Selina ICSE Solutions for Class 9 Chemistry – The Language of Chemistry
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Selina ICSE Solutions for Class 9 Chemistry Chapter 4 The Language of Chemistry
Page No: 8
A symbol is the short form which stands for the atom of a specific element or the abbreviations used for the names of elements.
- It represents a specific element.
- It represents one atom of an element.
- A symbol represents how many atoms are present in its one gram (gm) atom.
- 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.
(a) H stands for one atom of hydrogen.
(b) H2 stands for one molecule of hydrogen.
(c) 2H stands for two atoms of hydrogen.
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
Solution 7 (a)
- Valency of Na is +1 because it can lose one electron.
- 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.
Solution 7 (b)
If an element exhibits two different positive valencies, then
- for the lower valency, use the suffix -OUS at the end of the name of the metal
- for the higher valency, use the suffix -IC at the end of the name of the metal.
|Element||Lower valency||Higher valency|
|Ferrum (Iron)||Ferrous (Fe2+)||Ferric (Fe3+)|
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.
It also indicates the fixed proportion by weight in which atoms combine.
(i) The positive and negative radicals are written side by side (+ve first) with their charge as a superscript on the right side.
(ii) Charges are then interchanged and written as a subscript.
(iii) The final formula is written without the sign of charge, e.g. Hg2O
Solution 10 (a)
Acid radical: The electronegative or negatively charged radical is called an acid radical.
Examples: Cl–, O2-
Solution 10 (b)
Basic radical: The electropositive or positively charged radical is called a basic radical.
Examples: K+, Na+
|(a) Boric acid||xvi. H3BO3|
|(b) Phosphoric acid||xvii. H3PO4|
|(c) Nitrous acid||xv. HNO2|
|(d) Nitric acid||xiv. HNO3|
|(e) Sulphurous acid||xiii. H2SO3|
|(f) Sulphuric acid||xviii. H2SO4|
|(g) (a) Hydrochloric acid||xii. HCl|
|(h) Silica (sand)||ii. SiO2|
|(i) Caustic soda
|(j) Caustic potash (potassium hydroxide)||iv. KOH|
| (k) Washing soda
(l) Baking soda
| (m) Lime stone
|(n) Water||viii. H2O|
|(o) Hydrogen sulphide||vii. H2S|
|(p) Ammonia||xi. NH3|
|(q) Phosphine||ix. PH3|
|(r) Methane||x. CH4|
|Acidic radical||Basic radical|
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
Chemical names of compounds:
- Ca3(PO4)2 – Calcium phosphate
- K2CO3 – Potassium carbonate
- K2MnO4 – Potassium manganate
- Mn3(BO3)2 – Manganese (II) borate
- Mg(HCO3)2 – Magnesium hydrogen carbonate
- Na4Fe(CN)6 – Sodium ferrocyanide
- Ba(ClO3)2 – Barium chlorate
- Ag2SO3 – Silver sulphite
- (CH3COO)2Pb – Lead acetate
- Na2SiO3 – Sodium silicate
- NaClO – Sodium hypochlorite
- NaClO2 – Sodium chlorite
- NaClO3 – Sodium chlorate
- NaClO4 – Sodium perchlorate
Solution 18 (a)
iii. The formula of a compound represents a molecule.
Solution 18 (b)
iii. The correct formula of aluminium oxide is Al2O3.
Solution 18 (c)
iv. The valency of nitrogen in nitrogen dioxide (NO2) is four.
Page No: 13
Balanced chemical equations:
- 3Fe + 4H2O → Fe3O4 + 4H2
- 3Ca + N2 → Ca3N2
- Zn + 2KOH → K2ZnO2 + H2
- Fe2O3 + 3CO → 2Fe + 3CO2
- 3PbO + 2NH3 → 3Pb + 3H2O + N2
- 2Pb3O4 → 6PbO + O2
- 2PbS + 3O2 → 2PbO + 2SO2
- S + 2H2SO4 → 3SO2 + 2H2O
- S + 6HNO3 → H2SO4 + 6NO2 + 2H2O
- MnO2 + 4HCl → MnCl2 + 2H2O + Cl2
- C + 2H2SO4 → CO2 + H2O + SO2
- 2KOH + Cl2 → KCl + KClO + H2O
- 2NO2 + H2O → HNO2 + HNO3
- Pb3O4 + 8HCl → 3PbCl2 + 4H2O + Cl2
- 2H2O + 2Cl2 → 4HCl + O2
- 2NaHCO3 → Na2CO3 + H2O + CO2
- 2HNO3 + H2S → 2NO2 + 2H2O + S
- P + 5HNO3 → 5NO2 + H2O + H3PO4
Page No: 17
(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.
(a) This equation conveys the following information:
- The actual result of a chemical change.
- Substances take part in a reaction, and substances are formed as a result of the reaction.
- Here, one molecule of zinc and one molecule of sulphuric acid react to give one molecule of zinc sulphate and one molecule of hydrogen.
- 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.
- 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
- 22.4 litres of hydrogen are formed at STP.
(b) This equation conveys the following information:
- Magnesium reacts with hydrochloric acid to form magnesium chloride and hydrogen gas.
- 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.
- Hydrogen produced at STP is 22.4 litres.
