Foundation and Fundamentals

Everything you can touch, breathe or drink is matter, and chemistry is the science of what matter is made of and how it changes. This short opening unit fixes the vocabulary and the counting rules that the other sixteen units will use without further comment: atom, molecule, radical, atomic mass unit, empirical formula, molecular formula. Get these exactly right now and the stoichiometry in Unit 2 becomes arithmetic instead of guesswork.

1.1 General introduction of chemistry

Matter is anything that has mass and occupies space. Chemistry studies matter at the level of its particles — atoms, molecules and ions — and explains bulk behaviour (why iron rusts, why ice floats, why a lemon tastes sour) in terms of what those particles do.

The subject grew out of alchemy, the medieval search for a way to turn base metals into gold. It became a quantitative science when Antoine Lavoisier (1743–1794) began weighing his reactants and products and stated the law of conservation of mass; he is called the father of modern chemistry. John Dalton's atomic theory (1808) then gave the weighings a particle explanation, and the modern subject follows from there.

Branch	Deals with
Physical chemistry	Principles and laws governing chemical change — energy, rates, equilibrium, structure
Inorganic chemistry	Elements and their compounds other than hydrocarbons and their derivatives
Organic chemistry	Compounds of carbon: hydrocarbons and their derivatives
Analytical chemistry	Detection (qualitative) and estimation (quantitative) of substances
Biochemistry	Chemical processes inside living organisms
Industrial / applied chemistry	Manufacture of chemicals on a commercial scale
Nuclear chemistry	Changes in the nucleus: radioactivity, fission, fusion
Environmental chemistry	Chemical species in air, water and soil, and pollution

1.2 Importance and scope of chemistry

Chemistry is called the central science because physics, biology, geology, agriculture, medicine and engineering all borrow from it.

Field	What chemistry contributes
Agriculture	Fertilizers (urea, DAP, potash), pesticides, soil-pH correction with lime
Medicine	Drug synthesis — antibiotics, analgesics, vaccines, anaesthetics
Industry	Cement, glass, paper, soap and detergents, paints, plastics, textiles
Energy	Petroleum refining, LPG, biogas, batteries, solar-cell materials
Food	Preservatives, iodisation of salt, vitamins, food testing for adulteration
Environment	Water purification, monitoring of SO₂ and CO₂, ozone-layer studies
Materials	Alloys, semiconductors, ceramics, nano-materials, polymers

In Nepal the scope is very concrete. Cement plants at Udayapur and Hetauda convert limestone (CaCO₃) into clinker; iodised salt distributed by the Salt Trading Corporation has removed goitre from most hill districts; arsenic test kits in the Terai screen tube-well water; and the country imports several hundred thousand tonnes of urea and DAP every year, so knowing the nitrogen content of a fertilizer bag is a genuinely useful piece of chemistry.

1.3 Basic concepts

Atoms and molecules

The number of atoms present in one molecule of a substance is its atomicity.

Atomicity	Meaning	Examples
Monoatomic	1 atom per molecule	He, Ne, Ar, Na (vapour)
Diatomic	2 atoms	H₂, O₂, N₂, Cl₂, HCl, CO
Triatomic	3 atoms	O₃, H₂O, CO₂
Tetra-atomic	4 atoms	NH₃, P₄
Polyatomic	many atoms	S₈, C₆H₁₂O₆

Relative masses and the atomic mass unit

An atom of oxygen weighs about 2.66 × 10⁻²³ g. Numbers like this are useless in the laboratory, so chemists compare masses instead of quoting them. The agreed standard is the carbon-12 atom.

Relative atomic mass of an element is the number of times one atom of that element is heavier than one-twelfth of a carbon-12 atom. It is a pure ratio, so it has no unit. Expressed in u it is called the atomic mass.

Relative molecular mass is defined in the same way for a molecule, and is obtained simply by adding the atomic masses of all the atoms in the formula. For ionic compounds, which have no discrete molecules, the same sum is called the formula mass.

Radicals

Valency	Basic radicals (cations)	Acid radicals (anions)
1	Na⁺, K⁺, NH₄⁺, Ag⁺	Cl⁻, NO₃⁻, OH⁻, HCO₃⁻
2	Mg²⁺, Ca²⁺, Fe²⁺, Cu²⁺, Zn²⁺	SO₄²⁻, CO₃²⁻, O²⁻, S²⁻
3	Al³⁺, Fe³⁺	PO₄³⁻, N³⁻

A formula is written by criss-crossing the valencies and reducing: Al³⁺ with SO₄²⁻ gives Al₂(SO₄)₃; Ca²⁺ with PO₄³⁻ gives Ca₃(PO₄)₂.

