Class 9 SELINA Solutions Chemistry Chapter 1: The Language of Chemistry

iii. The formula of a compound represents a molecule.

iii. The correct formula of aluminium oxide is Al₂O₃.

iv. The valency of nitrogen in nitrogen dioxide (NO₂) is four.

a. H - Hydrogen atom

b. H₂ - Dihydrogen or hydrogen gas

c. 2H - Two hydrogen atoms

d. 2H₂ - Two dihydrogen molecule

Chemical names of compounds:

1. Ca₃(PO₄)₂ - Calcium phosphate 
2. K₂CO₃ - Potassium carbonate
3. K₂MnO₄ - Potassium manganate  
4. Mn₃(BO₃)₂ - Manganese (II) borate
5. Mg(HCO₃)₂ - Magnesium hydrogen carbonate 
6. Na₄Fe(CN)₆ - Sodium ferrocyanide
7. Ba(ClO₃)₂ - Barium chlorate  
8. Ag₂SO₃ - Silver sulphite
9. (CH₃COO)₂Pb - Lead acetate 
10. Na₂SiO₃ - Sodium silicate 

Valencies of aluminium, ammonium and zinc are 3, 1 and 2, respectively.

The valency of sulphate is 2.

Hence, chemical formulae of the sulphates of aluminium, ammonium and zinc are Al₂(SO₄)₃, (NH₄)₂SO₄ and ZnSO₄.

Formula of the compound = A₂B₃

1. KClO- Potassium hypochlorite
2. KClO₂- Potassium chlorite
3. KClO₃- Potassium chlorate
4. KClO₄- Potassium perchlorate

The valency of metal M is 3. So, the formulae are as follows:

1. Chloride - MCl₃
2. Oxide - M₂O₃
3. Phosphate - M(PO₄)
4. Acetate - M(CH₃COO)₃ 

A symbol is the short form which stands for the atom of a specific element or the abbreviations used for the names of elements.

i. It represents a specific element.
ii. It represents one atom of an element.
iii. A symbol represents how many atoms are present in its one gram (gm) atom.
iv. It represents the number of times an atom is heavier than one atomic mass unit (amu) taken as a standard.

In most cases, the first letter of the name of the element is taken as the symbol for that element and written in capitals (e.g. for sulphur, we use the symbol S). In cases where the first letter has already been adopted, we use a symbol derived from the Latin name (e.g. for sodium/Natrium, we use the symbol Na). In some cases, we use the initial letter in capital together with a small letter from its name (e.g. for silicon, we use the symbol Si). 

The full form of IUPAC is International Union of Pure and Applied Chemistry.

Names of the elements:

Au - Gold 

Pb - Lead

Sn - Tin

Hg - Mercury

Co stands for Cobalt. If we write CO, then it would mean that it is a compound containing two non-metal ions, i.e. carbon and oxygen, which forms carbon monoxide gas. 

The number of atoms of an element that join together to form a molecule of that element is known as its atomicity.

Diatomic molecules: H₂, O₂, N₂, Cl₂

1. Valency of Na is +1 because it can lose one electron.
2. Valency of O is -2 because it can accept two electrons.

Variable valency: It is the combining capacity of an element in which the metal loses more electrons from a shell next to a valence shell in addition to electrons of the valence shell.

If an element exhibits two different positive valencies, then  

 i. for the lower valency, use the suffix -OUS at the end of the name of the metal

 ii. for the higher valency, use the suffix -IC at the end of the name of the metal.

Example:

1. Chemical formula: The chemical formula of a substance (element or compound) is a symbolic representation of the actual number of atoms present in one molecule of that substance.
2. Significance of the molecular formula:

 i. It represents both molecule and molecular mass of the compound.

 ii. It represents the respective number of different atoms present in one molecule of the compound.

 iii. It represents the ratios of the respective masses of the elements present in the compound.e.g. Hg₂O 1. Hg¹⁺O^2-

2.

3. Hg₂O

Acid radical: The electronegative or negatively charged radical is called an acid radical.  

Examples: Cl^-, O^2- 

Basic radical: The electropositive or positively charged radical is called a basic radical.  