(a) A poly-atomic ion is a charged ion composed of two or more atoms covalently bounded that can be carbonate (CO32-) and sulphate (SO42-)
(b) The fundamental laws which are involved in every equation are:
- A chemical equation consists of formulae of reactants connected by plus sign (+) and arrow (→) followed by the formulae of products connected by plus sign (+).
- The sign of an arrow (→) is to read ‘to form’. It also shows the direction in which reaction is predominant.
(a) Valency of fluorine in CaF2 is -1.
(b) Valency of sulphur in SF6 is -6.
(c) Valency of phosphorus in PH3 is +3.
(d) Valency of carbon in CH4 is +4.
(e) Valency of nitrogen in the given compounds:
- N2O3 = N is +3
- N2O5 = N is +5
- NO2 = N is +4
- NO = N is +2
According to law of conservation of mass, “matter can neither be created nor be destroyed in a chemical reaction”. This is possible only, if total number of atoms on the reactants side is equals to total number of atoms on products side. Thus, a chemical reaction should be always balanced.
Let us consider an example,
Fe + H2O → Fe3O4 + H2
In this equation number of atoms on both sides is not the same, the equation is not balanced.
The balanced form of this equation is given by,
3Fe + 4H2O → Fe3O4 + 4H2
(a) The atomic mass unit (amu) is defined as 1/12th of the mass of an atom of carbon.
1 a.m.u. = 1.67 x 10-24 gm = 1.67 x 10-27 kg
1 gm mass = 6.02 x 1023 a.m.u. and 1 kg mass = 6.02 x 1026 a.m.u. (b)
- The relative molecular mass of = CuSO45H2O
= 63.5 + 32 + (16 x 4) + 5 (2 + 16)
= 159.5 + 90 = 249.5
- The relative molecular mass of = (NH4)2CO3 = N2H8CO3
= 14 x 2 + 1 x 8 + 12 + 3 x 16
= 28 + 8 + 12 + 48 = 96
- The relative molecular mass of = (NH2)2CO = N2H4CO
= 2 x 14 + 1 x 4 + 12 + 16
= 28 + 4 + 12 + 16 = 60
- The relative molecular mass of = Mg3N2 = 3 x 24 + 2 x 14 = 72 + 28 = 100
(a) (iii) Berzelius
(b) (i) One
(c) (iii) Fe2(SO4)3
(d) (i) 1: 8
(e) (ii) Ca(HCO3)2
(a) A molecular formula represent The Molecule of an element or of a Compound.
(b) The molecular formula of water (H2O) represents 18 parts by mass of water.
(c) A balanced equation obeys the law of conservation of mass wherever unbalanced equation does not obey this law.
(d) CO and Co represent carbon-monoxide and cobalt respectively.
- Relative molecular mass of CHCl3
= 12 + 1 + (3 × 35.5)
= 12 + 1 + 106.5
- Relative molecular mass of (NH4)2 Cr2O7
= (14 × 2) + (1× 8) + (52 × 2) + (16 × 7)
= 28 + 8 + 104 + 112
- Relative molecular mass of CuSO4· 5H2O
= 63.5 + 32 + (16 × 4) + 5(2 + 16)
= 159.5 + 90
- Relative molecular mass of (NH4)2SO4
= (2 × 14) + (8 × 1) + 32 + (4 × 16)
= 28 + 8 + 32 + 64
- Relative molecular mass of CH3COONa
= (12 × 2) + (1× 3) + (16 × 2) + 23
= 24 + 3 + 32 + 23
- Potassium chlorate (KClO3)
= 39.1+ 35.5 + (16 × 3)
= 39.1+ 35.5 + 48
- Ammonium chloroplatinate (NH4)2PtCl6
= (14 × 2) + (1 × 8) + 195.08 + (35.5 × 6)
= 28 + 8 + 195.08 + 213
(a) Benzene (C6H6)
Relative molecular mass of MgSO4·7H2O
=24 + 32 + (16 × 4) + 7(2 + 16)
=24 + 32 + 64 + 126
26 g of Epsom salt contains 126 g of water of crystallisation.
Hence, 100 g of Epsom salt contains
The % of H2O in MgSO4·7H2O = 51.2
(a) Relative molecular mass of Ca(H2PO4)2
= 40.07 + (1 × 4) + (30.9 × 2) + (16 × 8)
= 40.07 + 4 + 61.8 + 128
233.87 g Ca(H2PO4)2 contains 61.8 g P
So, 100 g Ca(H2PO4)2 contains
The % of P in Ca(H2PO4)2 is 26.42%.
(b) Relative molecular mass of Ca3(PO4)2
= (40.07 × 3) + (30.9 × 2) + (16 × 8)
= 120.21 + 61.8 + 128
310.01 g Ca3(PO4)2 contains 61.8 g P
So, 100 g Ca(H2PO4)2 contains
The % of P in Ca(H2PO4)2 is 19.93%.
Relative molecular mass of KClO3
= 39.09 + 35.5 + (3 × 16)
= 122.59 g
The percentages of K, Cl and O in KClO3 are 31.9%, 28.9% and 39.1%, respectively.
Relative molecular mass of urea is
|Element||No. of atoms||Atomic mass||Total|