1. (i) Sodium hydrogencarbonate = NaHCO₃
2. (ii) Sulfur dioxide = SO₂
3. (iii) Ferric chloride = FeCl₃
4. (iv) Cuprous oxide = Cu₂O

1. (i) Sodium oxide: Na (valency 1), O (valency 2). Cross over valencies: Na₂O₁ → Na₂O.
2. (ii) Aluminium chloride: Al (valency 3), Cl (valency 1). Cross over: Al₁Cl₃ → AlCl₃.
3. (iii) Sodium Sulphide: Na (valency 1), S (valency 2). Cross over: Na₂S₁ → Na₂S.
4. (iv) Magnesium hydroxide: Mg (valency 2), OH (valency 1). Cross over: Mg₁(OH)₂ → Mg(OH)₂.

1. Atoms are very small, smaller than anything that we can imagine or compare with.
2. For example, more than millions of atoms when stacked together would make a layer as thick as a sheet of paper.

1. Mass of 6.022 × 10²³ molecules (1 mole) = 6.022×10²³ × (4.65×10⁻²³) = 28 g.
2. The substance could be carbon monoxide (CO), with molecular mass = 12 + 16 = 28 u, matching this value.

1. A chemical formula is the representation of the elements present in a compound using symbols, along with the number of atoms of each element (as subscript numbers).
2. For example, a molecule of water (compound) contains 2 atoms of hydrogen and 1 atom of oxygen, hence its chemical formula is H₂O.

1. When a solution is formed by dissolving salt in water, the total mass of the solution remains equal to the sum of the masses of salt and water used.
2. This shows that there is practically no change in mass during this physical change, indicating that mass is conserved in the process of solution formation.

1. Gram atomic mass of Na = 23 g, so 23 g of Na represents 1 mole.
2. 11.5 g of Na represents = (1 mol × 11.5 g)/23.0 g = 0.5 mol.

1. In all chemical reactions, there is only an exchange/rearrangement of reactants taking place when products are formed.
2. Since there is no loss or gain of mass in this process, chemical reactions always follow the law of conservation of mass.

1. The mass of a molecule is the mass of a single molecule — also known as its actual mass.
2. The molecular mass is the mass of 1 mole of that molecule, i.e. equal to the combined mass of Avogadro's number (6.022 × 10²³) of molecules, expressed in grams.

1. Gram molar mass of NaOH = 23 + 16 + 1 = 40 g. So 40 g of NaOH = 1 mol.
2. 1 g of NaOH = 1/40 mol. So 4 g of NaOH = (1/40) × 4 = 0.1 mole.

1. (i) 28 g of He: moles = 28/4 = 7 mol.
2. (ii) 46 g of Na: moles = 46/23 = 2 mol.
3. (iii) 60 g of Ca: moles = 60/40 = 1.5 mol.

1. (a) True. An ion is indeed a positively or negatively charged particle.
2. (b) False. A pure chemical compound always consists of the same elements combined together in a fixed proportion by mass, not a "different" proportion.
3. (c) False. The correct formula of Nickel sulphide is NiS (nickel has valency 2, sulphide has valency 2, giving a 1:1 ratio).

1. (a) Grams — 1 mole = atomic/molecular mass expressed in grams.
2. (b) Molecule — a molecule is a group of two or more atoms chemically bonded together.
3. (c) Symbol — a symbol is a letter or sketch that stands for something else.

1. Each formula is found by balancing the total positive and negative charge between the cation and anion, criss-crossing valencies where needed and simplifying ratios.

1. By the Law of Constant Proportions, the ratio of elements in a compound is always fixed. Carbon : Oxygen = 3 : 4.
2. 3 parts of C = 9 g, so 1 part = 9/3 = 3 g.
3. Oxygen (4 parts) = 4 × 3 = 12 g.

1. (i) Aluminium nitrate: Al³⁺ and NO₃⁻. Balancing charges (1 Al needs 3 NO₃): Al(NO₃)₃.
2. (ii) Calcium oxide: Ca²⁺ and O²⁻. Charges balance 1:1: CaO.
3. (iii) Ferric oxide: Fe³⁺ and O²⁻. Balancing charges (2 Fe gives 6+, 3 O gives 6−): Fe₂O₃.

1. The electronic configuration of fluorine (atomic number 9) is 2, 7 — so each fluorine atom has 7 valence electrons and needs 1 more electron to complete its octet.
2. To achieve stability, two fluorine atoms share one pair of electrons, forming a single covalent bond. This way, each atom attains a stable octet.
3. Structure: F−F, with each fluorine atom having one shared pair (single bond) and three lone pairs of electrons.