Examples: K⁺, Na⁺ 

1. Sodium sulphate - Na₂SO₄
   There are two sodium atoms, one sulphur atom and four oxygen atoms.

2. Quick lime - CaO
   There is one calcium atom and one oxygen atom.

3. Baking soda - NaHCO₃
   There is one sodium, carbon and hydrogen atom and three oxygen atoms.

4. Ammonia - NH₃
   There is one nitrogen atom and three hydrogen atoms.

5. Ammonium dichromate - (NH₄)Cr₂O₇
   There two ammonium atoms, two chromium atoms and seven oxygen atoms.

A chemical equation is the symbolic representation of a chemical reaction using the symbols and formulae of the substances involved in the reaction.

A chemical equation needs to be balanced because a chemical reaction is just a rearrangement of atoms.

Atoms themselves are neither created nor destroyed during the course of a chemical reaction.

The chemical equation needs to be balanced to follow the law of conservation of mass.

A solid metal zinc reacts with hydrochloric acid in the aqueous state to produce zinc chloride in the aqueous state and hydrogen gas.

• The chemical equation given in question 2 does not give the time taken for the completion of the reaction.
• Also, it does not give information about whether heat is absorbed or evolved during the reaction.

1. C + O₂→ CO₂
2. N₂ + O₂→ 2NO
3. 3Ca + N₂→ Ca₃N₂
4. CaO + CO₂→ CaCO₃
5. Mg + H₂SO₄→ MgSO₄ + H₂↑ 
6. Na + H₂O → NaOH + H₂↑

Balanced chemical equations:

1. 3Fe + 4H₂O → Fe₃O₄ + 4H₂
2. 3Ca + N₂ → Ca₃N₂
3. Zn + 2KOH → K₂ZnO₂ + H₂
4. Fe₂O₃ + 3CO → 2Fe + 3CO₂
5. 3PbO + 2NH₃ → 3Pb + 3H₂O + N₂
6. 2Pb₃O₄ → 6PbO + O₂
7. 2PbS + 3O₂ → 2PbO + 2SO₂
8. S + 2H₂SO₄ → 3SO₂ + 2H₂O
9. S + 6HNO₃ → H₂SO₄ + 6NO₂ + 2H₂O
10. MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂
11. C + 2H₂SO₄ → CO₂ + H₂O + SO₂
12. 2KOH + Cl₂ → KCl + KClO + H₂O
13. 2NO₂ + H₂O → HNO₂ + HNO₃
14. Pb₃O₄ + 8HCl → 3PbCl₂ + 4H₂O + Cl₂
15. 2H₂O + 2Cl₂ → 4HCl + O₂
16. 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
17. 2HNO₃ + H₂S → 2NO₂ + 2H₂O + S
18. P + 5HNO₃ → 5NO₂ + H₂O + H₃PO₄ 
19. Zn + 4HNO₃  → Zn(NO₃)₂ + 2H₂O + 2NO₂

iii. Berzelius

i. One

iii. Fe₂(SO₄)₃

i. 1:8

ii. Ca(HCO₃)₂

Correct option - (iii): C, Ca, Cu, Cd

Solution:

The correct atomic symbols for carbon, calcium, copper and cadmium is C, Ca, Cu and Cd.

Correct option - (iii): Mercury

Solution:

Hydraygyrum is the Latin name of Mercury.

Correct option - (iv): 2 and 3

Solution:

Correct option - (ii): 2

Solution:

Calcium is divalent, hence it will bond with two molecules of phosphate anion.

Correct option - (iii): 4

Solution:

Carbon is tetravalent.

1. Dalton used symbol [O] for oxygen,[H] for hydrogen.
2. Symbol represents gram atom(s) of an element.
3. Symbolic expression for a molecule is called molecular formula.
4. Sodium chloride has two radicals. Sodium is a basic radical, while chloride is an acid radical.
5. Valency of phosphorus in PCl₃, is 3 and in PCl₅ is 5.
6. Valency of iron in FeCl₂ is 2 and in FeCl₃ it is 3.
7. Formula of iron (III) carbonate is Fe₂[CO₃]₃.

