3. Atoms and Molecules

3. Atoms and Molecules

Text-book questiuons with solutions.

From Page 32-33 (NCERT Book)

Q1.  In a reaction, 5.3 g of sodium carbonate reacted with 6g of ethanoic acid. The products were 2.2g of carbon dioxide, 0.9g water and 8.2g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.

Answer: 

sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water

Mass of Sodium carbonate = 5.3g 

Mass of ethanoic Acid = 6g

Mass of Sodium ethanoic acid = 8.2g 

Mass of water = 0.9g  

We knock that according to the law of conservation of mass-

Mass of reactants = Mass of products

5.3g + 6g = 8.2g + 2.2g + 0.9g 

11.3g = 11.3g 

Hence, the given observations are satisfied the law of conservation of mass.

Q2.  Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?

Answer: Given ratio of Hydrogen and Oxyden - 1:8 

It means- 1g of Hydrogen gas required 8g Oxygen to reaction with Hydrogen

Therefore, 3g of Hydrogen gas required = 3 x 8 = 24g 

So, 24g Oxygen is required to react completely with Hydrogen gas.

Q3. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?

Answer: The second postulate of Dalton's atomic theory is the result of the law of conservation of mass. 

That is as follow- 

"Atoms are indivisible particles, which can neither be created nor be destroyed in a chemical reaction". 

Q4. Which postulate of Dalton’s atomic theory can explain the law of definite proportions?

Answer: 

The sixth postulate of Dalton's atomic theory explains the law of definite proportions. 

That is as follow- 

"The ralative number and kind of atoms in a given compound remains constatnt".

From Page 35 (NCERT Book)

Q1. Define the atomic mass unit.

Answer: 

An atomic mass unit or  amu is one twelfth of the mass of an unbound atom of carbon -12 isotope. This is approximetely 1.67377 x 10 ^-27 kilogram (kg). 

The relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to 1/12th the mass of one carbon-12 atom.

Atomic mass unit is abbreviated as 'amu' but on IUPAC latest recommendation, it is writen as 'u'. 

Q2. Why is it not possible to see an atom with naked eyes?

Answer: Atoms are very small tiny particles. It is so small that can not be seen with a nacked eyes. Also, an another reason - the atom of an element does not exist independently. 

From Page 39 (NCERT Book)

Q1. Write down the formulae of
(i) sodium oxide
(ii) aluminium chloride
(iii) sodium suphide
(iv) magnesium hydroxide

Answer: 

(i) Sodium oxide : Na₂O

(ii) Aluminium chloride : AlCl₃

(iii) Sodium Sulphate : Na₂S

(iv) Magnesium hydroxide : Mg(OH)₂

Q2. Write down the names of compounds represented by the

 

From Page 40 (NCERT Book)

Q1. Calculate the molecular masses of

        H2, O2, Cl2, CO2, CH4, C2H6, C2H4, NH3, CH3OH.

Q2. Calculate the formula unit masses of ZnO, Na2O, K2CO3, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.

From Page 42 (NCERT Book)

Q1. If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?

Q2. Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of Na = 23 u, Fe = 56 u)?

 

Exercises

Q1. A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.
Answer: Boron and oxygen compound —> Boron + Oxygen
0.24 g —> 0.096 g + 0.144 g
 

Q2. When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer?
Answer: The reaction of burning of carbon in oxygen may be written as:
It shows that 12 g of carbon bums in 32 g oxygen to form 44 g of carbon dioxide. Therefore 3 g of carbon reacts with 8 g of oxygen to form 11 g of carbon dioxide. It is given that 3.0 g of carbon is burnt with 8 g of oxygen to produce 11.0 g of CO2. Consequently 11.0 g of carbon dioxide will be formed when 3.0 g of C is burnt in 50 g of oxygen consuming 8 g of oxygen, leaving behind 50 – 8 = 42 g of O2. The answer governs the law of constant proportion.

