CBSE - Atoms and Molecules

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

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

Answer

In the given reaction, sodium carbonate reacts with ethanoic acid to produce sodium ethanoate, carbon dioxide, and water.
Sodium Carbonate + Ethanoic acid → sodium ethanoate + carbon dioxide + water

Mass of sodium carbonate = 5.3 g (Given)
Mass of ethanoic acid = 6 g (Given)
Mass of sodium ethanoate = 8.2 g (Given)
Mass of carbon dioxide = 2.2 g (Given)
Mass of water = 0.9 g (Given)

Now, total mass before the reaction = (5.3 + 6) g
= 11.3 g

And, total mass after the reaction = (8.2 + 2.2 + 0.9) g
= 11.3 g

∴ Total mass before the reaction = Total mass after the reaction
 

Hence, the given observations are in agreement with the law of conservation of mass.

It is given that the ratio of hydrogen and oxygen by mass to form water is 1:8.

Then, the mass of oxygen gas required to react completely with 1 g of hydrogen gas is 8 g.

Therefore, the mass of oxygen gas required to react completely with 3 g of hydrogen gas is 8 × 3 g = 24 g.

The postulate of Dalton :"Atoms are indivisible particles, which can not be created or destroyed in a chemical reaction" is the result of the law of conservation of mass.

The postulate of Dalton, “The relative number and kinds of atoms are constant in a given compound”, can explain the law of definite proportions.

Mass unit equal to exactly one-twelfth the mass of one atom of carbon-12 is called one atomic mass unit. It is written as 'u'.

The size of an atom is so small that it is not possible to see it with naked eyes. Also, the atom of an element does not exist independently

(i) sodium oxide

→ Na₂O

(ii) aluminium chloride

→ AlCl₃

(iii) sodium sulphide

→ Na₂S

(iv) magnesium hydroxide

→ Mg(OH)₂

(i) Al₂(SO₄)₃

→ Aluminium sulphate

(ii) CaCl₂

→ Calcium chloride

(iii) K₂SO₄

→ Potassium sulphate

(iv) KNO₃

→ Potassium nitrate

(v) CaCO₃

→ Calcium carbonate

The chemical formula of a compound is a symbolic representation of its composition.

How many atoms are present in a
(i) H₂S molecule and
(ii) PO₄^3-ion?

Ans

(i) In an H₂S molecule, three atoms are present; two of hydrogen and one of sulphur.

(ii) In a PO₄^3- ion, five atoms are present; one of phosphorus and four of oxygen.

→ Molecular mass of H₂ = 2 × Atomic mass of H
= 2 × 1
= 2 u

→ Molecular mass of O₂ = 2 × Atomic mass of O
= 2 × 16
= 32 u

→ Molecular mass of Cl₂= 2 × Atomic mass of Cl
= 2 × 35.5
= 71 u

→ Molecular mass of CO₂= Atomic mass of C + 2 × Atomic mass of O
= 12 + 2 × 16
= 44 u

→ Molecular mass of CH₄ = Atomic mass of C + 4 × Atomic mass of H
= 12 + 4 × 1
= 16 u

→ Molecular mass of C₂H₆ = 2× Atomic mass of C + 6× Atomic mass of H
= 2 × 12 + 6 × 1
= 30 u

→ Molecular mass of C₂H₄= 2 x Atomic mass of C + 4 × Atomic mass of H
= 2 × 12 + 4 × 1
= 28 u

→ Molecular mass of NH₃ = Atomic mass of N + 3 × Atomic mass of H
= 14 + 3×1
= 17 u

→ Molecular mass of CH₃OH = Atomic mass of C + 3 × Atomic mass of H + Atomic mass of O + Atomic mass of H
= 12 + 3×1 + 8 + 1
= 24 u

→ Formula unit mass of ZnO = Atomic mass of Zn + Atomic mass of O
= 65 + 16
= 81 u

→ Formula unit mass of Na₂O = 2 × Atomic mass of Na + Atomic mass of O
= 2 × 23 + 16
= 62 u

