# Atoms Molecules

## NCERT In Text Solution

Question 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, 0.9 g 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 → sodium ethanoate + carbon dioxide + water

Answer: Total Mass of Reactants = 5.3 + 6 = 11.3 g
Total Mass of Products = 2.2 + 0.9 + 8.2 = 11.3 g

Here; Total mass of reactants = Total mass of products

Hence, these observations are in agreement with the law of conservation of mass.

Question 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 3 g of hydrogen gas?

Answer: H/O = 1/8
Or, 3/O = 1/8
Or, O = 3 xx 8 = 24 g

Hence, 24 g of oxygen is required to completely react with 3 g of hydrogen.

Question 3: Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?

Answer: The relative number of and kinds of atoms are constant in a given compound.

Question 4: Which postulate of Dalton’s atomic theory can explain the law of definite proportions?

Answer: Atoms combine in the ratio of small whole numbers to form compounds.

Question 5: Define the atomic mass unit.

Answer: One atomic mass unit is a mass unit equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12.

Question 6: Why is it not possible to see an atom with naked eyes?

Answer: Atoms are very small in size. Hence, it is not possible to see an atom with naked eye.

Question 7: Write down the formulae of

(a) sodium oxide

(b) aluminium chloride

(c) sodium suphide

(d) magnesium hydroxide

Question 8: Write down the names of compounds represented by the following formulae:

(a) Al2(SO4)3

(b) CaCl2

(c) K2SO4

(d) KNO3

(e) CaCO3

Question 9: What is meant by the term chemical formula?

Answer: The symbolic representation of a compound is called chemical formula.

Question 10: How many atoms are present in a
(a) H2S molecule and
(b) PO43– ion?

Question 11: Calculate the molecular masses of H2, O2, Cl2, CO2, CH4, C2H6, C2H4, NH3, CH3OH.

= 2 xx 1 + 1 xx 16 = 18 u

Molecular mass of O2:
= 2 xx 16 = 32 u

Molecular mass of Cl2:
= 2 xx 35.5 = 71 u

Molecular mass of CO2:
= 1 xx 12 + 2 xx 16 = 44 u

Molecular mass of CH4:
= 1 xx 12 + 4 xx 1 = 16 u

Molecular mass of C2H6:
=2 xx 12 + 6 xx 1 = 30 u

Molecular mass of C2H4:
=2 xx 12 + 4 xx 1 = 28 u

Molecular mass of NH3:
=1 xx 14 + 3 xx 1 = 17 u

Molecular mass of CH3OH:
= 1 xx 12 + 3 xx 1 + 1 xx 16 + 1 xx 1 = 32 u

Question 12: Calculate the formula unit masses of ZnO, Na2O, K2CO3, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.

Answer: Formula unit mass of ZnO:
= 1 xx 65 + 1 xx 16 = 81 u

Formula unit mass of Na2O:
=2 xx 23 + 1 xx 16 = 62 u

Formula unit mass of K2CO3:
=2 xx 39 + 1 xx 12 + 3 xx 16 = 78 + 12 + 48 = 138 u

Question 13: If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?

Answer: Number of atoms in one mole of carbon atom = 6.022 x 1023
Weight of 1 mole carbon atoms = 12 g
So, weight of 1 atom of carbon = 12/(6.022 xx 10^23) = 1.9926 xx 10^-23 g

Question 14: Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of Na = 23 u, Fe = 56 u)?

Answer: Atomic mass of sodium = 23 u
So, 23 g of sodium contains 1 mole of sodium atoms
Hence, 100 g of sodium contains 100/23 mole of sodium atoms

Atomic mass of iron = 56 u
So, 56 g of iron contains 1 mole of iron atoms
Hence, 100 g of iron contains 100/56 mole of sodium atoms

Here; 100/23 is greater than 100/56 because numerator is same but denominator is smaller in case of sodium.
So, 100 g of sodium contains more atoms than 100 g of iron.