1. (i) M loses 2 electrons to form M²⁺; O gains 2 electrons to form O²⁻. Balancing charges gives the formula MO.
2. (ii) Formed by transfer of electrons from metal to non-metal, so it is an ionic bond.
3. (iii) The compound dissolves slightly in water, forming free M²⁺ and O²⁻ ions. These ions move freely in solution and carry electric current, so its aqueous solution conducts electricity (good conductor).

1. (i) ²⁷Al (neutral atom): Electrons = Protons = 13. Neutrons = mass number − protons = 27 − 13 = 14.
2. (ii) ⁸⁰Br⁻ (extra 1 electron, negative charge): Electrons = 35 + 1 = 36. Neutrons = 80 − 35 = 45.
3. (iii) ²⁰¹Hg²⁺ (loss of 2 electrons, positive charge): Electrons = 80 − 2 = 78. Neutrons = 201 − 80 = 121.

1. This behaviour indicates the compound has an ionic bond.
2. In the solid state, ions are fixed in position in a crystal lattice, so they cannot move and hence do not conduct electricity.
3. When dissolved in water, the ionic compound dissociates into free ions that can move and carry electric current, allowing it to conduct electricity.

1. Atoms combine to form compounds mainly in two ways: sharing electrons (forming covalent bonds) or transferring electrons (forming ionic bonds).
2. In sharing, atoms form covalent bonds by contributing electrons to a shared pair. In transfer, one atom donates electrons to another, forming positively and negatively charged ions held together by electrostatic attraction (ionic bonds).
3. Both methods help atoms achieve a stable electronic configuration.

1. Elements combine to form compounds in fixed, simple whole-number ratios of their atoms.
2. This means the types and numbers of atoms in a given compound remain constant, regardless of the compound's source or method of preparation.
3. For example, water always contains two hydrogen atoms combined with one oxygen atom, showing a fixed and definite composition.

1. Atoms of the same element have the same mass and identical chemical properties.
2. This similarity arises because they have the same number of protons and electrons, which determine their behaviour in chemical reactions.
3. As a result, atoms of a particular element react in the same way under similar conditions, leading to consistent and predictable outcomes during chemical processes.

1. (a) Mass of sodium atoms carried by Krish = 5 × 23 = 115 g. Mass of carbon atoms carried by Raunak = 5 × 12 = 60 g. So Krish's container is heavier than Raunak's.
2. (b) Both containers have the same number of atoms, since they have the same number of moles of atoms (1 mole always contains 6.022 × 10²³ atoms regardless of the element).

1. (i) No, ionic compounds do not conduct electricity in solid form, because their ions are fixed in position and cannot move freely to carry electric current.
2. (ii) Ionic compounds have high melting points due to strong electrostatic forces of attraction between oppositely charged ions. A large amount of energy is required to break these forces and melt the compound.
3. (iii) Ionic compounds are generally soluble in water but insoluble in organic solvents. Covalent compounds are usually insoluble in water but dissolve in organic solvents like kerosene and petrol due to their non-polar nature.

OR

1. Covalent compounds do not conduct electricity because they do not form ions in solution. Without free-moving charged particles, electrical current cannot pass through these substances.

1. (i) The gas is Nitrogen gas (N₂). Nitrite ion = NO₂⁻. Nitrate ion = NO₃⁻.
2. (ii) 1 mole of N₂ gas = 6.022 × 10²³ molecules = 2 × 6.022 × 10²³ atoms of N = 12.044 × 10²³ atoms. So this quantity of atoms corresponds to 1 mole of N₂ gas.
3. (iii) Since an H atom contains only protons contributing to its mass, mass of one atom of H = 1 amu (this doesn't depend on the value chosen for Avogadro's number).

OR

1. Molecular mass of NH₄NO₃ = 14 + (1×4) + 14 + (3×16) = 14 + 4 + 14 + 48 = 80 u.

1. (i) The atomicity of an element is the number of atoms that constitute a molecule of that element.
2. (ii) Oxygen (O₂) is an example of a diatomic molecule.
3. (iii) Oxygen (O₂) is diatomic because it has two atoms in a molecule. Ozone (O₃) has three oxygen atoms in a molecule, so it has an atomicity of three.

OR

1. Monoatomic elements: Argon (Ar), Helium (He). Diatomic elements: Hydrogen (H₂), Nitrogen (N₂).

1. (i) Molar mass of N₂ = 28 g/mol. Mass = 28 × 0.5 = 14 g.
2. (ii) Molar mass of N atom = 14 g/mol. Mass = 14 × 0.5 = 7 g.
3. (iii) Number of moles = given particles/Avogadro's number = 3.011×10²³/6.022×10²³ = 0.5 mol. Mass = 14 × 0.5 = 7 g.

