Solution of Chapter 3. Atoms and Molecules (NCERT - Science Book)

In a reaction, 5.3 g of sodium carbonate reacted with 6 g ethanoic acid. The products were 2.2 g of carbon dioxide, o.9 g of water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.

Hydrogen and oxygen combine in the ratio of 1 : 8 by mass to from water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?

Which postulate of Dalton's atomic theory is the result of the law of conservation of mass?

Which postulate of Dalton's atomic theory can explain the law of definite proportions?

Define the atomic mass unit.

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

Write down the formulae of:

(i) Sodium oxide

(ii) Aluminum chloride

(iii) Sodium sulphide

(iv) Magnesium hydroxide

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

(i) Al₂(S0₄)₃

(ii) CaCl₂

(iii) K₂S0₄

(iv) KN0₃

(v) CaC0₃

What is meant by the term chemical formula?

How many atoms are present in a:

(i) H₂S molecule, and

(ii) PO^3-₄ ion?

Calculate the molecular masses of H₂, O₂, Cl₂, C0₂, CH₄, C₂H₆, C₂H₄, NH₃, CH₃0H (Atomic masses: H = 1; 0 = 16; Cl = 35.5; C = 12; N = 14)

Calculate the formula unit masses of ZnO, Na₂O, K₂C0₃

(Given: Atomic masses of Zn = 65 u; Na = 23 u; K = 39 u; C = 12u and O = 16 u)

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

Which has more number of atoms, 100 grams of sodium or 100 grams of iron? (Given: Atomic masses of Na = 23u, Fe = 56u)

A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.

When 3.0 g of carbon in burnt in 8.00 g of oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer?

What are polyatomic ions? Give examples.

Write the chemical formulae of the following:

(a) Magnesium chloride

(b) Calcium oxide

(c) Copper Nitrate

(d) Aluminium chloride

(e) Calcium carbonate

Give the names of the elements present in the following compounds:

(a) Quick lime

(b) Hydrogen bromide

(c) Baking soda

(d) Potassium sulphate

Calculate the molar masses of the following substances:

(a) Ethyne, C₂H₂

(b) Sulphur molecule, S₈

(c) Phosphorus molecule, P₄

(d) Hydrochloric acid, HCl

(e) Nitric acid, HNO₃

What is the mass of:

(a) 1 mole of nitrogen atoms

(b) 4 moles of aluminium atoms?

(c) 10 moles of sodium sulphite (Na₂S0₃)?

(Atomic masses: N = 14 u, AI= 27 u, Na = 23 u, S = 32 u and 0 = 16 u)

Convert the following into moles.(Atomic masses: 0 = 16u, H = 1u & C = 12u)

(a) 12 g of oxygen gas

(b) 20 g of water

(c) 22 g of carbon dioxide

What is the mass of:

(a) 0.2 mole of oxygen atoms?

(b) 0.5 mole of water molecules?

Calculate the number of molecules of sulphur (S₈) present in 16g of solid sulphur (Atomic mass of S = 32)

Calculate the number of aluminium ions present in 0.051 g of aluminium oxide. (Hint. The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u)