Q3. What are poly atomic ions? Give examples.
Answer: The ions which contain more than one atoms (same kind or may be of different kind) and behave as a single unit are called polyatomic ions e.g., OH–, SO42-, CO32-.

Q4. Write the chemical formulae of the following:
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate.
Answer: (a) Magnesium chloride
Symbol —> Mg Cl
Change —> +2 -1
Formula —> MgCl2
(b) Calcium oxide
Symbol —> Ca O
Charge —> +2 -2
Formula —> CaO
(c) Copper nitrate
Symbol —> Cu NO
Change +2 -1
Formula -4 CU(N03)2
(d) Aluminium chloride
Symbol —> Al Cl
Change —> +3 -1
Formula —> AlCl3
(d) Calcium carbonate
Symbol —> Ca CO3
Change —> +2 -2
Formula —> CaC03

Q5. Give the names of the elements present in the following compounds:
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
Answer: (a) Quick lime —> Calcium oxide
Elements —> Calcium and oxygen
(b) Hydrogen bromide
Elements —> Hydrogen and bromine
(c) Baking powder —> Sodium hydrogen carbonate
Elements —> Sodium, hydrogen, carbon and oxygen
(d) Potassium sulphate
Elements —> Potassium, sulphur and oxygen

Q6. Calculate the molar mass of the following substances.
(a) Ethyne, C2H2
(b) Sulphur molecule, S8
(c) Phosphorus molecule, P4 (Atomic mass of phosphorus = 31)
(d) Hydrochloric acid, HCl
(e) Nitric acid, HNO3
Answer: The molar mass of the following: [Unit is ‘g’]
(a) Ethyne, C2H2 = 2 x 12 + 2 x 1 = 24 + 2 = 26 g
(b) Sulphur molecule, S8 = 8 x 32 = 256 g
(c) Phosphorus molecule, P4=4 x 31 = i24g
(d) Hydrochloric acid, HCl = 1 x 1 + 1 x 35.5 = 1 + 35.5 = 36.5 g
(e) Nitric acid, HN03 = 1 x 1 + 1 x 14 + 3 x 16 = 1 + 14 + 48 = 63 g

Q7. What is the mass of
(a) 1 mole of nitrogen atoms?
(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)?
(c) 10 moles of sodium sulphite (Na2S03)?
Answer: (a) Mass of 1 mole of nitrogen atoms = 14 g
(b) 4 moles of aluminium atoms
Mass of 1 mole of aluminium atoms = 27 g
∴ Mass of 4 moles of aluminium atoms = 27 x 4 = 108 g
(c) 10 moles of sodium sulphite (Na2SO3)
Mass of 1 mole of Na2SO3 = 2 x 23 + 32 + 3 x 16 = 46 + 32 + 48 = 126 g
∴ Mass of 10 moles of Na2SO3 = 126 x 10 = 1260 g

Q8. Convert into mole.
(a) 12 g of oxygen gas
(b) 20 g of water
(c) 22 g of Carbon dioxide.
Answer: (a) Given mass of oxygen gas = 12 g
Molar mass of oxygen gas (O2) = 32 g
Mole of oxygen gas 12/32 = 0.375 mole
(b) Given mass of water = 20 g
Molar mass of water (H2O) = (2 x 1) + 16 = 18 g
Mole of water = 20/18 = 1.12 mole
(c) Given mass of Carbon dioxide = 22 g
Molar mass of carbon dioxide (CO2) = (1 x 12) + (2 x 16)
= 12 + 32 = 44 g
∴ Mole of carbon dioxide = 22/44 = 0.5 mole

Q9. What is the mass of:
(a) 0.2 mole of oxygen atoms?
(b) 0.5 mole of water molecules?
Answer: (a) Mole of Oxygen atoms = 0.2 mole
Molar mass of oxygen atoms = 16 g
Mass of oxygen atoms = 16 x 0.2 = 3.2 g
(b) Mole of water molecule = 0.5 mole
Molar mass of water molecules = 2 x 1 + 16= 18 g .
Mass of H2O = 18 x 0.5 = 9 g

Q10. Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur.
Answer:Molar mass of S8 sulphur = 256 g = 6.022 x 1023 molecule

Given ma:ss of sulphur = 16 g

Q11. Calculate the number of aluminium ions present in 0.051 g of aluminium oxide. (Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u)
Answer: Molar mass of aluminium oxide Al203
= (2 x 27) + (3 x 16)
= 54 + 48 = 102 g.
 

