NCERT Solutions for Class 9 Science Chapter 3 Atoms And Molecules

Q1 ) 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 \(\rightarrow \) Sodium acetate + carbon dioxide + water

Answer :

Sodium carbonate + Acetic acid \(\rightarrow \) Sodium acetate + Carbon dioxide + Water

(5.3g) + (6g) \(\rightarrow \) (8.2g) + (2.2g) + (0.9g)

\(\Rightarrow \) 11g = 11g

\(\Rightarrow \) LHS = RHS

\(\therefore \) Mass of reactant = Mass of product

which satisfies the law of conservation of mass.

Q2 ) 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 :

Given, ratio of H : O by mass to form water = 1 : 8
\(\Rightarrow \) 1 g of hydrogen required 8 g of oxygen to react completely.

We have to find the mass of oxygen that is required to completely react with 3 g of hydrogen.

So, 3 g of hydrogen will require \( (8 × 3) \)g of oxygen

\(\therefore \) 24 g of oxygen is required to completely react with 3 g of hydrogen.

Q3 ) Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?

Answer :

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 be destroyed”.

Q4 ) Which postulate of Dalton’s atomic theory can explain the law of definite proportions?

Answer :

The postulate of Dalton’s atomic theory that can explain the law of definite proportions is, " The relative number and kinds of atoms in a given compound remain constant."

Q1 ) Define the atomic mass unit?

Answer :

One atomic mass is equal to 1/12th the mass of one atom of carbon-12 i.e., 1 amu = \( 1 \over 12 \) ^th mass of one atom of carbon.

2. Why is it not possible to see an atom with naked eyes?

Answer :

We cannot see atoms with naked eye because their size is extremely small, i.e., their size is in nanometers such that 1nm = 10^-9 m . They can be only seen with the help of an electron microscope which has an extremely high resolution.

Q1 ) Write down the formulae of
(i) sodium oxide

(ii) aluminium chloride

(iii) sodium sulphide

(iv) magnesium hydroxide

Answer :

The formulae are:

(i) sodium oxide \(\rightarrow \) Na\( _2 \)O

(ii) aluminium chloride \(\rightarrow \) AlCl\( _3 \)

(iii) sodium sulphide \(\rightarrow \) Na\( _2 \)S

(iv) magnesium hydroxide \(\rightarrow \)Mg (OH)\( _2 \)

Q2 ) Write down the names of compounds represented by the following formulae:
(i) Al\( _2 \)(SO\( _4 \))\( _3 \)

(ii) CaCl\( _2 \)

(iii) K\( _2 \)SO\( _4 \)

(iv) KNO\( _3 \)

(v) CaCO\( _3 \)

Answer :

The names of the compounds are:

(i) Al\( _2 \)(SO\( _4 \))\( _3 \) \(\rightarrow \) Aluminium sulphate

(ii) CaCl\( _2 \) \(\rightarrow \) Calcium chloride

(iii) K\( _2 \)SO\( _4 \) \(\rightarrow \) Potassium sulphate

(iv) KNO\( _3 \) \(\rightarrow \) Potassium nitrate

(v) CaCO\( _3 \) \(\rightarrow \) Calcium carbonate

Q3 ) What is meant by the term chemical formula?

Answer :

A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule by using chemical element symbols, numbers etc. Chemical formula of a chemical compound is the symbolic representation of its composition. E.g. chemical formula of water is H\( _2 \)O.

Q4 ) How many atoms are present in a
(i) H\( _2 \)S molecule and

(ii) PO\( _4 \)\(^{3-} \) ion?

Answer :

The number of atoms present are:
(i) H\( _2 \)S molecule has 2 atoms of hydrogen and 1 atom of sulphur hence H\( _2 \)S molecule a total of 3 atoms.

(ii) PO\( _4 \)\(^{3-} \) ion has 1 atom of phosphorus and 4 atoms of oxygen hence PO\( _4 \)\(^{3-} \) ion has a total of 5 atoms.