The correct statements are -

1. The number of elements present in a molecule is represented by molecular formula.
2. The molecular formula of water is HO. HO is the molecular formula of hydrogen peroxide.
3. A molecule of sulfur is not monoatomic. It consists of 8 atoms.
4. Co represents cobalt and CO represents carbon monoxide.
5. The formula of iron (III) oxide is Fe₂O₃.

The empirical formula is:

1. Benzene (C₆H₆) = CH
2. Glucose (C₆H₁₂O₆) = CH₆O
3. Acetylene (C₂H₂) = CH
4. Acetic acid (CH₃COOH) = C₂H₂O

Valency of fluorine in CaF₂ is -1. 

Valency of sulphur in SF₆ is -6.

Valency of phosphorus in PH₃ is +3.

Valency of carbon in CH₄ is +4.

Valency of Manganese in MnO₂ is +4.

Valency of Copper in Cu₂O is +1

Valency of Magnesium in Mg₃N₂ is +2

Valency of nitrogen in the given compounds:

1. N₂O₃  = N is +3 
2. N₂O₅ = N is +5
3. NO₂ = N is +4
4. NO = N is +2

According to the law of conservation of mass, 'matter can neither be created nor can it be destroyed'. This is possible only if the total number of atoms on the reactants side is equal to the total number of atoms on the products side. Thus, a chemical reaction should always be balanced.

e.g.  KNO₃ → KNO₂ + O₂

In this equation, the number of atoms on both sides is not the same, and the equation is not balanced.

The balanced form of this equation is

2KNO₃ → 2KNO₂ + O₂

Atomic mass unit (amu) is equal to one-twelfth the mass of an atom of carbon-12 (atomic mass of carbon taken as 12).

2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

2KHCO₃ + H₂SO₄ → K₂SO₄ + 2CO₂ + 2H₂O

Fe + H₂SO₄ → FeSO₄ + H₂

Cl₂ + SO₂ + 2H₂O → H₂SO₄ + 2HCl

2AgNO₃ → 2Ag + 2NO₂ + O₂

3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 2NO + 4H₂O

BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

2ZnS + 3O₂ → 2ZnO + 2SO₂

Al₄C₃ + 12H₂O → 4Al(OH)₃ + 3CH₄

 4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂

2KMnO₄ + HCl → 2KCl + 2MnCl₂ + 5Cl₂ + 8H₂O

Al₂(SO₄)₃ + 8NaOH → 3Na₂SO₄ + 2NaAlO₂ + 4H₂O

2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂

2K₂Cr₂O₇ + 8H₂SO₄ → 2K₂SO₄ + 2Cr₂(SO₄)₃ + 8H₂O + 3O₂

K₂Cr₂O₇ + 14HCl → 2KCl + 2CrCl₃ + 7H₂O + 3Cl₂

S + HNO₃ → H₂SO₄ + NO₂ + H₂O 

2NaCl + MnO₂ + 3H₂SO₄ → 2NaHSO₄ + MnSO₄ + 2H₂O + Cl₂

a. NaCl + AgNO₃ → NaNO₃ + AgCl↓

b. It is a balanced equation.

c. Weights of reactants: NaCl - 58.44, AgNO₃ - 169.87 

    Weights of products: NaNO₃ - 84.99, AgCl - 143.32

    NaCl + AgNO₃ → NaNO₃  + AgCl

    (23+35.5) + (108+14+48) → (23+14+48) + (108+35.5)

    58.5 + 170 → 85 + 143.5

    228.5 g → 228.5 g

d. Law of conservation of mass: Matter is neither created nor destroyed in the course of a chemical reaction.

This equation conveys the following information:

1. The actual result of a chemical change.
2. Substances take part in a reaction, and substances are formed as a result of the reaction.
3. Here, one molecule of zinc and one molecule of sulphuric acid react to give one molecule of zinc sulphate and one molecule of hydrogen.
4. Composition of respective molecules, i.e. one molecule of sulphuric acid contains two atoms of hydrogen, one atom of sulphur and four atoms of oxygen.
5. Relative molecular masses of different substances, i.e. molecular mass of

Zn = 65

H₂SO₄ = (2+32+64) = 98

ZnSO₄ = (65+32+64) = 161  

H₂ = 2

6. 22.4 litres of hydrogen are formed at STP.

