JKBOSE 9th Class Science Solutions Chapter 9 Atoms And Molecules

◆ Atom. It is the smallest or ultimate particle of an element that takes part in chemical reactions. It may or may not exist independently.

◆ Molecule. It is the smallest or ultimate particle of a substance (element or compound) which can exist freely.

◆ Chemical formula. It shows constituent elements of a substance and number of atoms of each element present in one molecule of it.

◆ Valency. It is the combining capacity of an atom of an element and is numerically equal to the number of hydrogen atoms or number of chlorine atoms or double the number of oxygen atoms with which one atom of the element can combine.

◆ Variable valency. When an element shows more than one valency, it is said to have variable valency.

◆ Ion. It is an atom or group of atoms carrying some charge and can exist freely in solution.

◆ Radical. It is an atom or group of atoms having positive or negative charge and behaves as a single unit in chemical reactions. e.g. Na⁺, SO^2-/4, NO5^-3, NH^+-4 etc.

◆ Simple radical. A radical which is made up of only one kind of atoms is called a simple radical. e.g. Na⁺, Cl^– etc.

◆ Compound radical. A radical which is made up of more than one kind of atoms is called a compound radical. e.g. NH^+-4, SO^2-/4, CO^2-/3 etc.

◆ Polyatomic ion. It is an ion having more than one atom.

◆ The chemical formula of Ionic compound is determined by the charge on each ion.

◆ Law of conservation of mass. It states that matter (or mass) can neither be created nor destroyed during any known physical or chemical change.

◆ Law of definite proportions (or Law of constant compositions). It states that a pure chemical compound is always found to be made up of same elements combined together in the same fixed ratio of mass.

◆ Atomic mass unit (a.m.u. or u). It is 1/12th of the mass of one atom carbon (C^12- isotope). 1 a.m.u. = 1.66 × 10-²⁴ g = 1.66 × 10²⁷kg.

◆ Relative atomic mass (RAM). It is the average relative mass of one atom of element as compared to one atom of carbon-12 taken as 12 a.m.u.

◆ Relative molecular mass (RMM). It is the average relative mass of one molecule of a substance as compared to one atom of carbon-12 taken as 12 a.m.u.

◆ Avogadro’s number or constant is the number of atoms in 12 g of carbon-12. It is denoted by  N₀ or N_A and its value is 6.022 × 10²³, N₀ = 6.022 × 10²³.

◆ Mole. It is the amount of substance that contains the same number of particles (atoms/ions/molecules/formula units.) etc. as there are atoms in 12 g of carbon-12.

◆ Molar mass of substance is the mass of one mole of a substance.

◆ S.T.P. stands for standard temperature (0°C or 273K) and standard pressure (1 atmosphere).

◆ Molar volume. It is the volume occupied by one mole of a gas and its value at S.T.P./N.T.P. is 22.4 litres.

Ans.— Sodium     +    Ethanoic    →     Sodium     +      Carbon     +     Water

            Carbonate       acid                   ethanoate          dioxide

Mass of sodium carbonate = 5.3 g

Mass of ethanoic acid = 6.0 g

Total mass of the reactants = 5.3 + 6.0 = 11.3 g

Mass of carbon dioxide = 2.2 g

Mass of water = 0.9 g

Mass of sodium ethanoate = 8.2 g

Total mass of the products = 2.2 + 0.9 + 8.2 = 11.3 g

Since total mass of the reactants = Total mass of the products

Ans.— One g of hydrogen gas reacts with oxygen = 8 g

3 g of hydrogen gas will react with oxygen = 3 × 8 = 24 g

Ans.— Law of conservation of mass is based upon the postulate that atoms are indivisible particles and can neither be created nor destroyed during any chemical reaction.

Ans.— Atoms of various elements combine in simple whole number but fixed ratio to form compound atoms (molecules).

Ans.— One atomic mass unit is a mass unit and it is equal to 1/12th of mass of an atom of carbon-12.

1 a.m.u. = 1.66 × 10^-27 kg

Ans.— This is because atoms are very-very small. The radius of an atom is of the order of 10^-10 m.

Ans.— (i) Sodium oxide

Ans.— (i) Aluminium sulphate, (ii) Calcium chloride, (iii) Potassium sulphate

(iv) Potassium nitrate, (v) Calcium carbonate.

Ans.— Chemical formula. The chemical formula of a substance indicates its constituent elements and number of atoms of each combining element present in one molecule of it.

Ans.— (i) An H₂S molecule represents

(a) two atoms of hydrogen and

(b) one atom of sulphur.

(ii) A PO³/₄ ion represents

(a) one atom of P

(b) three atoms of O.

