# NCERT Class 9 Science Solutions: Chapter 3 –Atoms and Molecules Part 1

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

A.

In the given reaction, sodium carbonate reacts with ethanoic acid to produce sodium ethanoate, carbon dioxide, and water.

Mass of sodium carbonate (Given)

Mass of ethanoic acid (Given)

Mass of sodium ethanoate (Given)

Mass of carbon dioxide (Given)

Mass of water (Given)

Now, total mass before the reaction

And, total mass before the reaction

Hence, the given observations are in agreement with the law of conservation of mass.

Question 2:

Q. Hydrogen and oxygen combine in the ration of by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?

A.

It is given that the ration of hydrogen and oxygen by mass to from water is

Then, the mass of oxygen gas required to react completely with 1 g of hydrogen gas is.

Therefore, the mass of oxygen gas required to react completely with 3 g of hydrogen gas is

Question 3:

Q. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?

A. The postulate of Dalton’s atomic theory which is a result of the law of conservation of mass is:

Atoms are indivisible particles, which can neither be created nor destroyed in a chemical reaction.

Question 4:

Q. Which postulate of Dalton’s atomic theory can explain the law of definite proportions?

A. The postulate of Dalton’s atomic theory which can explain the law of definite proportion is:

The relative number and kind of atoms in a given compound remains constant.

Question 1:

Q. Define atomic mass unit.

A. Mass unit equal to exactly one- twelfth the mass of one atom of carbon-12 is called one atomic mass unit. It is written as ‘u’.

Question 2:

Q. why is it not possible to see an atom with naked eyes?

A. The size of an atom is so small that it is not possible to see it with naked eyes. Also, the atom of an element does not exist independently.