 

3. Atoms and Molecules

3. Atoms and Molecules

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Atoms and Molecules — Answers (Class 9)

उत्तर:

1. सभी पदार्थ अति-सूক্ষ्म कणों (atoms) से बने होते हैं।

2. किसी विशिष्ट तत्व के सभी atoms गुण (mass, properties) में समान होते हैं और अलग-अलग तत्वों के atoms अलग होते हैं।

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उत्तर:
किसी molecule का molecular mass (मौलिक द्रव्यमान) = उस molecule में उपस्थित समस्त atoms के atomic masses का tổng (sum)। यह atomic mass units (u) में लिखा जाता है। उदाहरण: H₂O का molecular mass = 2×1 + 16 = 18 u.

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उत्तर:
Avogadro संख्या = 6.022×10236.022\times 10^{23}6.022×1023. यह एक mole में उपस्थित कणों (atoms/molecules) की संख्या है।

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उत्तर:
H₂O में कुल 3 atoms होते हैं — 2 hydrogen और 1 oxygen।

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उत्तर:
यह नियम Joseph Proust ने दिया था। (Proust’s Law)

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उत्तर:

• Molecule: एक साथ जुड़े हुए दो या अधिक atoms का समूह (same element के भी हो सकते हैं, जैसे O₂)।

• Compound: दो या अधिक विभिन्न तत्वों के atoms से बना पदार्थ जिसका निश्चित संरचना और गुण होते हैं (जैसे H₂O, NaCl)।

संक्षेप: हर compound molecule है पर हर molecule compound नहीं होता (यदि molecule में अलग-अलग तत्व न हों तो वह compound नहीं होगा)।

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गणना:
CO₂ = 1×C + 2×O = 1×12+2×161\times 12 + 2\times 161×12+2×16.
=12+32=44= 12 + 32 = 44=12+32=44 u.
उत्तर: 44 u

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गणना:
Water (H₂O) का molecular mass = 2×1+16=182\times1 + 16 = 182×1+16=18 g/mol.
Moles = mass / molar mass = 18 g÷18 g/mol=118\ \text{g} \div 18\ \text{g/mol} = 118 g÷18 g/mol=1 mol.
उत्तर: 1 mole

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गणना:
Moles = 9 g÷18 g/mol=0.59\ \text{g} \div 18\ \text{g/mol} = 0.59 g÷18 g/mol=0.5 mol.
उत्तर: 0.5 mol

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स्पष्टीकरण (mass-ratio method):
मान लीजिए कि magnesium और oxygen कुछ निश्चित mass ratio में मिलते हैं — वास्तविक atomic masses से दिखाएँ:

• Mg का atomic mass ≈ 24 (प्रायः 24.3)

• O का atomic mass = 16

यदि हम तुलना के लिए 24 g Mg और 16 g O लें:

• Mg के moles = 24÷24=124 \div 24 = 124÷24=1 mol

• O के moles = 16÷16=116 \div 16 = 116÷16=1 mol
  mole का अनुपात = 1 : 1 → सबसे सरल whole-number ratio 1:1 → formula = MgO.

