Test: Laws Of Chemical Combination - Class 9 MCQ

The postulates of Dalton’s Atomic theory that is the result of the law of conservation of mass in the relative numbers and kinds of atoms are constant in a given compound. Atoms cannot be created nor destroyed in a chemical reaction.

According to the law of conservation of mass:

• The mass of reactants equals the mass of products.
• In this case:

Mass of compound A: 5 g

Mass of compound B: 10 g

Thus, the mass of compound C can be calculated as follows:

• Mass of compound C = Mass of compound A + Mass of compound B
• Mass of compound C = 5 g + 10 g
• Mass of compound C = 15 g

The correct answer is D: Both A and B.

• Precipitation Reaction: The reaction AgNO₃(aq) + KCl(aq) → AgCl(s) + KNO₃(aq) produces a solid precipitate.
• Double Displacement Reaction: The same reaction involves the exchange of ions between the two compounds.
• Explanation: This reaction exhibits both characteristics, forming a precipitate (A) and involving the double displacement of ions (B). Therefore, it is classified as both a precipitation and a double displacement reaction.

The differences in atomic masses and chemical properties of sulphur and phosphorus can be explained by:

• Dalton’s Atomic Theory suggests that elements are made of atoms, each with a specific mass. This theory helps explain why different elements, like sulphur and phosphorus, have different atomic masses.
• According to Dalton, atoms of different elements have distinct properties, which explains the variation in their chemical behaviour.

Formula unit mass is calculated similarly to molecular mass.

For ZnCl₂, the calculation is as follows:

• Atomic mass of Zn: 65.3
• Atomic mass of Cl: 35.5
• Since there are two chlorine atoms, we multiply by 2: 35.5 x 2 = 71

Now, add these values together:

• 65.3 + 71 = 136.3

Thus, the formula unit mass of ZnCl₂ is 136.3 u.

• The law of conservation of mass states that in a chemical reaction, the total mass of the reactants is equal to the total mass of the products.
• This implies that matter cannot be created or destroyed, but can only change from one form to another.
• Thus, during chemical reactions, matter is conserved.
• This principle is essential for understanding chemical processes and stoichiometry.

Dalton’s Atomic Theory was proposed by John Dalton and is fundamental to our understanding of matter. The key points of this theory are:

• All matter consists of tiny particles called atoms.
• Atoms cannot be divided into smaller parts; they are indivisible.
• Atoms of the same element are identical in mass and chemical properties.
• Atoms of different elements have distinct masses and properties.
• Atoms combine in small whole number ratios to form compounds.
• The composition of a compound is always consistent, meaning the number and types of atoms remain constant.

Dalton's theory was significant because it explained the law of conservation of mass and the law of definite proportions. It marked a turning point in the study of matter, laying the groundwork for modern chemistry.

Law of Conservation of Mass states that mass is neither created nor destroyed during a chemical reaction.

This principle was proposed by Antoine Lavoisier and later confirmed through experiments by Landolt.

Key points include:

• The total mass of reactants equals the total mass of products.
• In a closed system, mass remains constant throughout the reaction.
• This law is fundamental in understanding chemical reactions and balancing equations.

Additionally, Lavoisier observed that compounds consist of elements in fixed proportions, known as the Law of Constant Proportions. For example:

• In water, the ratio of hydrogen to oxygen is always 1:8 by mass.
• This consistency holds true regardless of the water's source.

In summary, the Law of Conservation of Mass is essential for grasping how substances interact in chemistry.

The balancing of chemical equations is based on the law of conservation of mass. 
The law of conservation of mass states that mass can be neither created nor destroyed in a chemical reaction. Total mass of reactants is equal to total mass of products.

• Option (A) states that the number of atoms in a given compound is fixed, which is a postulate of Dalton's atomic theory.
• Option (C) claims that atoms can neither be created nor destroyed, also a postulate of Dalton's atomic theory.
• Option (D) asserts that atoms cannot be divided, which is another postulate of Dalton's atomic theory.

However, option (B) is not a postulate of Dalton's atomic theory. It suggests that atoms of the same elements cannot combine in more than one ratio to form multiple compounds, which is incorrect.

The decomposition of water (H₂O) produces:

• 1 gram of hydrogen (H₂)
• 8 grams of oxygen (O₂)

This outcome is consistent with the law of conservation of mass, which states that mass cannot be created or destroyed in a chemical reaction.

According to the Law of Conservation of mass, the total mass of the reactants must equal the total mass of the products.

• The mass of the reactants is conserved during a chemical reaction.
• This means that no mass is lost or gained; it is simply transformed.
• For example, if you start with 10 grams of reactants, you will end with 10 grams of products.

This principle is fundamental in chemistry and ensures that all chemical equations are balanced.