→ Formula unit mass of K₂CO₃= 2 × Atomic mass of K + Atomic mass of C + 3 × Atomic mass of O
= 2 × 39 + 12 + 2 × 16 

= 78 + 12 + 32 

= 122 u

One mole of carbon atoms weighs 12 g (Given)
i.e., mass of 1 mole of carbon atoms = 12 g
Then, mass of 6.022 × 10²³ number of carbon atoms = 12 g
Therefore, mass of 1 atom of carbon = 12 ÷ (6.022 × 10²³)
= 1.9926 x 10^-23 g

Atomic mass of Na = 23 u (Given)
Then, gram atomic mass of Na = 23 g
Now, 23 g of Na contains = 6.022 × 10²³ g number of atoms
Thus, 100 g of Na contains = 6.022 × 10²³ / 23×100 number of atoms
= 2.6182 × 10²⁴ number of atoms

Again, atomic mass of Fe = 56 u (Given)
Then, gram atomic mass of Fe = 56 g

Now, 56 g of Fe contains = 6.022 × 10²³ g number of atoms

Thus, 100 g of Fe contains = 6.022 × 10²³ / 56 × 100 number of atoms
= 1.0753 × 10²⁴ number of atoms

Therefore, 100 grams of sodium contain more number of atoms than 100 grams of iron.

Total mass of Compound = 0.24 g (Given)
Mass of boron = 0.096 g (Given)
Mass of oxygen = 0.144 g (Given)

Thus, percentage of boron by weight in the compound = 0.096 / 0.24 × 100%

= 40%

And, percentage of oxygen by weight in the compound = 0.144 / 0.24 × 100% = 60%

3.0 g of carbon combines with 8.0 g of oxygen to give 11.0 of carbon dioxide.
 

If 3 g of carbon is burnt in 50 g of oxygen, then 3 g of carbon will react with 8 g of oxygen. The remaining 42 g of oxygen will be left un-reactive.
In this case also, only 11 g of carbon dioxide will be formed.
The above answer is governed by the law of constant proportions.

A polyatomic ion is a group of atoms carrying a charge (positive or negative). For example, Nitrate (NO₃^-) , hydroxide ion (OH - )

(a) Magnesium chloride

→ MgCl₂

(b) Calcium oxide

→ CaO

(c) Copper nitrate

→ Cu (NO₃)₂

(d) Aluminium chloride

→ AlCl₃

(e) Calcium carbonate

→ CaCO₃

(a) Quick lime
→ Calcium and oxygen

(b) Hydrogen bromide
→ Hydrogen and bromine

(c) Baking powder
→ Sodium, hydrogen, carbon, and oxygen

(d) Potassium sulphate
→ Potassium, sulphur, and oxygen

(a) Ethyne, C₂H₂
→ Molar mass of ethyne, C₂H₂ = 2 × 12 + 2 × 1 = 26 g

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

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

(d) Hydrochloric acid, HCl
→ Molar mass of hydrochloric acid, HCl = 1 + 35.5 = 36.5 g

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

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 (Na₂SO₃)?

Answer

(a) The mass of 1 mole of nitrogen atoms is 14 g.
 

(b) The mass of 4 moles of aluminium atoms is (4 × 27) g = 108 g

(c) The mass of 10 moles of sodium sulphite (Na₂SO₃) is
10 × [2 × 23 + 32 + 3 × 16] g = 10 × 126 g = 1260 g

Convert into mole.

(a) 12 g of oxygen gas
(b) 20 g of water
(c) 22 g of carbon dioxide

Answer

(a) 32 g of oxygen gas = 1 mole
Then, 12 g of oxygen gas = 12 / 32 mole = 0.375 mole

(b) 18 g of water = 1 mole
Then, 20 g of water = 20 / 18 mole = 1.111 mole

(c) 44 g of carbon dioxide = 1 mole
Then, 22 g of carbon dioxide = 22 / 44 mole = 0.5 mole

What is the mass of:

(a) 0.2 mole of oxygen atoms?
(b) 0.5 mole of water molecules?