This equation conveys the following information:

1. Magnesium reacts with hydrochloric acid to form magnesium chloride and hydrogen gas.
2. 24 g of magnesium reacts with 2(1 + 35.5) = 73 g of hydrochloric acid to produce (24 + 71), i.e. 95 g of magnesium chloride.
3. Hydrogen produced at STP is 22.4 litres. 

A polyatomic ion is a charged ion composed of two or more covalently bounded atoms. Examples: Carbonate (CO₃^2-) and sulphate (SO₄^2-)

Fundamental laws which are involved in every equation:

1. A chemical equation consists of formulae of reactants connected by a plus sign (+) and arrow (→) followed by the formulae of products connected by the plus sign (+).
2. The sign of an arrow (→) is to read 'to form'. It also shows the direction in which the reaction is predominant.

The fundamental law followed by every equation is 'Law of Conservation of Mass'.

The symbolic representation of a molecule is known as formula or molecular formula.

A molecular formula also known as chemical formula employs symbols to denote the molecule of an element or of a compound.

• The chemical formula of a compound provides information about the types of atoms that make up the compound, as well as the ratio in which they are combined.

• The elements that make up the compound: The chemical symbols of the elements present in the compound are used to write the formula.

For example,

The chemical formula H₂O tells us that the compound is made up of two hydrogen atoms (H) and one oxygen atom (O).

• A molecule of an element may contain one or more atoms of it.

For example,

A molecule of elements hydrogen, oxygen, nitrogen, chlorine, bromine and iodine, contains two atoms and are written as H₂, O₂, N₂, Cl₂, Br₂ and I₂ respectively.

• In case of a compound, the molecule containing different atoms united in a certain fixed ratio is represented by placing symbols of the elements present in it side by side indicating their numbers written in subscripts.

For example,

NH₄Cl represents one molecule of ammonium chloride containing one atom of nitrogen, four atoms of hydrogen and one atom of chlorine.

Na₂CO₃ denotes one molecule of sodium carbonate which contains two atoms of sodium, one atom of carbon and three atoms of oxygen.

2H₂O represents two molecules of water i.e. dihydrogen oxide, each molecule containing two atoms of hydrogen and one atom of oxygen.

Thus, by looking at a formula, we understand the ratio in which the different atoms are united to form the molecule.

 The molecular formula of a compound has quantitative significance. It represents:

(i) Both the molecule and the molecular mass of the compound.

(ii) The respective numbers of different atoms present in one molecule of the compound.

(iii) The ratios of the respective masses of the elements present in the compound.

For example, the formula CO₂ means that:

(i) The molecular formula of carbon dioxide is CO₂.

(ii) Each molecule contains one carbon atom joined by chemical bonds with two oxygen atoms.

(iii) The molecular mass of carbon dioxide is 44, given that the atomic mass of carbon is 12 and that of oxygen is 16.

A radical is an atom or a group of atoms of the same or of different elements that behaves as a single unit with a positive or negative charge.

A radical is called a simple radical when it is an atom only like sodium (Na⁺) and magnesium (Mg²⁺). It is known as a compound radical when it is made up of a group of two or more different atoms (polyatomic) like sulphate (SO₄^2-) which is made up of one sulphur atom and four oxygen atoms.

A radical with positive charge is a cation, e.g., NH⁴⁺ (ammonium ion), Na⁺ (sodium ion) and a radical with negative charge is an anion, e.g. Cl⁻ (chloride), CO₃^2- (carbonate).An acid reacts with a base to produce salt and water as a result of neutralization.

Potassium hydroxide + Hydrochloric acid → Potassium chloride + Water

 (Base) (Acid) (Salt) 

In the formation of potassium chloride, the potassium radical has been contributed by the base potassium hydroxide and is therefore called a basic radical while the chloride radical has been contributed by hydrochloric acid and is, therefore, termed an acid radical.

(a)  X and Y

Valence shell electrons in X and Y are 3 and 7 respectively.

