Class 09 : 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: $\frac{H}{O}=\frac{1}{8}$
Or, $\frac{3}{O}=\frac{1}{8}$
Or, $O=3\times 8=24$ 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

Answer: Na2O

(b) aluminium chloride

Answer: AlCl3

(c) sodium suphide

Answer: Na2S

(d) magnesium hydroxide

Answer: Mg(OH)2

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

(a) Al2(SO4)3

Answer: Aluminium sulphate

(b) CaCl2

Answer: Calcium Chloride

(c) K2SO4

Answer: Potassium sulphate

(d) KNO3

Answer: Potassium nitrate

(e) CaCO3

Answer: Calcium carbonate

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?

Answer: (a) 3, (b) 5

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

Answer: Molecular mass of H2:
$=2\times 1+1\times 16=18$ u

Molecular mass of O2:
$=2\times 16=32$ u

Molecular mass of Cl2:
$=2\times 35.5=71$ u

Molecular mass of CO2:
$=1\times 12+2\times 16=44$ u

Molecular mass of CH4:
$=1\times 12+4\times 1=16$ u

Molecular mass of C2H6:
$=2\times 12+6\times 1=30$ u

Molecular mass of C2H4:
$=2\times 12+4\times 1=28$ u

Molecular mass of NH3:
$=1\times 14+3\times 1=17$ u

Molecular mass of CH3OH:
$=1\times 12+3\times 1+1\times 16+1\times 1=32$ 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:
$=1\times 65+1\times 16=81$ u

Formula unit mass of Na2O:
$=2\times 23+1\times 16=62$ u

Formula unit mass of K2CO3:
$=2\times 39+1\times 12+3\times 16=78+12+48=138$ 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 $=\frac{12}{6.022\times {10}^{23}}=1.9926\times {10}^{-23}$ 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.

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