Answer

(a) Mass of one mole of oxygen atoms = 16 g
Then, mass of 0.2 mole of oxygen atoms = 0.2 × 16g = 3.2 g

(b) Mass of one mole of water molecule = 18 g

Then, mass of 0.5 mole of water molecules = 0.5 × 18 g = 9 g

1 mole of solid sulphur (S8) = 8 × 32 g = 256 g
i.e., 256 g of solid sulphur contains = 6.022 × 10²³ molecules
Then, 16 g of solid sulphur contains = 6.022 × 10²³ / 256  = 16 molecules
= 3.76375 × 10²² molecules

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

mole of aluminium oxide (Al₂O₃) = 2 × 27 + 3 × 16
= 102 g
i.e., 102 g of Al₂O₃= 6.022 × 10²³molecules of Al₂O₃
Then, 0.051 g of Al₂O₃contains = 6.022 × 10²³ / 102 × 0.051 molecules
= 3.011 × 10²⁰ molecules of Al₂O₃

The number of aluminium ions (Al3+) present in one molecule of aluminium oxide is 2.

Therefore, the number of aluminium ions (Al³⁺) present in 3.011 × 10²⁰molecules (0.051 g ) of aluminium oxide (Al₂O₃) = 2 × 3.011 × 10²⁰
= 6.022 × 10²⁰

1. Which of the following correctly represents 360 g of water?
(i) 2 moles of H₂0
(ii) 20 moles of water
(iii) 6.022 � 10²³ molecules of water
(iv) 1.2044�10²⁵ molecules of water
(a) (i)
(b) (i) and (iv)
(c) (ii) and (iii)
(d) (ii) and (iv)
Ans. (d) (ii) and (iv)

2. Which of the following statements is not true about an atom?
(a) Atoms are not able to exist independently
(b) Atoms are the basic units from which molecules and ions are formed
(c) Atoms are always neutral in nature
(d) Atoms aggregate in large numbers to form the matter that we can see, feel or touch
Ans. (a) Atoms are not able to exist independently.
Explanation: Atoms of inert gases exist independently, i.e. as single atom.


3. The chemical symbol for nitrogen gas is
(a) Ni
(b) N₂
(c) N⁺
(d) N
Ans. (b) N₂

4. The chemical symbol for sodium is
(a) So
(b) Sd
(c) NA
(d) Na
Ans. (d) Na

5. Which of the following would weigh the highest?
(a) 0.2 mole of sucrose (C₁₂H₂₂O₁₁)
(b) 2 moles of CO₂
(c) 2 moles of CaCO₃
(d) 10 moles of H₂O
Ans. (c) 2 moles of CaCO₃

Explanation: Weight of 1 mole of each compound can be calculated as follows:

(a) Sucrose =  12 � 12+  22 � 1 +11 � 16 = 144  + 22  +  176 =342 g
Or 0.2 M Sucrose =  342 � 0.2  = 68.4 g
(b) 1 M carbon dioxide =    1 � 12 + 2 � 16  = 44 g
Or 2 M CO₂ = � = 44 2 88 g
(c) 1 M CaCO₃ =    1 � 10 +  1  � 12 + 3 � 16 = 100 g
Or 2 M CaCO₃ =   100 �2 = 200 g
(d) 1 M H₂O=     2 � 1+  1 � 16 = 18g
Or 10 M water =  18 �10 = 180g

6. Which of the following has maximum number of atoms?
(a) 18 g of H₂O
(b) 18 g of O₂
(c) 18 g of CO₂
(d) 18 g of CH₄
Ans. (d) 18 g of CH₄
Explanation: Number of atoms


7. Which of the following contains maximum number of molecules?
(a) 1g CO₂
(b) 1g N₂
(c) 1g H₂
(d) 1g CH₄
Ans. (c) 1g H₂

= 3.011 �10 ²³
Molar mass of other molecules are much higher than given mass, so number of molecules in them will be less than that in hydrogen.