Ans.— Molecular mass of H₂ = 2 × 1 = 2 u x

Molecular mass of O₂ = 2 × 16 = 32 u

Molecular mass of Cl₂ = 2 × 35.5 = 71.0 u

Molecular mass of CO₂ = 1 × 12 + 2 × 16 = 12 + 32 = 44 u

Molecular mass of CH₄ = 12 + 4 × 1 = 12 + 4 = 16 u

Molecular mass of C₂H₆ = 2 × 12 + 6 × 1 = 24 + 6 = 30 u

Molecular mass of C₂H₄ = 2 × 12 + 4 × 1 = 24 + 4 = 28 u

Molecular mass of NH₃ = 14 + 3 × 1 = 14 + 3 = 17 u

Molecular mass of CH₃OH = 12 + 3 × 1 + 16 + 1 = 12 + 3 + 16 + 1 = 32 u

Ans.— Formula unit mass of ZnO = 65 + 16 = 81 u

Formula unit mass of Na₂O = 2 × 23 + 16 = 46 + 16 = 62 u

Formula unit mass of K₂CO₃ = 2 × 39 + 12 + 3 × 16 = 78 + 12 + 48 = 138 u

Ans.— 1 mole of carbon has mass = 12 g

∴ Mass of 1 atom of carbon = 12 / 6.023 × 10²³ X

= 1.9 x 10^-23 g

Ans.— Atomic mass of sodium = 23 u

Ans.—  Weight of compound = 0.24 g

Weight of Boron = 0.096 g

Weight of oxygen = 0.144 g

∴ % age of B = 0.096 / 0.24 × 100 = 40

% age of O=  0.144 / 0.240 × 100 = 60

Ans.—  3.00 g of carbon combines with 8.00 g of oxygen to produce 11.00 g of carbon dioxide

                                                         C + O₂ → CO₂

When 3.00 g of carbon is burnt in 50.00 g of oxygen, 11.00 g of carbon dioxide is produced. This is based upon law of constant compositions.

Ans.— Polyatomic ions. A group of atoms carrying a charge (+ve or -ve) is called polyatomic ion.

Examples : SO^2/4, SO^2/3, NH⁺⁴ etc.

(d) Potassium sulphate (K₂SO₄). The elements present in it are Potassium, Sulphur and Oxygen.

Ans.— (a) Molar mass of ethyne, C₂H₂ = 2 × 12 + 2 × 1 = 24 + 2 = 26 g

(b) Molar mass of sulphur molecule, S₈ = 8 × 32 = 256 g

(c) Molar mass of Phosphorus molecule, P₄ = 4 × 31 = 124 g

(d) Molar mass of hydrochloric acid, HCI = 1 + 35.5 = 36.5 g

(e) Molar mass of nitric acid, HNO₃ = 1 + 14 + 3 × 16 = 1 + 14 + 48 = 63 g

Ans.— (a) Mass of 1 mole of Nitrogen atoms = 14 g

(b) Mass of 1 mole of Al atoms = 27 g

∴ Mass of 4 moles of Al atoms = 4 × 27 = 108 g

(c) Mass of 1 mole of sodium sulphite, Na₂SO₃

= 2 × 23 + 32 + 3 × 16

= 46 + 32 + 48 = 126 g

∴ Mass of 10 moles of Na₂SO₃ = 10 × 126 = 1260 g

Ans.— We know that, No. of moles of a substance = Mass (g) / Molar mass

(a) No. of moles in 12 g of oxygen gas (O₂) 12/32 = 0.375

(b) No. of moles in 20 g of water (H₂O)= 20/18 = 10/9 = 1.11

(c) No. of moles in 22 g of carbon dioxide (CO₂) = 22/44 = 1/2 = 0.5

Ans.— (a) 0.2 mole of oxygen atoms :

Mass of 1 mole of oxygen atoms = 16 g

Mass of 0.2 mole of oxygen atoms = 0.2 × 16 = 3.2 g

Mass of 1 mole of water molecule (H₂O)

∴ Mass of 0.5 mole of water molecules = 0.5 × 18 = 9 g

Ans.—  Mass of solid sulphur (S₈) = 16 g

Mass of 1 mole of sulphur (S₈) = 8 × 32 = 256 g

No. of atoms in 16 g of solid sulphur = 16/256 × 8 × 6.023 × 10²³

= 3.0115 × 10²³

Ans.— (A) Positive ions or cations or Electropositive radicals :

Ans.—(a) Dalton’s atomic theory was given by John Dalton in 1808 to explain the nature of an atom and the chemical combination in the atoms. The main points of the theory are :

1. All matter is made up of a large number of extremely small particles called atoms.

2. An atom is indivisible.

3. Atoms of a particular element are identical in all respects i.e. they have same mass and similar properties.

4. Atoms of the different elements are different i.e. they have different masses and properties.

5. Two or more atoms of the same or different elements combine to form compound atoms or molecules of compounds.

6. The number and the nature of atoms in a molecule always remains the same.

7. Atoms of the different elements combine in a simple whole number but fixed ratios to form compound atoms (now called molecules).

8. Atoms of the same element can combine to form two or more compounds.

9. Atom can neither be created nor be destroyed.

1. Atom is no longer indivisible.

2. Atoms of the same element may not be always identical.

3. Atoms of different elements may have same atomic masses.

4. It does not explain the cause of chemical combination.

Ans.— (i) Prepare 5% solution of barium chloride and sodium sulphate in separate beaker.

(ii) Take a small amount of solution of barium chloride in a conical flask and a small amount of solution of sodium sulphate in an ignition tube.

(iii) Suspend the ignition tube carefully in the conical flask with a thread and cork it.

