Class 11 Chemistry • Foundation and Fundamentals
Language of Chemistry
Original Nepal eNotes title • Current NEB/CDC Unit 1 aligned
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Current curriculum alignment: The original Nepal eNotes page uses the older title Language of Chemistry. The current Grade 11 Chemistry curriculum organizes the matching introductory content as Unit 1 – Foundation and Fundamentals (2 teaching hours): general introduction, importance and scope of chemistry, atoms and molecules, relative atomic/molecular/formula masses, atomic mass unit, radicals/ions, molecular and empirical formulae, and percentage composition from a molecular formula. Detailed mole calculations, laws of stoichiometry, limiting reagent, yield and empirical-formula calculation from percentage data belong to Unit 2 – Stoichiometry.
1. What is Chemistry?
Chemistry
Chemistry is the branch of science concerned with the composition, structure, properties and transformations of matter and the energy changes associated with chemical processes.
Chemistry is the branch of science concerned with the composition, structure, properties and transformations of matter and the energy changes associated with chemical processes.
Chemical symbols, formulae, equations and relative masses form a precise language that allows chemists to communicate the identity and composition of substances.
Diagram 1: Composition, structure, properties and chemical change
2. Importance and Scope of Chemistry
Medicine
Drugs, disinfectants, diagnostics and biomolecules.
Agriculture
Soil chemistry, nutrients and fertilizers.
Industry
Metals, polymers, cement, glass, dyes and fuels.
Environment
Pollution analysis, water treatment and waste control.
Food
Composition, preservation, fermentation and quality control.
Energy & Materials
Batteries, fuels, corrosion control and advanced materials.
Exam Important
When asked about the scope of chemistry, connect the field to real chemical analysis, manufacture, safety, health, agriculture, environment or materials—not just a list of industries.
When asked about the scope of chemistry, connect the field to real chemical analysis, manufacture, safety, health, agriculture, environment or materials—not just a list of industries.
3. Atoms, Molecules and Ions
Atom
The smallest electrically neutral particle of an element that retains the chemical identity of that element.
The smallest electrically neutral particle of an element that retains the chemical identity of that element.
Molecule
An electrically neutral group of two or more atoms held together by chemical bonds and capable of existing as a discrete entity.
An electrically neutral group of two or more atoms held together by chemical bonds and capable of existing as a discrete entity.
Ion
An atom or bonded group of atoms carrying a net electric charge.
An atom or bonded group of atoms carrying a net electric charge.
| Species | Meaning | Examples |
|---|---|---|
| Atom | Neutral elemental particle | Na, He, C |
| Molecule | Neutral bonded particle | H₂, O₂, H₂O, CO₂ |
| Cation | Positive ion | Na⁺, Ca²⁺, NH₄⁺ |
| Anion | Negative ion | Cl⁻, NO₃⁻, SO₄²⁻ |
Diagram 2: Atom, molecule and ion
Important terminology
Ionic solids such as NaCl do not consist of discrete NaCl molecules; they are represented by formula units.
Ionic solids such as NaCl do not consist of discrete NaCl molecules; they are represented by formula units.
4. Radicals / Polyatomic Ions
Older school chemistry often uses the term compound radical for a charged group of atoms acting as a unit. Modern terminology usually calls these polyatomic ions.
| Name | Formula | Charge |
|---|---|---|
| Ammonium | NH₄⁺ | +1 |
| Hydroxide | OH⁻ | −1 |
| Nitrate | NO₃⁻ | −1 |
| Carbonate | CO₃²⁻ | −2 |
| Sulfate | SO₄²⁻ | −2 |
| Phosphate | PO₄³⁻ | −3 |
5. Atomic Mass Unit (amu or u)
Unified atomic mass unit
One unified atomic mass unit is one-twelfth of the mass of a neutral carbon-12 atom in its ground state.
One unified atomic mass unit is one-twelfth of the mass of a neutral carbon-12 atom in its ground state.
1 u = 1/12 × mass of one ¹²C atom1 u ≈ 1.66054 × 10⁻²⁷ kg
The older notation amu is still common in school materials; the modern symbol is u.
Diagram 3: Definition of 1 u
6. Relative Atomic Mass
Relative atomic mass, Ar
The weighted mean mass of an atom of an element compared with one-twelfth of the mass of a carbon-12 atom.
