CBSE Class 9 Science Chapter 3: Atoms and Molecules NCERT Solutions

The Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction. To verify this law with the given data, we need to calculate the total mass of the reactants and the total mass of the products and check if they are equal.

Given:

• Mass of sodium carbonate (reactant) = 5.3 g
• Mass of ethanoic acid (reactant) = 6 g
• Mass of sodium ethanoate (product) = 8.2 g
• Mass of carbon dioxide (product) = 2.2 g
• Mass of water (product) = 0.9 g

First, calculate the total mass of the reactants:

Total mass of reactants = Mass of sodium carbonate + Mass of ethanoic acid

$$=\; 5.3\backslash text\{\; g\}\; +\; 6\backslash text\{\; g\}\; =\; 11.3\backslash text\{\; g\}$$

Next, calculate the total mass of the products:

Total mass of products = Mass of sodium ethanoate + Mass of carbon dioxide + Mass of water

$$=\; 8.2\backslash text\{\; g\}\; +\; 2.2\backslash text\{\; g\}\; +\; 0.9\backslash text\{\; g\}\; =\; 11.3\backslash text\{\; g\}$$

Comparing the total mass of reactants and products:

Total mass of reactants (11.3 g) = Total mass of products (11.3 g)

Since the total mass of the reactants is equal to the total mass of the products, these observations are in agreement with the Law of Conservation of Mass.

The problem states that hydrogen and oxygen combine in a mass ratio of 1:8 to form water. This means that for every 1 gram of hydrogen that reacts, 8 grams of oxygen are required.

We are given that we have 3 grams of hydrogen gas that needs to react completely.

Using the given ratio:

If 1 g of hydrogen requires 8 g of oxygen,

Then, 3 g of hydrogen will require:

$$3\; \backslash text\{\; g\; hydrogen\}\; \backslash times\; \backslash frac\{8\; \backslash text\{\; g\; oxygen\}\}\{1\; \backslash text\{\; g\; hydrogen\}\}\; =\; 24\; \backslash text\{\; g\; oxygen\}$$

Therefore, 24 grams of oxygen gas would be required to react completely with 3 grams of hydrogen gas.

Dalton's atomic theory proposed several postulates about atoms. The postulate that directly arises from and explains the Law of Conservation of Mass is:

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

This postulate implies that in any chemical reaction, the atoms present initially are just rearranged to form new substances, but the total number of atoms remains constant. Since each atom has a specific mass, the total mass of the reactants must equal the total mass of the products, thus upholding the Law of Conservation of Mass.

The Law of Definite Proportions states that a given chemical compound always contains the same elements in the same proportion by mass, regardless of its source or method of preparation.

Dalton's atomic theory provides an explanation for this law through the following postulate:

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

This means that a specific compound is always formed from the same types of atoms combined in the same fixed numerical ratio. For example, a water molecule always consists of two hydrogen atoms and one oxygen atom, leading to a constant mass ratio of hydrogen to oxygen in water.