ADDITIONAL QUESTIONS AND ANSWERS:
SHORT QUESTIONS:
1. Define law of conservation of mass.
Ans: In a chemical reaction mass can neither be created nor destroyed.
E.g. 2Na + Cl2 → 2NaCl
2 x 23 + 2 x 35.5 → 2(23 + 35.5)
2. Explain law of constant proportion.
Ans: In a chemical substance the elements are always present in definite proportion by mass.
Eg., In water, the ratio of the mass of hydrogen to the mass of oxygen H:O is always 1:8
3. Who coined the term atom.
Ans: Dalton coined the term atom.
4. Define (i) atom (ii) molecule (iii) atomicity (iv) atomic mass unit.
Ans: (i) The smallest particle of matter, which can take part in a chemical reaction is called atom.
(ii) The smallest particle of an element or compound which can exist independently is called molecule
(iii) The number of atoms constituting a molecule is known as its atomicity.
(iv) The sum of the atomic masses of all the atoms in a molecule of the substance is expressed in atomic mass unit.
5. How do atoms exist.
Ans: Atoms exist in the form of atoms, molecules or ions.
6. Give the atomicity of phosphorous and nitrogen.
Ans: The atomicity of phosphorus is P4 (4).The atomicity of nitrogen is N2 (2).
7. What is an ion.
Ans: Charged atom is called as an ion. The ion can be positively charged called cation or negatively charged called anion.
8. Give one example of cation and anion.
Ans: Cation is Na+. Anion is Cl–
9. Find the molecular mass of H20.
Ans: Molecular mass of H20 = (2 x 1) + (16) = 2 + 16 = 18 u
10. Give the chemical formula for Ammonium Sulphate.
Ans: Ammonium Sulphate NH4+SO42- Chemical formula → (NH4)2S04.
11. What is Avogadro’s constant.
Ans: The Avogadro’s constant (6.022 x 1023) is defined as the number of atoms that are present in exactly 12 g of Carbon−12.
12. Initially the atomic mass unit was taken as 1/16th of the mass of naturally occurring oxygen atom by scientists. Why was naturally occurring oxygen atom initially chosen by scientists?
Ans. It is because oxygen reacted with a large number of elements and using this atomic mass unit, masses of most of the atoms of elements was arrived at as whole numbers.
13.Which element was later chosen by scientists
Ans. The element later chosen by scientists was Carbon−12
14. Define atomicity . Give an example of each of monoatomic , diatomic , tetra- atomic and polyatomic molecules .
Ans : Atomicity is defined as the number of atoms present in a molecule .
He is monoatomic , H2 is diatomic , P4 is tetra atomic and S8 is polyatomic molecules
15. How would you differentiate between a molecule of an element and a molecule of a compound ? Write one example of each type .
Ans : Molecule of an element contains the same kind of atoms. P4 is a molecule of element which contains all four atoms of phosphorus . Molecule of compound contains two or more kinds of atoms
e.g. H2O is a molecule of compound which contains 2 atoms of hydrogen and 1 atom of oxygen .
16. Give one word for the following :
i) Positively charged ion
ii) A group of atoms carrying a charge .
Ans : i) Cation ii) Polyatomic ion .
17. You are provided with a fine white coloured powder which is either sugar or salt . How would you identify it without testing ?
Ans : Heat the given substance . If it turns black on heating, then it is sugar , otherwise salt because sugar will lose water on heating and black coloured carbon will be left back .
18. Does the solubility of a substance change with temperature ? Explain with the help of an example .
Ans : Yes ,the solubility of solid in liquid increases with increase in temperature , e.g. cold water can dissolve less amount of sugar , whereas hot water can dissolve more amount of sugar .
19. Define the law of constant proportion .
Ans : A compound prepared by any method contains the same elements in the fixed ratio by mass is the law of constant proportion .
20. What is (i) an atom (ii) a molecule
Ans : The smallest particle which may or may not exist in free state in nature but takes part in a chemical reaction . It is made up of atoms . It exists in nature in the free state .