उत्तर: MgO

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गणना/सिद्धि:
Water का वास्तविक mass-composition (H₂O): H का कुल mass = 2×1=22\times1 = 22×1=2 और O = 16 → H : O by mass = 2:16=1:82 : 16 = 1 : 82:16=1:8.
दिए गए उदाहरण में भी H : O = 2:16=1:82 : 16 = 1 : 82:16=1:8.
यह सुनिश्चित करता है कि किसी भी sample में H और O का mass-ratio स्थिर है — यही Law of Constant Proportion है।

उत्तर: हाँ, दिया हुआ मिश्रण उसी स्थिर अनुपात (1:8) में है, अतः नियम संतुष्ट होता है।

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उत्तर (valency method):

• Na (sodium) की valency = 1 (यह 1+ ion देता है)।

• Cl (chlorine) की valency = 1 (यह 1– ion बनाता है)।
  दोनों की valencies 1 और 1 हैं → एक-एक atom लेते हैं → formula = NaCl.

(वैकल्पिक mass-ratio से: Na और Cl के atomic masses से simplest whole number ratio भी 1:1 देगा।)

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स्पष्टीकरण:
6.022×10236.022\times10^{23}6.022×1023 molecules = 1 mole (Avogadro संख्या)। CO₂ का molar mass हमने Q7 में निकाला = 44 g/mol.
इसलिए mass = 1 mol×44 g/mol=441\ \text{mol}\times 44\ \text{g/mol} = 441 mol×44 g/mol=44 g.
उत्तर: 44 g

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गणना:
O₂ का molar mass = 2×16=322\times 16 = 322×16=32 g/mol.
Mass = moles × molar mass = 5×32=1605 \times 32 = 1605×32=160 g.
उत्तर: 160 g

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स्पष्टीकरण:
1 mole किसी element का mass उसके atomic mass (g/mol) के बराबर होता है। यदि 1 mole = 32 g → atomic mass = 32 u (या 32 g/mol)।
उत्तर: 32 u (32 g/mol)

3. Atoms and Molecules

Additional -Questions:

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Q1. What is atomicity?

Ans: The number of atoms contained in a molecule of a substance is called its atomicity. 

Q2. How molecules are categrised on the base of atomicity?

Ans:

1. Monoatomic Molecules (Molecule having 1 atom)

2. Diatomic Molecules (molecule having 2 atoms)

3. Triatomic Molecules (molecules having 3 atoms)

4. Tetraatomic Molecules (molecules having 4 atoms)

5. Ployatomic Molecules (molecules having more than 4 molecules) 

1. Atom

• The smallest particle of an element that cannot be divided further by chemical means.

• It retains all the properties of that element.

• Example: H (Hydrogen atom), O (Oxygen atom).

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2. Molecule

• A group of two or more atoms (same or different) chemically bonded together.

• Example: H₂ (hydrogen molecule), H₂O (water molecule).

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3. Ion

• A charged particle formed when an atom or molecule loses or gains electrons.

• Positive ions → Cations (formed by loss of electrons).

• Negative ions → Anions (formed by gain of electrons).

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4. Atomicity

• The number of atoms present in one molecule of an element.

• Example:

  • O₂ → Atomicity = 2

  • P₄ → Atomicity = 4

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5. Anion

• A negatively charged ion.

• Formed by gain of electrons.

• Example: Cl⁻, O²⁻.

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6. Cation

• A positively charged ion.

• Formed by loss of electrons.

• Example: Na⁺, Ca²⁺.

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7. One mole atom

• The amount of a substance containing 6.022 × 10²³ atoms of that element.

• Example: 1 mole of carbon = 6.022 × 10²³ atoms of carbon.

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8. One mole molecule

• The amount of a substance containing 6.022 × 10²³ molecules.

• Example: 1 mole of water (H₂O) = 6.022 × 10²³ molecules of H₂O.

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9. Monoatomic

• A molecule that contains only one atom.

• Example: Noble gases (He, Ne, Ar).

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10. Diatomic

• A molecule that contains two atoms.

• Example: H₂, O₂, Cl₂.

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11. Triatomic

• A molecule that contains three atoms.

• Example: O₃ (ozone), H₂O (water).

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12. Polyatomic

• A molecule that contains more than three atoms.

• Example: P₄ (phosphorus), CH₄ (methane), C₆H₁₂O₆ (glucose).