Molecular formula and empirical formula

They are connected by one integer n:

Compound	Molecular formula	Empirical formula	n
Water	H₂O	H₂O	1
Hydrogen peroxide	H₂O₂	HO	2
Benzene	C₆H₆	CH	6
Ethyne	C₂H₂	CH	2
Glucose	C₆H₁₂O₆	CH₂O	6
Acetic acid	C₂H₄O₂	CH₂O	2

1.4 Percentage composition from molecular formula

The percentage composition of a compound is the mass of each element present in 100 g of the compound. From the molecular formula:

The percentages of all elements must add up to 100 (allow ±0.1 for rounding) — that is your free check on the arithmetic.

Chapter summary

• Chemistry studies the composition, structure, properties and transformations of matter; its main branches are physical, inorganic, organic, analytical, biochemistry, industrial, nuclear and environmental.
• An atom is the smallest particle of an element taking part in a reaction; a molecule is the smallest particle able to exist independently.
• 1 atomic mass unit = 1/12 of the mass of one ¹²C atom = 1.66054 × 10⁻²⁴ g.
• Relative atomic and molecular masses are ratios and carry no unit; molecular mass is the sum of the atomic masses in the formula.
• A radical is a charged atom or group of atoms that acts as one unit; formulae are built by criss-crossing valencies.
• Molecular formula = n × empirical formula, with n = \text{molecular mass} / \text{empirical formula mass}.
• Percentage of X = (atomic mass of X × number of X atoms) / molecular mass × 100; all percentages must total 100.

Practice questions

Group A — Multiple choice (1 mark each)

1. One atomic mass unit is equal to [1]amass of one ¹²C atomb1/12 of the mass of one ¹²C atomcmass of one hydrogen atomd1/16 of the mass of one oxygen atom
2. The empirical formula of hydrogen peroxide (H₂O₂) is [1]aH₂O₂bHOcH₂OdHO₂
3. The percentage of nitrogen in ammonium nitrate, NH₄NO₃, is [1]a17.5 %b21.2 %c35.0 %d46.7 %
4. Which of the following is a triatomic molecule? [1]aHebCl₂cO₃dP₄
5. Relative atomic mass has the unit [1]agrambamuckg mol⁻¹dno unit

Answers to Group A

1. (b) — that is the definition of the amu on the carbon-12 scale.

2. (b) — divide both subscripts by the HCF 2.

3. (c) — M = 80 and 2 N atoms give 28, so 28/80 × 100 = 35.0 %.

4. (c) — ozone has three atoms per molecule.

5. (d) — it is a ratio of two masses, so the units cancel.

Group B — Short answer (5 marks each)

1. Define atom, molecule and radical, giving one example of each. Why can an atom of oxygen not exist freely while a molecule of oxygen can? [5]
2. What is meant by atomic mass unit? Calculate the mass in grams of one molecule of carbon dioxide. [5]
3. Distinguish between empirical formula and molecular formula with two examples each. A compound has the empirical formula CH₂ and molecular mass 56; find its molecular formula. [5]
4. Calculate the percentage composition of calcium carbonate, CaCO₃, and hence the mass of calcium oxide obtainable from 1 tonne of pure limestone. (Ca = 40, C = 12, O = 16) [5]

Answers to Group B

2. 1 u = 1/12 of the mass of a ¹²C atom = 1.66054 × 10⁻²⁴ g. For CO₂, M = 12 + 32 = 44, so one molecule has mass 44 × 1.66054 × 10⁻²⁴ g = 7.31 × 10⁻²³ g.

3. Empirical formula mass of CH₂ = 12 + 2 = 14, so n = 56/14 = 4 and the molecular formula is C₄H₈.

4. M(CaCO₃) = 40 + 12 + 48 = 100. %Ca = 40 %, %C = 12 %, %O = 48 %. On heating, CaCO₃ → CaO + CO₂. Every 100 g of CaCO₃ gives 56 g of CaO, so 1 tonne (1000 kg) gives 560 kg of CaO.

Group C — Long answer (8 marks each)

1. (a) Explain why chemistry is called the central science, listing four fields in which it is applied with one concrete example from Nepal in each. [4](b) State the relation between empirical formula, molecular formula and molecular mass, and use it to show that acetic acid and glucose share an empirical formula. [4]
2. A farmer can buy either urea, CO(NH₂)₂, at Rs 1,000 per 50 kg bag or ammonium sulphate, (NH₄)₂SO₄, at Rs 600 per 50 kg bag. [8]aCalculate the percentage of nitrogen in each.bCalculate the mass of nitrogen in one bag of each.cFind the cost per kilogram of nitrogen for each and say which is cheaper.

Answer to Group C question 2

(a) Urea: M = 60, N = 28, so %N = 28/60 × 100 = 46.67 %. Ammonium sulphate: M = 132, N = 28, so %N = 28/132 × 100 = 21.21 %.

(b) Urea bag: 0.4667 × 50 = 23.33 kg of N. Ammonium sulphate bag: 0.2121 × 50 = 10.61 kg of N.

(c) Urea: Rs 1000 / 23.33 kg = Rs 42.9 per kg of N. Ammonium sulphate: Rs 600 / 10.61 kg = Rs 56.6 per kg of N. Urea is the cheaper source of nitrogen, even though the bag costs more.