Mass of one atom of oxygen is

3.42 g of sucrose are dissolved in 18 g of water in a beaker. The number of oxygen atoms in the solution are

(a) 6.68 x 10²³
(b) 6.09 x 10²²
(c) 6.022 x 10²³
(d) 6.022 x 10²¹

Ans. (a) 6.68 x 10²³

10. A change in the physical state can be brought about
(a) only when energy is given to the system
(b) only when energy is taken out from the system
(c) when energy is either given to, or taken out from the system
(d) without any energy change
Ans. (c) when energy is either given to, or taken out from the system
Explanation: When a solid changes into liquid, it takes energy. When a liquid changes into solid, it releases energy.

Which of the following represent correct chemical formula? Name it.
(a) CaCl
(b) BiPO₄
(c) NaSO₄
(d) NaS
Ans. (b) In BiPO₄; with ions are trivalent and hence this shows the correct chemical formula.
In option (a); Ca is bivalent while chlorine is monovalent. In options (c) and (d); Na is
monovalent while sulphate and Sulphur are bivalent. 

(a) Copper (II) bromide
Ans. CuBr₂
(b) Aluminium (III) nitrate
Ans. Al(NO₃)₃
(c) Calcium (II) phosphate
Ans. Ca₃(PO4)₂
(d) Iron (III) sulphide
Ans. Fe₂S₃
(e) Mercury (II) chloride
Ans. HgCl₂
(f) Magesium (II) acetate
Ans. Mg(CH₃COO)₂

Write the molecular formulae of all the compounds that can be formed by the combination of following ions
Cu²⁺ , Na⁺, Fe³⁺, Cl^- , SO₄^2- , PO₄ ^3-

Ans. CuCl₂/ CuSO₄/ Cu₃(PO₄)₂
NaCl/ Na₂SO₄/ Na₃PO₄
FeCl₃/ Fe₂(SO₄)3 / FePO₄

Write the cations and anions present (if any) in the following compounds
(a) CH₃COONa
(b) NaCl
(c) H₂
(d) NH₄NO₃

(a) ANION  ->CH₃COO^-  CATIONS ->Na⁺

(b) ANION  ->Cl^- CATIONS ->Na⁺
(c) Its a covalent compund    
(d) ANION  ->NO₃^- CATIONS ->NH₄⁺

(a) Calcium and fluorine
Ans. CaF₂
(b) Hydrogen and sulphur
Ans. H₂S
(c)Nitrogen and hydrogen
Ans. NH₃
(d)Carbon and chlorine
Ans. CCI₄
(e)Sodium and oxygen
Ans. Na₂O
(f) Carbon and oxygen
Ans. CO, CO₂

Which of the following symbols of elements are incorrect? Give their correct
symbols
(a) Cobalt CO
(b) Carbon c
(c) Aluminium AL
(d) Helium He
(e) Sodium So
Ans. (a) Incorrect, the correct symbol of cobalt is Co
(b) Incorrect, the correct symbol of carbon is C
(c) Incorrect, the correct symbol of aluminium is Al
(d) Correct (He)
(e) Incorrect, the correct symbol of sodium is Na 

Give the chemical formulae for the following compounds and compute the ratio by mass of the combining elements in each one of them. (You may use appendix-III). 
(a) Ammonia
(b) Carbon monoxide
(c) Hydrogen chloride
(d) Aluminium fluoride
(e) Magnesium sulphide
Ans. (a) NH3; ratio N : H = 14 : 3
 (b) CO; ratio C : O = 3 : 4
 (c) HCl; ratio H : Cl = 1 : 35.5 or 2 : 71
 (d) AlF₃; ratio Al : F = 9 : 19
 (e) MgS; ratio Mg : S = 3 : 4 

(a) CO₂³⁻
Ans. 4
(b) PO₄³⁻
Ans. 5
(c) P₂O₅
 Ans. 7
 (d) CO
 Ans. 2

Yes, it is a temperature dependent property. The solubility generally, increases with increase in temperature. For example, you can dissolve more sugar in hot water than in cold water.

