9th Class (CBSE) Science: Atoms and Molecules

Question: Which postulate of Dalton’s atomic theory can explain the law of definite proportions?

Answer: The relative number and kinds of atoms are constant in a given compound.

Question: Define the atomic mass unit.

Answer: One atomic mass unit is equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12. The relative atomic masses of all elements have been found with respect to an atom of carbon-12.

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

Answer: Atom is too small to be seen with naked eyes. It is measured in nanometres.

1 m = 109 nm

Question: Write down the formulae of
(i) Sodium oxide
(ii) Aluminium chloride
(iii) Sodium sulphide
(iv) Magnesium hydroxide

Answer: The formulae are

Question: What is meant by the term chemical formula?

Answer: The chemical formula of the compound is a symbolic representation of its composition, e.g., chemical formula of sodium chloride is NaCl.

Question: How many atoms are present in a

1. H₂S molecule and
2. P0₄^3- ion?

Answer:

1. H₂S —> 3 atoms are present
2.  P0₄^3- —> 5 atoms are present

Question: Calculate the molecular masses of H₂, O₂, Cl₂, C0₂, CH₄, C₂H₂,NH₃, CH₃OH.

Answer: The molecular masses are:

Question:Calculate the formula unit masses of ZnO, Na₂O, K₂C0₃, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.

Answer: The formula unit mass of

1. ZnO = 65 u + 16 u = 81 u
2. Na₂O = (23 u x 2) + 16 u = 46 u + 16 u = 62 u
3. K₂C0₃ = (39 u x 2) + 12 u + 16 u x 3

= 78 u + 12 u + 48 u = 138 u

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

Answer:

Question: 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: