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Atoms and Molecules Class 9 CBSE Test paper Solved

17/10/2015

1. Q. Why the symbol of iron is is Fe and sodium is Na?

Ans: symbol of some have been taken from the names of elements in Latin, German or Greek. For example, the symbol of iron is Fe from its Latin name ferrum, sodium is Na from natrium.

2. Q. Mention the difference between N2 and 2N?

Ans: N2 represent one molecules of nitrogen consist of two atoms and 2N represent two seprate atoms of nitrogen

3. Q. To identify the Law of Conservation of Mass, a student added 10 ml of 5% barium chloride solution into 10 ml of 5% sodium sulphate solution. Which one of the following is the correct observation?

(a) A clear solution is formed         (b) The solution turns to red colour
(c) Yellow precipitate is formed       (d) White precipitate is formed

Hint: Aqueous solution of barium chloride reacts with that of sodium sulphate to form a white precipitate of barium sulphate. Ans: (d)

4. Q. To identify the Law of Conservation of Mass, a student added 10 ml of 5% barium chloride solution into 10 ml of 5% sodium sulphate solution. Which one of the following is the correct observation?

(a) A clear solution is formed         (b) The solution turns to red colour
(c) Yellow precipitate is formed       (d) White precipitate is formed
Hint: Aqueous solution of barium chloride reacts with that of sodium sulphate to form a white precipitate of barium sulphate. Ans: (d)

5.Q. Define atomic number and mass number. Which one of them is a more fundamental attribute of element? State the reason.

Ans: Atomic number is the total number of protons present in the nucleus of an atom. Mass number is the sum of protons and neutrons present in the nucleus of an atom.
Atomic number is the fundamental attribute of elements because all elements are characterised by their atomic numbers. When elements react, their atoms either lose or gain electrons but their atomic number remains the same.

6.Q. A student noted down the following precautions for the experiment "To verify the law of conservation of mass in a chemical reaction"

(i) Weighing should be done carefully.
(ii) Flask should be corked tightly.
(iii) Solutions should not get mixed before the reaction.
(iv) Mixing of two solutions must be done quickly
The precaution which needs to be corrected is:

(a) (i)                   (b) (ii)            (c) (iii)            (d) (iv)

Ans: (d)

7.Q. Nitrogen and Hydrogen atoms combine in the ratio 14 : 3 by mass to form ammonia molecule. Find the formulae of ammonia molecule by calculating the molar ratio.

Ans: Atomic mass of 1-Nitrogen atom = 14 grams
Atomic mass of 1- Hydrogen atom = 1
Nitrogen and hydrogen combine in the ratio 14: 3 by mass.
Therefore one Nitrogen atom combines with three Hydrogen atoms and molecular weight of the compound formed is 17.
The formulae of ammonia molecule = NH₃ (ammonia).

8. Q. Mention two advantages of Dalton’s atomic theory.

Ans: two advantages of Dalton’s atomic theory

Dalton’s atomic theory provided an explanation for the law of conservation of mass and the law of definite proportions.
According to Dalton’s atomic theory, all matter (whether an element, a compound or a mixture), is composed of small particles, called atoms.

9.Q. Elaborate the basic difference between the formula unit mass and the molecular mass.

Ans: The only difference is that we use the word formula unit for those substances whose constituent particles are ions.

10.Q. While searching for various atomic mass units, scientists initially took 1/16 of the mass of an atom of naturally occurring oxygen as the unit. State two reasons

Ans: Scientists initially took 1/16 of the mass of an atom of naturally occurring oxygen as the unit because of following two reasons

Oxygen reacted with a large number of elements and formed compounds.
This atomic mass unit gave masses of most of the elements as whole numbers.

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