(a) F₂
(b) NO₂
(c) N₂O
(d) C₂H₆
(e) P₄
(f) H₂O₂
(g) P₄O₁₀
(H) O₃
(i) HCl
(j) CH₄
(k) He
(l) Ag
Ans. (a) 2 atom
(b) 3 atom
(c) 3 atom
(d) 8 atom
(e) 4 atom
(f) 4 atom
(g) 14 atom
(h) 3 atom
(i) 2 atom
(j) 5 atom
(k) 1 atom (noble gases are always monoatomic)
(l) Polyatomic (because metals are bound by metallic bond and any measurable quantity
of a metal can contain millions of atoms.)

On heating the powder, it will char if it is a sugar. Alternatively, the powder may be dissolved in water and checked for its conduction of electricity. If it conducts, it is a salt.

Number of moles = mass/molar mass = 12 g/24 g = 0.5 mol

(a) 5 moles of CO₂ and 5 moles of H₂O do not have the same mass.
Ans. CO₂ has molar mass = 44g mol^–1
5 moles of CO₂ have molar mass = 44 × 5 = 220 g
H₂O has molar mass = 18 g mol^–1
5 moles of H₂O have mass = 18 × 5 g = 90 g

Ans. Number of moles in 240g Ca metal 240 /40 = 6

Number of moles in 240g of Mg metal 240 10

Ratio 6: 10 = 3: 5 
 

(a) CaCO₃
Ans. Ca: C: O^₃ = 40: 12: 16 × 3 = 40: 12: 48 = 10: 3: 12

(b) MgCl₂
Ans. Mg: CI₂ = 24: 35.55 × 2 = 24: 71

(c) H₂SO₄
Ans. H₂: S: O₄ = 1 × 2: 32: 16 × 4 = 2: 32: 64 = 1: 16: 32

(d) C₂H₅OH
Ans. C₂: H₆: O = 12 × 2: 1 × 6: 16 = 24: 6: 16 = 12: 3: 8

(e) NH₃
Ans. N: H₃ = 14: 1 × 3 = 14: 3

(f) Ca(OH)₂
Ans. Ca: O₂: H₂ = 40: 16 × 2: 1 × 2 = 40: 32: 2 = 20: 16: 1

Calcium chloride when dissolved in water dissociates into its ions according to the following equation.

 CaCl₂ (aq) →Ca²⁺ (aq) 2Cl⁻ (aq)   
Calculate the number of ions obtained from CaCl₂  when 222 g of it is  dissolved in water.
Ans. 1 mole of calcium chloride = 111g
 ∴ 222g of CaCl₂ is equivalent to 2 moles of CaCl₂ 
 
 Since 1 formula unit CaCl₂ gives 3 ions, therefore, 1 mol of CaCl₂ will give 3 moles of ions
2 moles of CaCl₂ would give 3 ×2 = 6 moles of ions.
No. of ions = No. of moles of ions × Avogadro number
 = 6 × 6.022 × 10²³
 = 36.132 × 10²³
 = 3.6132 × 10²⁴ ions 

A sodium atom and ion, differ by one electron. For 100 moles each of sodium atoms and ions there would be a difference of 100 moles of electrons.

Mass of 100 moles of electrons= 5.48002 g

Molar mass of HgS = 200.6 + 32 = 232.6 g mol^-1

Mass of Hg in 232.6 g of HgS = 200.6 g

Mass of Hg in 225 g of HgS 

One mole of screws weigh = 2.475 × 10²⁴ g = 2.475 × 10²¹ kg

Mass of earth is 2.4 ×10³ times the mass of screws

The earth is 2400 times heavier than one mole of screws.

1 mole of oxygen atoms = 6.023 × 10²³ atoms

∴ Number or moles of oxygen atoms

4.28 moles of oxygen atoms.

(a) Mass of sodium atoms carried by Krish = (5× 23)g =115 g
 While mass of carbon atom carried by Raunak = (5× 12)g =60 g
 Thus, Krish’s container is heavy
(b) Both the bags have same number of atoms as they have same number of moles of atoms. 

