NCERT QUESTIONS: CHAPTER-3 : ATOMS AND MOLECULES
NCERT INTEXT QUESTIONS(P-27)
- In a reaction, 5.3g of sodium carbonate reacted with 6 g of acetic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium acetate. Show that these observations are in agreement with the law of conservation of mass.
Sodium carbonate + acetic acid → Sodium acetate + carbon dioxide + water
Sodium carbonate + acetic acid → Sodium acetate + carbon dioxide + water
5.3g 6g 8.2g 2.2g 0.9g
Total mass of reactants: 5.3g + 6g = 11.3 g
Total mass of products: 2.2g + 0.9 g + 8.2 g = 11.3 g
Hence the observations are in agreement with the law of conservation of mass.
- 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?
We know hydrogen and water mix in the ratio 1: 8.
For every 1 g of hydrogen, it is 8 g of oxygen.
Therefore, for 3 g of hydrogen, the quantity of oxygen = 3 x 8 = 24 g. Hence, 24 g of oxygen would be required for the complete reaction with 3 g of hydrogen gas.
- Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
“Atoms are indivisible particles that can neither be created nor destroyed”.
- Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
“The relative number and kinds of atoms are constant in a given compound.”
NCERT INTEXT QUESTIONS(P-30)
- Define the atomic mass unit.
One atomic mass unit is a mass unit (u), which is equal to (1/12) th of the mass of an atom of carbon-12. - Why is it not possible to see an atom with naked eyes?
Atom is too small to be seen with naked eyes, measured in nanometres (1nm = 10-9m) Further, atoms of most elements do not exist independently.
NCERT INTEXT QUESTIONS(P-34)
- Write down the formulae of
(i) sodium oxide
Symbol Na O
Valency 1 2
Formula= Na2O
(ii) aluminium chloride
Symbol Al Cl
Valency 3 1
Formula= AlCl3
(iii) sodium sulphide
Symbol Na S
Valency 1 2
Formula= Na2S
(iv) magnesium hydroxide
Symbol Mg OH
Valency 2 1
Formula= Mg (OH)2
- Write down the names of compounds represented by the following formulae:
(i) Al2(SO4)3 Aluminium sulphate
(ii) CaCl2 Calcium chloride
(iii) K2SO4 Potassium sulphate
(iv) KNO3 Potassium nitrate
(v) CaCO3 Calcium carbonate
- What is meant by the term chemical formula?
The chemical formula is the symbolic representation of a chemical compound. For example- The chemical formula of hydrochloric acid is HCl.
- How many atoms are present in
(i) H2S molecule
H2S molecule has 2 atoms of hydrogen and 1 atom of sulphur. Atomicity=3
(ii) PO43- ion?
PO43- ion has 1 atom of phosphorus and 4 atoms of oxygen. Atomicity=5
NCERT INTEXT QUESTIONS(P-35)
- Calculate the molecular masses of H2 , O2, Cl2, CO2, CH4, C2H6, C2H4, NH3, CH3
- The molecular mass of H2– 2 x atomic mass of H = 2 x 1u = 2u
- The molecular mass of O2– 2 x atomic mass of O = 2 x 16u = 32u
- The molecular mass of Cl2– 2 x atomic mass of Cl = 2 x 35.5u = 71u
- The molecular mass of CO2– atomic mass of C + 2 x atomic mass of O = 12 + ( 2×16)u = 44u
- The molecular mass of CH4– atomic mass of C + 4 x atomic mass of H = 12 + ( 4 x 1)u = 16u
- The molecular mass of C2H6– 2 x atomic mass of C + 6 x atomic mass of H = (2 x 12) + (6 x 1)u=24+6=30u
- The molecular mass of C2H4– 2 x atomic mass of C + 4 x atomic mass of H = (2x 12) +(4 x 1)u=24+4=28u
- The molecular mass of NH3– atomic mass of N + 3 x atomic mass of H = (14 +3 x 1)u= 17u
- The molecular mass of CH3OH – atomic mass of C + 3x atomic mass of H + atomic mass of O + atomic mass of H = (12 + 3×1+16+1)u=(12+3+17)u = 32u
- Calculate the formula unit masses of ZnO, Na2 O, K2 CO3, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.
The formula unit mass of ZnO = Atomic mass of Zn + Atomic mass of O = 65u + 16u = 81u
The formula unit mass of Na2O = 2 x Atomic mass of Na + Atomic mass of O = (2 x 23)u + 16u = 46u + 16u = 62u
The formula unit mass of K2CO3 = 2 x Atomic mass of K + Atomic mass of C + 3 x Atomic mass of O = (2 x 39)u + 12u + (3 x 16)u = 78u + 12u + 48u = 138u
NCERT EXERCISES (P-36)
- A 0.24 g sample of a 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.
Mass of the sample compound = 0.24 g,
mass of boron = 0.096 g,
mass of oxygen = 0.144 g
Percentage of boron = (mass of boron)/(mass of compound) x 100 = (0.096 g )/(0.24 g ) x 100 = 40%
Percentage of oxygen = 100 – percentage of boron = 100 – 40 = 60%
- When 3.0 g of carbon is burnt in 8.00 g of 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?The reaction of burning carbon in oxygen may be written as:
C + O2 → CO2 12g 32g 44g 12 g of carbon bums in 32 g oxygen to form 44 g of carbon dioxide.
⸫3 g of carbon reacts with 8 g of oxygen to form 11 g of carbon dioxide. 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.
- What are polyatomic ions? Give examples.
Polyatomic ions: A group of atoms having an electric charge is called a polyatomic ion. Positively charged polyatomic ions are called polyatomic cations, and negatively charged polyatomic ions are called polyatomic anions.
Examples:Polyatomic cation: Ammonium ion, NH4+; Phosphonium ion, PH4+Polyatomic anion: Sulphate ion, SO42-; Nitrate ion, NO3–; Phosphate ion, PO43-
4. Write the chemical formula of the following.
(a) Magnesium chloride MgCl2
(b) Calcium oxide CaO
(c) Copper nitrate Cu (NO3)2
(d) Aluminium chloride AlCl3
(e) Calcium carbonate CaCO3
5. Give the names of the elements present in the following compounds.
(a) Quick lime Calcium and Oxygen (CaO)
(b) Hydrogen bromide Hydrogen and Bromine (HBr)
(c) Baking powder Sodium, Carbon, Hydrogen, Oxygen (NaHCO3)
(d) Potassium sulphate. Sulphur, Oxygen, and Potassium (K2SO4)
6. Calculate the molar mass of the following substances.
(a) Ethyne, C2H2
Molar mass of Ethyne C2H2= 2 x atomic mass of C+2 x atomic mass of H = (2×12)+(2×1)=24+2=26g
(b) Sulphur molecule, S8
Molar mass of Sulphur molecule S8 = 8 x atomic mass of S = 8 x 32 = 256 g
(c) Phosphorus molecule, P4 (Atomic mass of phosphorus = 31)
Molar mass of Phosphorus molecule, P4 = 4 x Mass of P = 4 x 31 = 124 g
(d) Hydrochloric acid, HCl
Molar mass of Hydrochloric acid, HCl = Mass of H+ Mass of Cl = 1+35.5 = 36.5g
(e) Nitric acid, HNO3
Molar mass of Nitric acid, HNO3 = Mass of H+ Mass of Nitrogen + 3 x Mass of O = 1 + 14 + 3×16 = 63 g