# Atoms and Molecules : NCERT Intext Questions

Page 32

Q.1      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,0.9g 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 $\to$ sodium ethanoate + carbon dioxide + water
Sol.       In a reaction, sodium carbonate  reacts with ethanoic acid to produce sodium ethanoate, carbondioxide, and water.
Sodium      +      Ethanoic $\to$ Sodium    +     Carbon    +    Water
Carbonate            acid            ethanoate        dioxide
Mass of sodium carbonate = 5.3g (Given)
Mass of ethanoic acid = 6g (Given)
Mass of sodium ethanoate = 8.2g (Given)
Mass of carbon dioxide = 2.2 (Given)
Mass of water = 0.9g (Given)
Now, total mass before the reaction = (5.3 + 6)g
= 11. 3g
and total mass after the reaction = (8.2 + 2.2 + 0.9)g
= 11.3g
Therefore, Total mass before the reaction = Total mass after the reaction
Hence, the given observations are in agreement with the law of conservation of mass.

Page 33

Q.2      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 3g of hydrogen gas?
Sol.      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 1g of hydrogen gas is 8g . Therefore, the mass of oxygen gas required to react completely with 3g of hydrogen gas is 8 × 3g = 24 g.

Q.3     Which postulate of Dalton's atomic theory is the result of the law of conservation of mass?
Sol.      The postulate of Dalton's atomic theory which is a result of the law of conservation of mass is : Atoms are indivisible particles, which can neither be created nor destroyed in a chemical reaction.

Q.4     Which postulate of Dalton's atomic theory can explain the law of definite proportions?
Sol.      The postulate of Dalton's atomic theory which can explain the law of definite proportion is: The relative number and kind of atoms in a given compound remains constant.

Page 35

Q.1     Define atomic mass unit.

Sol.      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'

Q.2     Why is it not possible to see an atom with naked eyes?
Sol.      The size of an atom is so small that it is not possible to see it with naked eyes. Also, atom of an element does not exist independently.

Page 39

Q.1     Write down the formula of

(i) sodium oxide
(ii) aluminium chloride
(iii) sodium suphide
(iv) magnesium hydroxide
Sol.     (i) Sodium oxide $\to$ $N{a_2}O$
(ii) Aluminium chloride $\to$ $AlC{l_3}$
(iii) Sodium suphide $\to$ $N{a_2}S$
(iv) Magnesium hydroxide $\to$ $Mg{(OH)_2}$

Q.2      Write down the names of compounds represented by the following formula:
(i) $A{l_2}{\left( {S{O_4}} \right)_3}$
(ii) $CaC{l_2}$
(iii) ${K_2}S{O_4}$
(iv) $KN{O_3}$
(v) $CaC{O_3}$
Sol.      (i) $Al{\left( {S{O_4}} \right)_3} \to$ Aluminium sulphate
(ii) $CaC{l_2} \to$ Calcium chloride
(iii) ${K_2}S{O_4} \to$ Potassium sulphate
(iv) $CaC{O_3} \to$ Calcium carbonate

Q.3     What is meant by the term chemical formula?
Sol.      The chemical formula of a compound means the symbolic representation of the composition of
a compound. From the chemical formula of a compound, we can know the number and kinds of atoms of different elements that constitute the compound. For example, from the chemical formula $C{o_2}$ of carbon dioxide, we come to know that one carbon atom and two oxygen atoms are chemically bonded together to form one molecule of the compound, carbon dioxide.

Q.4     How many atoms are present in a
(i) ${H_2}S$ molecule and
(ii) $PO_4^{3 - }$ ion?
Sol.      (i) In an ${H_2}S$ molecule, three atoms are present; two of hydrogen and one of sulphur.
(ii) In a $PO_4^{3 - }$- ion, five atoms are present; one of phosphorus and four of oxygen.

Page 40

Q.1     Calculate the molecular masses of ${H_2},{O_2},C{l_2},C{O_2},C{H_4},{C_2}{H_6},{C_2}{H_4},N{H_3},C{H_3}OH.$
Sol.      Molecular mass of ${H_2}$ = 2 × Atomic mass of H
= 2 × 1
= 2u
Molecular mass of ${O_2}$ = 2 × Atomic mass of O
= 2 × 16
= 32u
Molecular mass of $C{l_2}$ = 2 × Atomic mass of Cl
= 2 × 35.5
= 71 u
Molecular mass of $C{O_2}$ = Atomic mass of C + 2 × Atomic mass of O
= 12 + 2 × 16
= 44 u
Molecular mass of $C{H_4}$ = Atomic mass of C + 4 × Atomic mass of H
= 12 + 4 × 1
= 16 u
Molecular mass of ${C_2}{H_6}$ = 2 × Atomic mass of C + 6 × Atomic mass of H
= 2 × 12 + 6 × 1
= 30u
Molecular mass of ${C_2}{H_4}$ = 2 × Atomic mass of C + 4 × Atomic mass of H
= 2 × 12 + 4 × 1
= 28u
Molecular mass of $N{H_3}$ = Atomic mass of N + 3 × Atomic mass of H
= 14 + 3 × 1
=17 u
Molecular mass of $C{H_3}OH$ Atomic mass of C + 4 × Atomic mass of H + Atomic mass of O
= 12 + 4 × 1 + 16
= 32 u

Q.2     Calculate the formula unit masses of ZnO, $N{a_2}O,{K_2}C{O_3}$, given masses of Zn = 65u, Na= 23u, K =39u, C = 12u, and O = 16u.
Sol.      Formula unit mass of ZnO = Atomic mass of Zn + Atomic mass of O
= 65 + 16
= 81 u
Formula unit mass of $N{a_2}O$ = 2 × Atomic mass of Na + Atomic mass of O
= 2 × 23 + 16
= 62u
Formula unit mass of ${K_2}C{O_3}$ = 2 × Atomic mass of K + Atomic mass of C + 3 ×
Atomic mass of O
= 2 × 39 + 12 + 3 × 16
= 138u

Page 42

Q.1     If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?
Sol.      One mole of carbon atoms weighs 12g (Given)
i.e., mass of 1 mole of carbon atoms = 12g
Then, mass of 6.022× ${10^{23}}$ number of carbon atoms = 12g
Therefore, mass of 1 atom of carbon = ${{12} \over {6.022 \times {{10}^{23}}}}$g
= 1.9926 × ${{{10}^ - }^{23}}$g

Q.2     Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of Na = 23u, Fe =56 u)?
Sol.      Atomic mass of Na = 23u (Given)
Then, gram atomic mass of Na = 23g
Now, 23g of Na contains = 6.022×${{{10}^{23}}}$ number of atoms
Thus, 100g of Na contains = ${{6.022 \times {{10}^{23}}} \over {23}} \times 100$ number of atoms
= 2.6182 × ${{{10}^{24}}}$ number of atoms
Again, atomic mass of Fe = 56u (Given)
Then, gram atomic mass of Fe = 56g
Now, 56 g of Fe contains = ${6.022 \times {{10}^{23}}}$ number of atoms
Thus, 100 g of Fe ${{6.022 \times {{10}^{23}}} \over {56}} \times 100$ number of atoms
= 1.0753 × ${{{10}^{24}}}$ number of atoms
Therefore, 100 grams of sodium contain more number of atoms than 100 grams of iron.

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