L}$ o…Free
  3. Example 3Naturally occurring chlorine consists of two isotopes: $^{35}\text{Cl}$ (mass $34.97\ \text{u}$, abundance $75.77\%$) and $^{37}\text{Cl}$ (…Free
  4. Example 4Calculate the molecular mass of glucose, $\text{C}_6\text{H}_{12}\text{O}_6$, given the atomic masses $\text{C} = 12.01\ \text{u}$, $\text{H…Preview
  5. Example 5Calculate (a) the number of moles and (b) the number of molecules present in $9.8\ \text{g}$ of sulphuric acid, $\text{H}_2\text{SO}_4$ (mol…Preview
  6. Example 6How many atoms of sodium are present in $0.5\ \text{mol}$ of sodium metal? (Take Avogadro's number $N_A = 6.022 \times 10^{23}\ \text{mol}^{…Preview
  7. Example 7Calculate the percentage composition (by mass) of calcium, carbon and oxygen in calcium carbonate, $\text{CaCO}_3$ (molar mass $100.09\ \tex…Preview
  8. Example 8A compound of carbon, hydrogen and oxygen was found on analysis to contain $40.0\%$ carbon, $6.7\%$ hydrogen and $53.3\%$ oxygen by mass. De…Preview
  9. Example 9For the reaction $\text{N}_2(g) + 3\text{H}_2(g) \rightarrow 2\text{NH}_3(g)$, calculate the mass of ammonia ($\text{NH}_3$) that can be pro…Preview
  10. Example 10$4.0\ \text{g}$ of sodium hydroxide ($\text{NaOH}$, molar mass $40.0\ \text{g mol}^{-1}$) is dissolved in enough water to make $250\ \text{m…Preview
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  1. Q11Sample A of water is decomposed and found to contain $1\ \text{g}$ of hydrogen and $8\ \text{g}$ of oxygen. Sample B of water, obtained from…Free
  2. Q12Nitrogen forms two oxides. In the first oxide, $14\ \text{g}$ of nitrogen combines with $16\ \text{g}$ of oxygen. In the second oxide, $14\…Free
  3. Q13A closed flask of volume $2\ \text{L}$ contains oxygen gas, and an identical flask of the same volume, at the same temperature and pressure,…Free
  4. Q14State the main postulates of Dalton's atomic theory. Explain, using the theory, why the law of conservation of mass must always hold true du…Preview
  5. Q15Sodium chloride, $\text{NaCl}$, does not exist as discrete molecules but as an ionic lattice. Calculate its formula mass, given $\text{Na} =…Preview
  6. Q16Calculate the molecular mass of sulphuric acid, $\text{H}_2\text{SO}_4$, given $\text{H} = 1.008\ \text{u}$, $\text{S} = 32.07\ \text{u}$, $…Preview
  7. Q17Calculate the mass of $0.25\ \text{mol}$ of carbon dioxide gas, $\text{CO}_2$ (molar mass $44.01\ \text{g mol}^{-1}$).Preview
  8. Q18A sample of oxygen gas occupies $44.8\ \text{L}$ at STP ($22.4\ \text{L mol}^{-1}$). Calculate (a) the number of moles and (b) the mass of o…Preview
  9. Q19Calculate the number of moles of water in $90\ \text{g}$ of water, and hence the total number of hydrogen atoms present (molar mass of $\tex…Preview
  10. Q20Calculate the percentage composition (by mass) of carbon, hydrogen and oxygen in glucose, $\text{C}_6\text{H}_{12}\text{O}_6$ (molar mass $1…Preview
  11. Q21The empirical formula of a carbohydrate is $\text{CH}_2\text{O}$ (empirical formula mass $30.03\ \text{g mol}^{-1}$). If its molecular mass…Preview
  12. Q22$4.0\ \text{g}$ of hydrogen gas is mixed with $40.0\ \text{g}$ of oxygen gas and ignited to form water, $2\text{H}_2(g) + \text{O}_2(g) \rig…Preview
  13. Q23Magnesium burns in oxygen as $2\text{Mg}(s) + \text{O}_2(g) \rightarrow 2\text{MgO}(s)$. Calculate the mass of magnesium oxide formed when $…Preview
  14. Q24Methane burns in oxygen as $\text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(l)$. Calculate the volume of $\…Preview
  15. Q25$5.85\ \text{g}$ of sodium chloride ($\text{NaCl}$, molar mass $58.44\ \text{g mol}^{-1}$) is dissolved in $500\ \text{g}$ of water. Calcula…Preview
  16. Q26A solution is prepared by mixing $36\ \text{g}$ of water ($\text{H}_2\text{O}$, molar mass $18.02\ \text{g mol}^{-1}$) with $46\ \text{g}$ o…Preview
  17. Q27$20\ \text{g}$ of common salt ($\text{NaCl}$) is dissolved in $180\ \text{g}$ of water to give $200\ \text{g}$ of solution. Calculate the ma…Preview
  18. Q28$4.9\ \text{g}$ of sulphuric acid ($\text{H}_2\text{SO}_4$, molar mass $98.09\ \text{g mol}^{-1}$, $n\text{-factor} = 2$) is dissolved in wa…Preview