Q1 ) Calculate the molecular masses of H\( _2 \) , O\( _2 \) , Cl\( _2 \), CO\( _2 \), CH\( _4 \), C\( _2 \)H\( _6 \), C\( _2 \)H\( _4 \), NH\( _3 \), CH\( _3 \)OH.

Answer :

We know that molecular mass of a substance is the sum of the atomic masses of all the atoms in the molecule of the substance.

The molecular mass of H\( _2 \)  = atomic mass of H + atomic mass of H = 2 × atoms atomic mass of H = 2 × 1 = 2u

The molecular mass of O\( _2 \)  = 2 × atoms atomic mass of O = 2 × 16 = 32u

The molecular mass of Cl\( _2 \)  = 2 × atoms atomic mass of Cl = 2 × 35.5 = 71u

The molecular mass of CO\( _2 \)   = atomic mass of C + 2 × atomic mass of O = 12 + ( 2 × 16) = 44u

The molecular mass of CH\( _4 \)  = atomic mass of C + 4 × atomic mass of H = 12 + ( 4 × 1) = 16u

The molecular mass of C\( _2 \)H\( _6 \)  = 2 × atomic mass of C + 6 × atomic mass of H = (2 × 12) + (6 × 1) = 24 + 6 = 30u

The molecular mass of C\( _2 \)H\( _4 \)  = 2 × atomic mass of C + 4 × atomic mass of H = (2 × 12) + (4 × 1) = 24 + 4 = 28u

The molecular mass of NH\( _3 \)  = atomic mass of N + 3 × atomic mass of H = 14 + (3 × 1) = 17u

The molecular mass of CH\( _3 \)OH   = atomic mass of C + 3 × atomic mass of H + atomic mass of O + atomic mass of H = 12 + (3 × 1) + 16 + 1 = 12 + 3 + 17 = 32u

Q2 ) Calculate the formula unit masses of ZnO, Na\( _2 \)O, K\( _2 \)CO\( _3 \), given atomic masses of Zn = 65u, Na = 23 u, K=39u, C = 12u, and O=16u.

Answer :

Given, atomic masses of Zn = 65u, Na = 23u, K = 39u, C = 12u, O = 16u

We know that formula unit mass of a substance is the sum of the atomic masses of all the atoms in the formula unit of the substance.

So, the formula unit mass of ZnO  = Atomic mass of Zn + Atomic mass of O = 65 + 16 = 81u

The formula unit mass of Na\( _2 \)O  = 2 × Atomic mass of Na + Atomic mass of O = (2 × 23) +16 = 46 + 16 = 62u

The formula unit mass of K\( _2 \)CO\( _3 \)  = 2 × Atomic mass of K + Atomic mass of C + 3 × Atomic mass of O = (2 × 39) + 12 + (3 × 16) = 78 + 12 + 48 = 138u

Q1 ) If one mole of carbon atoms weighs 12grams, what is the mass (in grams) of 1 atom of carbon?

Answer :

Given, 1 mole of carbon weighs = 12g

1 mole of carbon has \( = 6.022 × 10^{23} \)atoms

i.e., \( 6.022 × 10^{23} \)atom = 12g

So, mass of 1 atom of carbon \( = \) \( 12 \over 6.022 × 10^{23} \) \( = 1.99 × 10^{-23} \)g

Q2 ) Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of Na = 23u, Fe = 56 u)?

Answer :

Given, Atomic mass of Na=23u

Atomic mass of Fe= 56u

Now,
23g of Na contains \( = 6.022 × 10^{23} \) atoms

1g of Na contains = \( 6.022 × 10^{23} \over 23 \)atoms

100g of Na contains = \( 6.022 × 10^{23} \over 23 \) × 100 \( = 2.6182 × 10^{24} \) atoms

Now, 56g of Fe contains = 6.022 × 1023 atoms

1g of Fe contains = \( 6.022 × 10^{23} \over 56 \)atoms

100g of Fe contains = \( 6.022 × 10^{23} \over 56 \) × 100 \( = 1.075 × 10^{24} \) atoms

Hence 100g of Na has more atoms.