1. What is the law of conservation of mass?

The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.

2. How can the law of conservation of mass be illustrated through an activity?

The activity involves mixing barium chloride and sodium sulphate solutions in a conical flask and observing that the mass of the flask remains the same before and after the reaction, demonstrating the conservation of mass.

3. What is the law of constant proportion?

The law of constant proportion states that in a chemical substance, the elements are always present in definite proportions by mass.

4. What are the postulates of Dalton’s atomic theory?

Dalton's atomic theory states that atoms are indivisible particles, identical atoms of a given element have the same mass and chemical properties, atoms of different elements have different properties, atoms combine in whole-number ratios, and the relative number and kinds of atoms are constant in a given compound.

5. How small are atoms and how is atomic radius measured?

Atoms are very small, smaller than anything that we can imagine or compare with, and atomic radius is measured in nanometers, where 1 nm = 10^-9 m.

6. What are Dalton’s symbols of elements and how are elements named today?

Dalton’s symbols of some elements were based on historical names and abbreviations, but today the International Union of Pure and Applied Chemistry (IUPAC) approves the names of elements.

7. What is the standard reference for measuring atomic masses and how is atomic mass measured?

The C-12 isotope is chosen as the standard reference for measuring atomic masses, and one atomic mass unit is equal to 1/12th of the mass of one atom of carbon-12. Atomic masses of elements are measured relative to carbon-12.

8. Why was C-12 chosen as the standard reference for measuring atomic masses?

C-12 was chosen as the standard reference because it gives nearly whole numbers for the atomic masses of other elements.

Example: In the reaction between hydrogen and oxygen to form water, the law of conservation of mass is demonstrated as the mass of the reactants is equal to the mass of the products.

9. What is a molecule?

A molecule is the smallest particle of an element or a compound that is capable of independent existence and shows all the properties of that substance. For example, a molecule of water (H₂O) consists of two hydrogen atoms and one oxygen atom.

10. What is the atomicity of an element?

The number of atoms present in a molecule of an element is called its atomicity. For example, the atomicity of helium is 1, since it exists as single atoms, while the atomicity of oxygen is 2, since it exists as a diatomic molecule (O₂).

11. What are ions?

Ions are charged particles that can be negatively or positively charged. Negatively charged ions are called anions, while positively charged ions are called cations. For example, the chloride ion (Cl^-) is an anion, while the sodium ion (Na⁺) is a cation.

12. How is a chemical formula of a compound written?

A chemical formula represents the composition of a compound using symbols and valencies of elements. For example, water is represented as H₂O, meaning it contains two hydrogen and one oxygen atom, with the charges balanced.

13. What is the molecular mass of a substance?

Molecular mass of a substance is the sum of atomic masses of all the atoms in a molecule of the substance. For example, the molecular mass of water (H₂O) is 18.015 grams per mole, which is the sum of the atomic masses of two hydrogen atoms and one oxygen atom.

14. What is the formula unit mass of a substance?

The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of the compound. For example, the formula unit mass of sodium chloride (NaCl) is 58.44 grams per mole, which is the sum of the atomic masses of one sodium atom and one chlorine atom.

15. What is mole concept?

One mole of a species (atom, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecular mass in grams. The number of particles present in 1 mole of a substance is fixed. For example, one mole of carbon atoms has a mass of 12 grams, while one mole of water molecules has a mass of 18.015 grams.

16. What is Avogadro's number?

Avogadro's number or Avogadro's constant (represented by N0) is the number of particles present in 1 mole of a substance. Its value is 6.022 x 1023. For example, there are 6.022 x 1023 water molecules in one mole of water.