21. Name the Indian philosopher who postulated the concept of paramanu.
Ans: Maharshi Kanada
22. Give one difference between cation and anion.
Ans: Anions are negatively charged ion. Cations are positively charged ion.
NUMERICAL PROBLEMS:
The atomic masses of the elements are given below: Hydrogen = lu, Carbon = 12u, Nitrogen = 14u, Oxygen = 16u, Sodium = 23u, Magnesium = 24u, Sulphur = 32u, Chlorine = 35.5u, Calcium = 40u, Zinc = 65u, Potassium = 39u, Aluminium = 27u Silver = 108u, lodine = 127u
The valency of ions is as under:
Sodium = 1+, Potassium = 1+, Oxide = 2−, Aluminium = 3+, Magnesium= 2+. Zinc = 2+. Sulphide = 2−, Ammonium = 1+, Hydroxide = 1− Carbonate = 2−,
Sulphate = 2−, Phosphate = 3+, Calcium = 2+.
1. Calculate the molecular mass of (i) HCl ii) ZnSO
Ans. (i) The molecular mass of HCl
= Atomic mass of H + Atomic mass of Cl = 1 +3
(ii) The molecular mass of ZnSO
= Atomic mass of Zn+Atomic mass of S+ (4 x Atomic mass of O
= 65 + 32 + 4 x 16
= 161 u
2. Calculate the gram molecular mass or molar mass of each of
i) Silver iodide (ii) Calcium hydroxide.
Ans. (i) Molar mass of Agl = 108+ 127 = 235 grams
(ii) Molar mass of Ca(OH)2= 40 + [2 x (16+1)] = 40 + 34 = 74 grams.
3. Calculate the formula unit mass of Calcium carbonate
Ans. Formula of Calcium carbonate is CaCO3.
Formula unit mass of Calcium carbonate
= Atomic mass of Ca + atomic mass of C + (3 x atomic mass no )
= 40 + 12 + (3 x 16) = 100 u
4. Calculate the mass of
(i) 3.011 x 1023 number of oxygen molecules (O2)
(ii) 6.022 X 1023 oxygen atoms (O).
Ans. (i) Number of moles = Given number/ Avogadro number = N/ No
= 3.011 x 1023 / 6.022 x 1023= 0.5 moles
Mass of (O2) = molar mass x number of moles
= 0.5 x 16 = 8g
(ii) Number of moles = Given number/ Avogadro number = N/ No
=6.022 x 1022/ 6.022 x 1022 = 0.1 mole
Mass of (O2) = molar mass x number of moles = 0.1 x 8 = 0.8 g
5. Calculate the number of moles for the following:
i) 60 g of Sodium hydroxide
ii) 49 g of Hydrogen sulphate (Sulphuric acid)
iii) 210 g of NaHC0,
Ans. (i) Number of moles = Given mass/ Molar mass
∴ Molar mass of NaOH = 23 + 16 +1 = 40g
∴ Number of moles = 60/40 = 1.5
(ii) Number of moles = Given mass/ Molar mass
∴ Molar mass of H, SO, = 2 + 32 +(4 x 16) = 98g
∴ Number of moles = 49 /98 = 0.5
(iii) Number of moles = Given mass/ Molar mass;
Molar mass of NaHCO3 = 23 + 1 + 12 + (3 x 16) = 84g
∴ Number of moles = 210/84 = 2.5
6. Calculate the mass of the following
(i) 6 moles of Aluminium
(ii) 0.1 moles of Chlorine gas (Cl2)
(iii) 5 moles of Potassium nitrate.
Ans: (i) Molar mass x number of moles
= 27 x 6 = 162 g
(ii) Mass = molar mass x number of moles
= 35.5 x 0.1 = 3.55 g
(iii) Mass = molar mass x number of moles
Molar mass of KNO3= 39 + 14 + (3 x 16) = 101 g.
Mass = 101 x 5 = 505 g