OR

1. Number of moles = 6.022×10²³/6.022×10²³ = 1 mol. Mass = 28 × 1 = 28 g.

1. (a) Atomicity
2. (b) Na₂CO₃, (NH₄)₂SO₄
3. (c) Polyatomic
4. (d) Symbol
5. (e) Formula unit mass of Ca₃(PO₄)₂ = (3×Ca) + (2×P) + (8×O) = (3×40) + (2×31) + (8×16) = 120 + 62 + 128 = 310 u.

1. (i) When sodium sulfate solution is mixed with barium chloride solution, a white precipitate is formed. This visible change indicates that a chemical reaction has taken place between the two solutions.
2. (ii) The precipitate formed is barium sulfate. It appears as a white solid and is insoluble in water, which is why it separates out from the solution.
3. (iii) Sodium sulfate + Barium chloride → Barium sulfate + Sodium chloride. This shows the two reactants combining to form a white precipitate of barium sulfate along with sodium chloride as the other product.

1. (i) A molecule is an electrically neutral group of two or more atoms that can exist independently. It shows all the properties of the substance it forms and remains stable under normal conditions.
2. (ii) Elements like helium exist as single atoms because their outermost shell is already complete with two electrons. This stable arrangement means they do not need to gain, lose, or share electrons, so they do not form molecules with other atoms.
3. (iii) Atoms combine to achieve stability mainly in two ways: by sharing electrons (forming covalent bonds) or by transferring electrons (one atom gives electrons, another gains them, forming ionic bonds). Both methods help atoms reach a stable electronic configuration.

1. (i) Law of conservation of mass
2. (ii) Polyatomic ion
3. (iii) Ca₃(PO₄)₂ = 3×40 + 2×31 + 8×16 = 310 u
4. (iv) Na₂CO₃ ; (NH₄)₂SO₄

1. (i) The mass of a compound is equal to the sum of the masses of the elements that combine to form it. This shows that no mass is lost or gained during the formation of a compound, and the total mass remains conserved.
2. (ii) A compound has properties that are very different from the elements that form it. For example, hydrogen is combustible and oxygen supports combustion, but water — formed from them — does not burn and can even extinguish fire. This shows that compounds have entirely new properties.
3. (iii) The formation of water provides clear evidence: the mass of water formed is equal to the combined masses of hydrogen and oxygen used in the reaction. This observation supports that mass is conserved when elements combine to form a compound.

1. (i) Element X has 6 valence electrons, so it needs 2 more electrons to complete its octet (8 electrons). To achieve stability, two atoms of X share electrons with each other, so each atom can complete its octet. Hence, X forms a diatomic molecule (X₂).
2. (ii) Since electrons are shared between two X atoms, the bond formed is a covalent bond (double bond).
3. (iii) Each X atom shares 2 electrons with the other: X=X (double covalent bond between two X atoms). Each X atom also has 2 lone pairs of electrons.
4. (iv) Y has 2 electrons in its second shell, so it can lose 2 electrons (forms Y²⁺, like Mg-type). X needs 2 electrons, so it gains them (forms X²⁻, like O-type). Therefore, one Y atom combines with one X atom: Y²⁺ + X²⁻ → YX (ionic compound formation).

1. (i) ZnO = Atomic mass of Zn + atomic mass of O = 65 + 16 = 81 u.
2. (ii) Na₂O = (Atomic mass of Na × 2) + atomic mass of O = (23×2) + 16 = 46 + 16 = 62 u.
3. (iii) K₂CO₃ = (Atomic mass of K × 2) + Atomic mass of C + (Atomic mass of O × 3) = (39×2) + 12 + (16×3) = 78 + 12 + 48 = 138 u.

1. Hydrocarbon is a compound of carbon and hydrogen. The amount reacted = 0.44 g. Let us calculate the mass of carbon and hydrogen present in the products (CO₂ and H₂O).

Calculation of mass of carbon (C)

1. Mass of CO₂ formed = 0.88 g. Molar mass: CO₂ = 44 g, C = 12 g.
2. 44.0 g of CO₂ contains C = 12.0 g. So 0.88 g of CO₂ contains C = 12.0 × (0.88/44.0) = 0.24 g.

Calculation of mass of hydrogen (H)

1. Mass of H₂O formed = 1.8 g. Molar mass: H₂O = 18 g, 2H = 2 g.
2. 18 g of H₂O contains H = 2.0 g. So 1.8 g of H₂O contains H = 2.0 × (1.8/18.0) = 0.20 g.

1. Total mass of C and H in the products = 0.24 + 0.20 = 0.44 g.
2. This mass matches the original mass of the hydrocarbon (0.44 g). The data is in agreement with the law of conservation of mass.