(iv) Weigh the flask carefully.

(v) Tilt and swirl the flask such that the two solutions get mixed and react.

BaCI_2(aq) + Na₂SO_4(aq) → BaSO_4(s) + 2NaCl_(aq)

(vi) Cool the flask to room temperature and again weigh it. The weight remains unchanged showing the truth of the law.

Ans.— A covalent compound made up of two elements is called a binary molecular compound. e.g. water (H₂O).

While writing the name of a binary molecular compound, first write the name of the element whose symbol appears on the left hand side in the molecular formula and write down the name of other element. The name of the other element should end with ide The prefixes di, tri, tetra, penta etc. are used to indicate 2, 3, 4, 5…. atoms of an element.

Some examples are :

Compound	Name
CO	Carbon monoxide
CO2	Carbon dioxide
HCI	Hydrogen chloride
HBr	Hydrogen bromide
ZnO	Zinc oxide
SO2	Sulphur dioxide
PCI3	Phosphorus trichloride
PCI5	Phosphorus pentachloride
NO2	Nitrogen dioxide

Ans.— First experiment

Weight of copper = 2.16 g

Weight of copper oxide = 2.70 g

Percentage of copper in copper oxide = 2.16/2.70 × 100 = 80

percentage of oxygen is copper oxide = 100 – 80 = 20

Weight of copper oxide = 1.15 g

Weight of copper = 0.90 g

Percentage of copper in copper oxide = 0.92/1.15 × 100 = 80

Percentage of copper CuO = 100 – 80 = 20

Since the percentage composition of copper oxide in the two experiments is the same.

The above results illustrate the law of definite proportions.

∴ Ratio by atoms in water molecule is H  : O

                                                              2  : 1

Ans.— In H₂O molecule

Ans.—(a) 12+ 32 = 44 U

(b) 12+ 4 = 16 U

Atom	Molecule
It is the smallest particle of an element which takes part in chemical reactions. It may or may not exist freely.	It is the smallest particle of a substance which can exist freely.
Generally it is highly reactive.	It is less reactive.
It is less stable.	It is more stable.
It can’t be sub-divided.	It can be sub-divided.

Ans.— (a) Chemical formula. The symbolic representation of a molecule of a substance is called its chemical formula. It indicates the actual number of atoms of various elements present in one molecule of a substance.

(b) NH₃ represents ammonia.

Ans.— (i) Sodium Carbonate (ii) Magnesium Chloride (iii) Aluminium Sulphate (iv) Potassium Sulphate (v) Nickel Sulphate (vi) Potassium Nitrate (vii) Calcium Carbonate.

Ans.— (a) Magnesium hydroxide.

(b) Ammonium sulphate.

Ans.— (a) Na₂CO₃ (b) CaCl₂

Ans.— On heating, if the powder chars, it is sugar otherwise it is salt.

Ans.— It is the atomic mass of an element expressed in grams.

Or

It is such a quantity of an element whose mass is numerically equal to its atomic mass expressed in a.m.u.

Ans.— Mole. One mole of a substance represent such a quantity of it whose mass is equal to its gram atomic mass (for elements) or gram molecular mass (for compounds).

Ans.— Simple radicals. A radical which contains only one atom is called simple radical. eg. Na⁺, Ba²⁺ etc.

Ans.—The radical which is derived from acid and carries negative charge is called acidic radical or electronegative radical or anion. e.g. CI^–, S^2-, PO₄³etc.

Ans.— A radical which is derived from base and carries positive charge is called basic radical or electropositive radical or cations. e.g. Na⁺, Mg²⁺, AI³⁺ etc.

Ans.— This shows that the element exists in the form of isotopes.

Ans.— Helium and neon.

Ans.— Hydrogen (H₂) and Oxygen (O₂).

Ans.—This is because it is simply a ratio.

Ans.— This is due to the existence of isotopes of an element having different masses.

Ans.— Symbols derived from English names: Cu (Copper), Cl (Chlorine) Symbols derived from Latin names: Au (Aurum), Na (Natrium).

Ans.— The solubility of a substance changes with temperature. Generally, the solubility of a substance increases with the increase in temperature. For example, more sugar dissolves in hot water than in cold water.

Ans.—It is the molecular mass of a substance expressed in grams.

Or

It is such a quantity of a substance whose mass in grams is numerically equal to its molecular mass expressed in u or amu.

Ans.— H₂ represents one molecule of H₂ and 2H represents two atoms of hydrogen.

(A) 44 kg

(B) 4.4 g

(C) 44 g

(D) None.

Ans.— (C) 44 g

(A) Ca, C, O

(B) Ca, O

(C) C, O

(D) None.

Ans.— (A) Ca, C, O

(A) 26 a.m.u.

(B) 26 g mol^-1

(A) Na⁺

(B) K⁺

(C) Mg²⁺

(D) Cl^–

Ans.— (D) Cl^–

(A) 28 g

(B) 14 g

(C) 10 kg

(D) 7 g.

Ans.— (B) 14 g

(A) 1.0076 g

(B) 1.66 x 10²⁴ g

(C) 1 g

(D) 0.00327 g.