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. This means the total mass of the reactants must be equal to the total mass of the products.

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, let's 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, let's 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 ratio of 1:8 by mass to form water. This means that for every 1 gram of hydrogen, 8 grams of oxygen are needed.

We are given 3 grams of hydrogen gas.

To find the mass of oxygen required, we can set up a proportion or use the given ratio directly.

According to the ratio, 1 g of hydrogen requires 8 g of oxygen.

Therefore, 3 g of hydrogen will require:

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

Alternatively, using the ratio 1:8:

Mass of oxygen = 8 × Mass of hydrogen

$$=\; 8\; \backslash times\; 3\backslash text\{\; g\}\; =\; 24\backslash text\{\; g\}$$

Thus, 24 g of oxygen gas would be required to react completely with 3 g of hydrogen gas.

Dalton's atomic theory proposed several postulates about the nature of atoms and chemical reactions. 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 means that during a chemical reaction, atoms are merely rearranged, and since atoms have mass, their total mass remains constant, thus conserving mass.

The Law of Definite Proportions states that a chemical compound always contains the same elements in the same proportion by mass, regardless of its origin.

The postulate of Dalton's atomic theory that explains this law is:

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

This means that a specific compound is always made up of the same types of atoms combined in the same fixed numerical ratio, which naturally leads to a constant proportion by mass.