The weighted mean mass of an atom of an element compared with one-twelfth of the mass of a carbon-12 atom.
Ar = average mass of an atom / (1/12 mass of ¹²C atom)
Ar is a ratio and therefore has no unit. Many values are non-integral because naturally occurring elements may contain several isotopes.
7. Relative Molecular Mass and Relative Formula Mass
Relative molecular mass, Mr
The mass of a molecule relative to one-twelfth of the mass of a carbon-12 atom.
The mass of a molecule relative to one-twelfth of the mass of a carbon-12 atom.
Mr = Σ(Ar × number of each atom)
H₂O: Mr = 2(1) + 16 = 18.
CO₂: Mr = 12 + 2(16) = 44.
Relative formula mass
The sum of relative atomic masses represented by a chemical formula; this is the preferred term for ionic formula units.
The sum of relative atomic masses represented by a chemical formula; this is the preferred term for ionic formula units.
NaCl: 23 + 35.5 = 58.5.
Common mistake
Relative molecular/formula mass is dimensionless. Molar mass, by contrast, has units such as g mol⁻¹.
Relative molecular/formula mass is dimensionless. Molar mass, by contrast, has units such as g mol⁻¹.
8. Valency, Charge Balance and Simple Formula Writing
For an ionic compound, total positive charge must equal total negative charge.
| Ions | Charge balance | Formula |
|---|---|---|
| Na⁺ + Cl⁻ | +1 −1 = 0 | NaCl |
| Ca²⁺ + Cl⁻ | +2 + 2(−1) = 0 | CaCl₂ |
| Al³⁺ + O²⁻ | 2(+3) + 3(−2) = 0 | Al₂O₃ |
| NH₄⁺ + SO₄²⁻ | 2(+1) −2 = 0 | (NH₄)₂SO₄ |
Diagram 4: Writing a neutral ionic formula
9. Molecular Formula
Molecular formula
Shows the actual number of atoms of each element present in one molecule.
Shows the actual number of atoms of each element present in one molecule.
- H₂O: 2 H and 1 O
- H₂O₂: 2 H and 2 O
- C₆H₁₂O₆: 6 C, 12 H and 6 O
10. Empirical Formula
Empirical formula
Shows the simplest whole-number ratio of atoms of the elements in a compound.
Shows the simplest whole-number ratio of atoms of the elements in a compound.
| Substance | Molecular formula | Empirical formula |
|---|---|---|
| Hydrogen peroxide | H₂O₂ | HO |
| Glucose | C₆H₁₂O₆ | CH₂O |
| Benzene | C₆H₆ | CH |
| Water | H₂O | H₂O |
Molecular formula = (Empirical formula)n
Syllabus boundary
Unit 1 includes the concepts of molecular and empirical formulae. Detailed calculation of empirical/molecular formula from percentage composition is treated in Unit 2 – Stoichiometry.
Unit 1 includes the concepts of molecular and empirical formulae. Detailed calculation of empirical/molecular formula from percentage composition is treated in Unit 2 – Stoichiometry.
Diagram 5: Molecular formula compared with empirical formula
11. Percentage Composition from a Molecular Formula
Percentage composition
The mass percentage of each constituent element in a compound, calculated from the formula and relative atomic masses.
The mass percentage of each constituent element in a compound, calculated from the formula and relative atomic masses.
% element = (mass contribution of element / total relative molecular or formula mass) × 100
- Write the correct formula.
- Calculate the total Mr or relative formula mass.
- Find the mass contribution of the required element.
- Divide by total mass and multiply by 100.
Diagram 6: Percentage-composition method
12. Worked Examples
Example 1 — Mᵣ of NH₃14 + 3(1) = 17
Example 2 — Formula mass of CaCO₃40 + 12 + 3(16) = 100
Example 3 — Empirical formula of C₂H₄
2:4 simplifies to 1:2 → CH₂.
Example 4 — Percentage composition of H₂O%H = [2(1)/18]×100 ≈ 11.1%%O = (16/18)×100 ≈ 88.9%
Example 5 — Percentage carbon in CO₂%C = (12/44)×100 ≈ 27.3%
Example 6 — Formula from ions
Ca²⁺ requires two NO₃⁻ ions → Ca(NO₃)₂.