3 × X = (8-7)×Y

By rearranging the equation,

 X/Y = 1/3

Same number of electrons has to be shared/donated.

Hence, the chemical formula is XY₃.

(b) X and Z

Valence shell electrons in X and Z are 3 and 6 respectively.

Same number of electrons has to be shared/donated.

3 × X = (8-6) × Y

X/Y = 2/3

Hence, the chemical formula of the compound is X₂Y₃.

(a)  

The number of carbon atoms = 6

The number of hydrogen atoms = 12

(b)  Molecular formula = C₆H₁₂

Empirical formula = CH₆

(c) Percentage composition of all the elements present in the compound:

Relative atomic mass of C6H12 = (6×12)+(12×1) = 84 amu

% of carbon in the compound

= (atomic mass of carbon / atomic mass of molecule) × 100 = (72/84)×100 = 85.7% is the percentage of carbon atoms in the compound.

% of hydrogen in the compound

= (atomic mass of hydrogen / atomic mass of molecule) × 100 = (12/84) × 100 = 14.3% is the percentage of hydrogen atoms in the compound.

Molecular mass of Na₂SO₄.10H₂O

= (2⨯23) + 32 +(4⨯16)+(10⨯18)

= 46 + 32 + 64 + 180

= 322

Molecular mass of (NH₄)₂CO₃

= (2 × 14) + (8 × 1) + 12 + (3 × 16)

= 28 + 8 + 12 + 48

= 96

Molecular mass of (NH₂)₂CO

= (14 × 2) + (4 × 1) + 12 + 16

= 28 + 4 + 12 + 16

= 60 

Molecular mass of Mg₃N₂

= (3 × 24) + (2 × 14)

= 72 + 28

= 100

1. CHCl₃ = (C)12 + (H)1+ (3Cl)3 x 35.5 = 119.5
2. (NH₄)₂Cr₂O₇ = (2N)28 + (8H)8 + (2Cr)2 x 51.9+ (7O)7 x 16 = 252
3. CuSO₄.5H₂O = (Cu)63.5 + (S)32 + (4O)64 + (5H₂O)5 x 18 = 249.5
4. (NH₄)₂SO₄ = (2N)28 + (8H)8 + (S)32 + (4O)64 = 132
5. CH₃COONa = (C)12 + (3H)3 + (C)12 + (2O)32 + (Na)23 = 82
6. Potassium chlorate KClO₃ = (K)39 + (Cl)35.5 + (3O)48 = 122.5
7. Ammonium chloroplatinate, (NH₄)₂PtCl₆ = (2N)28 + (8H)8 + (Pt)195 + (6Cl)35.5 x 6 = 444

Given, 

Mass of epsom salt, MgSO₄.7H₂O.= 246.36 g/mole

Mass of water = 18 g/mole

Mass of 7H₂O = 7⨯18 = 126 g/mole

Now we have to calculate the percentage mass of water in the epsom salt.

% mass of water =   

Now put all the given values in this formula, we get the percentage mass of water in the epsom salt.

% mass of water =   

Therefore, the percentage mass of water in the epsom salt is, 51.14 % 

1. Calcium hydrogen phosphate Ca(H₂PO₄)₂

Molecular mass of Ca(H₂PO₄)₂ = 234 g/mole

Atomic weight of P = 31

% of P in Ca(H₂PO₄)₂  =  

= 26.49 % of P

2. Calcium phosphate Ca₃(PO₄)₂

Molecular mass of Ca₃(PO₄)₂ = 310.17 g/mole

Atomic weight of P = 31

% of P in Ca(H₂PO₄)₂ = = 19.98 % of P 

Molecular mass of KClO₃ = 122.5 g

% of K = 39 /122.5 = 31.8%

% of Cl = 35.5/122.5 = 28.98%

% of O = 3 × 16/122.5 = 39.18%

Urea has the formula NH₂CONH₂, i.e. CON₂H₄

Molecular mass of urea = 60 g/mol

Atomic mass of C = 12 and number of atoms = 1 i.e. 12 g/mol of urea

Thus the % of C = 12/60 × 100 = 20 %