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Atoms And Molecules - Class 9 MCQ


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5 Questions MCQ Test - Atoms And Molecules

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Atoms And Molecules - Question 1

Which of the following correctly represents 360g of water?

(i) 2 moles of water
(ii) 20 moles of water
(iii) 6.022 × 1023 molecules of water
(iv) 1.2044 × 1025 molecules of water

Detailed Solution for Atoms And Molecules - Question 1

To determine which option correctly represents 360g of water, we need to understand the relationship between grams, moles, and molecules in water.
1. Moles of water:
- The molar mass of water (H2O) is approximately 18g/mol.
- To calculate the number of moles in 360g of water, we divide the mass by the molar mass:
Number of moles = Mass / Molar mass
Number of moles = 360g / 18g/mol
Number of moles = 20 moles
2. Molecules of water:
- Avogadro's number (6.022 × 10^23) represents the number of particles (atoms, molecules, or ions) in one mole of a substance.
- To calculate the number of molecules in 360g of water, we can use the following conversion:
Number of molecules = Number of moles × Avogadro's number
Number of molecules = 20 moles × 6.022 × 10^23
Number of molecules = 1.2044 × 10^25 molecules
Now, let's analyze the given options:
(i) 2 moles of water: This option represents a much smaller quantity of water (only 2 moles) compared to 360g, so it is incorrect.
(ii) 20 moles of water: This option correctly represents 360g of water, as calculated above.
(iii) 6.022 × 10^23 molecules of water: This option represents only one mole of water, which is much less than 360g, so it is incorrect.
(iv) 1.2044 × 10^25 molecules of water: This option also correctly represents 360g of water, as calculated above.
Therefore, the correct options are (ii) and (iv). Hence, the answer is option C: (ii) and (iv).
Atoms And Molecules - Question 2

Which of the following statements is not true about an atom?

Detailed Solution for Atoms And Molecules - Question 2
Explanation:
The incorrect statement is: B: Atoms aggregate in large numbers to form the matter that we can see, feel or touch.
Reasoning:
Atoms are the fundamental building blocks of matter, and they do not aggregate in large numbers to form the matter that we can see, feel, or touch. This statement is incorrect because atoms themselves combine to form molecules and compounds, which then aggregate in large numbers to form visible matter.
True statements about an atom:
A: Atoms are always neutral in nature.
- Atoms have a balanced number of protons and electrons, resulting in a neutral charge.
C: Atoms are not able to exist independently.
- Atoms are stable when they are bonded with other atoms to form molecules.
D: Atoms are the basic units from which molecules and ions are formed.
- Molecules are formed when two or more atoms chemically combine, and ions are formed when atoms gain or lose electrons.
Therefore, the correct answer is B: Atoms aggregate in large numbers to form the matter that we can see, feel or touch.
Atoms And Molecules - Question 3

Which of the following contains the maximum number of molecules?

Detailed Solution for Atoms And Molecules - Question 3

The maximum no. of molecules correspond to the maximum number of moles and those are in 1g of H2 , i.e. 0.5.

Atoms And Molecules - Question 4

An element X is divalent and another element Y is tetravalent. The compound formed by these two elements will be:

Detailed Solution for Atoms And Molecules - Question 4

To determine the compound formed by the divalent element X and the tetravalent element Y, we need to consider their valencies and how they combine.
Valencies of the elements:
- Element X is divalent, meaning it has a valency of 2.
- Element Y is tetravalent, meaning it has a valency of 4.
Combining the elements:
When elements combine, they form compounds by sharing or transferring electrons to achieve a stable electron configuration. The valency of an element indicates how many electrons it can gain, lose, or share to achieve stability.
To form a stable compound, the total positive charge of the cations (positively charged ions) must equal the total negative charge of the anions (negatively charged ions). In this case, since X is divalent (2+ charge) and Y is tetravalent (4+ charge), they will combine in a 2:4 ratio to balance the charges.
Compound formed:
Based on the valencies and combining ratio, the compound formed by the elements X and Y will be represented by the chemical formula X2Y. This means two atoms of X will combine with one atom of Y to form the compound.
Therefore, the correct answer is B: X2Y.
Atoms And Molecules - Question 5

3.42 g of sucrose is dissolved in 18 g of water in a beaker. The numbers of oxygen atoms in the solution are:

Detailed Solution for Atoms And Molecules - Question 5

The molecular formula of sucrose (C12H22O11) indicates that there are:
- 12 carbon atoms
- 22 hydrogen atoms
- 11 oxygen atoms
To find the number of oxygen atoms in the solution, we need to consider the molar mass of sucrose and the amount of sucrose dissolved in water.
1. Calculate the molar mass of sucrose:
- The molar mass of carbon (C) is 12.01 g/mol.
- The molar mass of hydrogen (H) is 1.008 g/mol.
- The molar mass of oxygen (O) is 16.00 g/mol.
- The molar mass of sucrose can be calculated as follows:
Molar mass of sucrose = (12 * 12.01 g/mol) + (22 * 1.008 g/mol) + (11 * 16.00 g/mol) = 342.34 g/mol
2. Calculate the number of moles of sucrose:
- Moles of sucrose = Mass of sucrose / Molar mass of sucrose
= 3.42 g / 342.34 g/mol
= 0.01 mol
3. Since the molecular formula of sucrose contains 11 oxygen atoms, the number of oxygen atoms in 0.01 mol of sucrose is:
- Number of oxygen atoms = 0.01 mol * 11
= 0.11 mol
4. Calculate the number of moles of water:
- Moles of water = Mass of water / Molar mass of water
= 18 g / 18.02 g/mol
= 0.999 mol
5. Since water does not contain any oxygen atoms, the number of oxygen atoms in 0.999 mol of water is 0.
6. Calculate the total number of oxygen atoms in the solution:
- Total number of oxygen atoms = Number of oxygen atoms in sucrose + Number of oxygen atoms in water
= 0.11 mol + 0
= 0.11 mol
7. Convert the number of moles to atoms:
- Avogadro's number states that 1 mole of any substance contains 6.022 x 10^23 particles (atoms, molecules, or ions).
- Number of oxygen atoms = 0.11 mol * 6.022 x 10^23 atoms/mol
Therefore, the number of oxygen atoms in the solution is approximately 6.68 x 10^23 (Option B).
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