32. Fill in the missing data in the Table 3.1

(a) kilo
 (b) deci
(c) centi
 (d) micro
 (e) nano
 (f) pico

(a) 5.84 × 10^-3 mg
Ans. 5.84 × 10^-9 Kg

(b) 58.34 g
Ans. 5.834 ×10^-2 Kg

(c) 0.584 g
Ans. 
5.84 ×10^-4 Kg

(d) 5.873 × 10^-21 g
Ans. 5.873 × 10²⁴ Kg

A Mg²⁺ ion and Mg atom differ by two electrons.
 10³ moles of Mg²⁺ and Mg atoms would differ by
 10³×2 moles of electrons 
 Mass of 2×10³ moles of electrons = 2 x 10³ x 6.023 x 10²³ x 9.1 x 10 ^-31 kg

1.096 x 10^-3 Kg

NH₃  would have more atoms.

5.85 g of NaCl 5.85 / 58.5 = 0.1 moles 

Or 0.1 moles of NaCl particle
 Each NaCl particle is equivalent to one Na⁺ one CI^-

⇒ 2 ions 
⇒Total moles of ions = 0.1 × 2
⇒2 moles 
No. of ions  =  0.2 × 6.022 × 10²³
⇒1.2042 × 10²³ ions 

One gram of gold sample will contain 90 /100 = 0.9  gram of gold 

Number of moles of gold =mass of gold / atomic mass of gold

0.9 / 197 = 0.0046
One moles of gold contains NA atoms = 6.022 x 10²³
∴0.0046 mole of gold will contain = 0.0046  x 6.022 x 10²³
= 2.77 x 10²¹ 

Atoms of different elements join together in definite proportions to form molecules of compounds. Examples— water, ammonia, carbondioxide. Compounds composed of metals and non-metals contain charged species. The charged species are known as ions. An ion is a charged particle and can be negatively or positively charged. A negatively charged ion is called an anion and the positively charged ion is called cation. Examples— sodium chloride, calcium oxide

Mass of 1 mole of aluminium atom = the molar mass of aluminium = 27 g mol^-1
 An aluminium atom needs to lose three electrons to become an ion, AI³⁺
 For one mole of AI³⁺ ion, three moles of electrons are to best lost. 
 
 The mass of three moles of electrons 3 x (9.1x10^-28) x 6.022 x10²³ g

 = 27.3 x 6.022 x10^-5 g
 
=164.400 x 10⁵ g
=.00164 g

Molar mass of Al3+ = (27 - 0.00164) gmol^-1
= 26.998gmol^-1

Difference = 27 – 26.9984 = 0.0016 g

(a) In a chemical reaction, the sum of the masses of the reactants and products remains
unchanged. This is called __________.
Ans. Law of conservation of mass
(b) A group of atoms carrying a fixed charge on them is called __________.
Ans. Polyatomic ion
(c) The formula unit mass of Ca₃(PO₄)₂ is __________.
Ans. (3 × atomic mass of Ca) + (2 × atomic mass of phosphorus) + (8 × atomic mass of oxygen) = 310
(d) Formula of sodium carbonate is and that of ammonium sulphate is __________.
Ans. Na₂ CO₃; (NH4)₂SO4

(a) Caustic potash
Ans. KOH
(39 + 16 + 1) = 56 g mol^-1
(b) Baking powder
Ans. NaHCO₃
23 + 1 + 12 + (3 × 16) = 84 g mol^-1
(c) Lime stone
Ans. CaCO₃
40 + 12 + (3 × 16) = 100 g mol^-1
(d) Caustic soda
Ans. NaOH
23 + 16 + 1 = 40 g mol^-1
(e) Ethanol
Ans. C₂H₅OH = C2H6O
2 × 12 + (6 × 1) + 16 = 46 g mol^-1
(f) Common salt
Ans. NaCI
23 + 35.5 = 58.5 g mol^-1