13. Common Exam Mistakes
- Incorrect symbol capitalization: CL instead of Cl, NA instead of Na.
- Confusing Co (cobalt) with CO (carbon monoxide).
- Calling every ionic formula a molecule.
- Confusing atoms, molecules and ions.
- Using “free radical” as a synonym for the syllabus word “radical”; the syllabus usage often means polyatomic ion.
- Giving Ar or Mr units. They are dimensionless ratios.
- Confusing relative molecular mass with molar mass in g mol⁻¹.
- Ignoring subscripts when adding relative atomic masses.
- Failing to reduce molecular subscripts to the simplest whole-number ratio for empirical formula.
- Writing H₂O₂ as its empirical formula instead of HO.
- Forgetting parentheses in Ca(NO₃)₂ or (NH₄)₂SO₄.
- Using the element mass contribution rather than total formula mass as the denominator in percentage composition.
- Forgetting ×100 in percentage composition.
- Failing to check that elemental percentages total approximately 100%.
- Mixing Unit 1 with full Stoichiometry topics such as limiting reagent and theoretical yield.
14. Important Exam Questions
Short Questions
- Define chemistry and state its importance.
- Define atom, molecule and ion.
- Differentiate cation and anion.
- What is a compound radical/polyatomic ion?
- Define 1 u.
- Define relative atomic mass.
- Why can Ar be fractional?
- Define relative molecular mass and relative formula mass.
- Differentiate Mr and molar mass.
- Define molecular formula and empirical formula.
- Find the empirical formula of H₂O₂, C₆H₆ and C₆H₁₂O₆.
- Write formulae from given ionic charges.
- Define percentage composition and write its formula.
Numerical Questions
- Calculate Mr of H₂O, NH₃, CO₂, CH₄ or H₂SO₄.
- Calculate relative formula mass of NaCl, CaCO₃ or Al₂O₃.
- Calculate percentage composition of H₂O.
- Calculate percentage carbon in CO₂.
- Calculate percentage nitrogen in NH₃.
- Calculate percentage composition of CaCO₃.
Exam Strategy
Master five anchors: scope of chemistry → atom/molecule/ion → relative masses → molecular vs empirical formula → percentage composition. Detailed mole and stoichiometric calculations belong to Unit 2.
Master five anchors: scope of chemistry → atom/molecule/ion → relative masses → molecular vs empirical formula → percentage composition. Detailed mole and stoichiometric calculations belong to Unit 2.
15. One-Minute Revision
- Original source title: Language of Chemistry; current Unit 1 title: Foundation and Fundamentals.
- Unit 1 is under General and Physical Chemistry.
- Chemistry studies composition, structure, properties and changes of matter.
- Atom = neutral elemental particle; molecule = neutral bonded group; ion = charged species.
- Cation is positive; anion is negative.
- Older “compound radical” generally corresponds to a polyatomic ion.
- 1 u = 1/12 mass of one carbon-12 atom.
- Ar, Mr and relative formula mass are dimensionless.
- Molecular formula gives actual atom numbers.
- Empirical formula gives the simplest whole-number ratio.
- H₂O₂ → HO; C₆H₁₂O₆ → CH₂O.
- Ionic formulae must have zero total charge.
- % element = element mass contribution / total formula mass × 100.
- Element percentages should total about 100%.
- Full stoichiometry belongs to Unit 2.
16. Diagram Practice
- Scope of chemistry concept map.
- Atom, molecule and ion.
- Carbon-12 reference for 1 u.
- Charge balance in an ionic formula.
- Molecular vs empirical formula.
- Percentage composition workflow.
Source handling: The original Nepal eNotes Language of Chemistry PDF remains embedded above using the verified Google Drive file. The current NEB/CDC Grade 11 Chemistry curriculum organizes the matching introductory material as Unit 1 – Foundation and Fundamentals. The typed section follows that current scope and deliberately leaves laws of stoichiometry, mole calculations, limiting reactants, yields and empirical/molecular-formula determination from percentage data to Unit 2 – Stoichiometry. Where the embedded PDF does not expose searchable text, this typed section is a syllabus-aligned study companion and is not claimed to be a word-for-word transcription.
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