ATOMS AND MOLECULES (BASIC CONCEPT AND ASSIGNMENT)

ATOMS AND MOLECULES(CONCEPT)

1. Law of conservation of mass : Mass can neither be created nor can it be destroyed in a chemical reaction.

2. Law of constant proportions : In a pure substance same elements are always present in a definite proportion by weight.

3. Atom : Smallest unit of an element, which may or may not exist independently, but always takes part in a chemical reaction.

4. Molecule : Smallest unit of an element or a compound which always exists independently and retains the complete physical and chemical properties of the element or the compound.

5. Atomicity of a molecule of an element : It is the number of atoms which constitute one molecule of an element.

6. Molecular formula : The symbolic representation of the kind and the actual number of atoms in one molecule of a pure substance, may be an element or a compound.

7. Atomic mass unit : The mass of 1/12 part of C-12 (isotope of carbon) is equivalent to one atomic mass unit.

8. Gram-atomic mass : The atomic mass of an element expressed in terms of grams.

9. Atomic mass : The number of times an atom of an element is heavier than 1/12 part of C-12 (isotope of carbon).

10. Molecular mass : The number of times a molecule of a pure substance is heavier than 1/12 part of C-12 (isotope of carbon).

11. Gram-molecular mass : The molecular mass of a pure substance expressed in grams.

12. Mole : A group of 6.022 × 10²³ particles (atoms, molecules, ions, electrons, protons, neutrons, etc.) of a substance is called mole.

13. Mole number : It is a number which states, how many times one molecular mass comes in certain mass of a substance in grams.

ASSIGNMENT

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 of water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.

2.      Calculate the molecular masses of :

(i)     H2 (ii) O2 (iii) Cl2 (iv) CO2 (v) CH4 (vi) C2H6 (vii)C2H4 (viii) NH3 and (ix) CH3OH.

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

4.      A 0.24 g sample of a 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.

5.      What is the mass of :

(a)    1 mole of nitrogen atoms?

(b)    4 moles of aluminium atoms?

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

7.      What is the difference between Na and Na+ in terms of number of electrons?

8.      State the law of constant proportions.

9.      State law of conservation of mass. If 12 g of carbon is burnt in the presence of 32 g of oxygen, how much carbon dioxide will be formed?

10.    Write chemical formula of :

(a)    Sodium carbonate (b) Ammonium chloride

11.     Write the names of compounds represented by the following formulae :

(i)     KNO₃ (ii) Al₂(SO₄)₃ (iii) MgCl₂ (iv) (NH₄)₂SO₄

12.    Mention the two postulates of Dalton's atomic theory.

13.    What is molecular mass of substance? And  Calculate molecular mass of carbon dioxide

14.    Compute the number of ions present in 5.85 g of sodium chloride. [Na = 23, Cl = 35.5]

15.    A gold sample contains 90% of gold and the rest copper. How many atoms of gold are present in one gram of this sample of gold? [Au = 97]