Atoms And Molecules, Science, Class 9 - Test - Class 9 MCQ

Atomicity of K₂Cr₂O₇:
The atomicity of a compound refers to the number of atoms present in a single molecule of that compound. In the case of K₂Cr₂O₇, we need to determine the total number of atoms in a single molecule.
Step 1: Identify the elements present in K₂Cr₂O₇:
- Potassium (K)
- Chromium (Cr)
- Oxygen (O)
Step 2: Determine the number of atoms for each element:
- Potassium (K) has a subscript of 2, indicating that there are 2 atoms of potassium in the compound.
- Chromium (Cr) also has a subscript of 2, indicating that there are 2 atoms of chromium in the compound.
- Oxygen (O) does not have a subscript, which means there is only 1 atom of oxygen in the compound.
Step 3: Calculate the total number of atoms:
- Add up the number of atoms for each element:
2 atoms of potassium (K) +
2 atoms of chromium (Cr) +
1 atom of oxygen (O) = 5 atoms in total
Therefore, the atomicity of K₂Cr₂O₇ is 5.

Formula for Quick Lime: CaO
Quick lime, also known as calcium oxide, is a chemical compound that is commonly used in various applications such as construction, agriculture, and chemical manufacturing. The formula for quick lime is CaO, which means it is composed of one calcium atom (Ca) and one oxygen atom (O).
Here is a detailed explanation of why the correct answer is option D: CaO.
Calcium Oxide (CaO):
- Quick lime is an inorganic compound that is obtained by heating limestone (CaCO3) at high temperatures in a process called calcination.
- During calcination, carbon dioxide (CO2) is released, leaving behind calcium oxide (CaO) as the end product.
- The chemical formula CaO represents the composition of quick lime, where Ca refers to the calcium atom and O refers to the oxygen atom.
- Calcium oxide is a white, crystalline solid with a high melting point and is highly reactive with water.
Other Options:
A: CaCl2 - Calcium chloride is a different compound that consists of one calcium atom and two chlorine atoms. It is not the formula for quick lime.
B: CaCO3 - Calcium carbonate is the chemical formula for limestone, which is used as a raw material in the production of quick lime. However, CaCO3 is not the formula for quick lime itself.
C: Ca(OH)2 - Calcium hydroxide, also known as slaked lime, is another compound that is obtained by adding water to quick lime. However, Ca(OH)2 is not the formula for quick lime itself.
In summary:
- The formula for quick lime is CaO, which represents the combination of one calcium atom and one oxygen atom.
- Quick lime is obtained by heating limestone (CaCO3) during the process of calcination.
- Options A, B, and C do not represent the correct formula for quick lime.
Therefore, the correct answer is D: CaO.

The symbol of cadmium is Cd.
Explanation:
Cadmium is a chemical element with the atomic number 48 and the symbol Cd. It is a soft, bluish-white metal that is found naturally in the Earth's crust.
To remember the symbol of cadmium, you can use the mnemonic "CD" which stands for Cadmium.
Here is a breakdown of the answer:
- Cadmium symbol: Cd
- Symbol options:
- Ca: This symbol represents the element calcium.
- Cu: This symbol represents the element copper.
- Cm: This symbol represents the element curium.
- Cd: This is the correct symbol for cadmium.
- The correct answer is D: Cd.

To determine the correct answer, we need to understand the basic properties of noble gases. Noble gases are a group of elements found in Group 18 of the periodic table. They include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn).
1. Noble gases are chemically inert:
Noble gases have a full complement of electrons in their outermost energy level, making them highly stable and chemically unreactive. This stability is due to their full valence electron shells, which makes them unlikely to form chemical bonds with other elements.
2. Noble gases exist as monatomic molecules:
Monatomic means that they exist as single atoms rather than bonded together as molecules. Each noble gas atom exists independently and does not form bonds with other atoms. This is because their outermost energy level is already filled with electrons, providing them with stability.
Therefore, the correct answer is A: monoatomic. Noble gas molecules are composed of single atoms rather than diatomic or triatomic molecules.

The valency of an element is the number of electrons it needs to gain or lose in order to achieve a stable electron configuration. In the case of nitrogen (N), it has an atomic number of 7 and an electronic configuration of 2, 5.
The valency of nitrogen in NH3 (ammonia) can be determined by looking at the Lewis structure or electron dot structure of the molecule. In the Lewis structure, nitrogen is surrounded by three hydrogen atoms, each sharing one electron with nitrogen.
Here's the Lewis structure of NH3:
H
|
H-N-H
|
H
From the Lewis structure, we can see that nitrogen has formed three covalent bonds with three hydrogen atoms, and it has one lone pair of electrons. The number of bonds formed by an atom determines its valency.
Thus, the valency of nitrogen in NH3 is 3 (B).

The molecular formula of ethanol is C2H5OH, which indicates that it consists of two carbon atoms (C2), six hydrogen atoms (H6), and one oxygen atom (O1).
To calculate the molecular mass of ethanol, we need to determine the atomic masses of carbon (C), hydrogen (H), and oxygen (O) and add them together according to the formula.
The atomic mass of carbon (C) is approximately 12u.
The atomic mass of hydrogen (H) is approximately 1u.
The atomic mass of oxygen (O) is approximately 16u.
Using these values, we can calculate the molecular mass of ethanol as follows:
2 carbon atoms: 2 * 12u = 24u
6 hydrogen atoms: 6 * 1u = 6u
1 oxygen atom: 1 * 16u = 16u
Adding these values together, we get:
Molecular mass of ethanol = 24u + 6u + 16u = 46u
Therefore, the molecular mass of ethanol is 46u.
Answer: A (46u)

Question: How many moles are present in 28g of nitrogen atoms?

To determine the number of moles present in 28g of nitrogen atoms, we need to use the molar mass of nitrogen.
Step 1: Find the molar mass of nitrogen.
- The atomic mass of nitrogen (N) is approximately 14.01 g/mol.
Step 2: Calculate the number of moles.
- Divide the given mass (28g) by the molar mass of nitrogen (14.01 g/mol).
Calculation:
- Moles = Mass/Molar mass
- Moles = 28g / 14.01 g/mol
- Moles ≈ 1.998 moles
Answer:
- There are approximately 2 moles of nitrogen atoms present in 28g of nitrogen atoms.

Question Analysis:
The question asks us to determine which compound among the given options has a molecular mass of 10^6.

To find the molecular mass of a compound, we need to sum up the atomic masses of all the elements present in the compound. Let's calculate the molecular masses of the given compounds:
A: CaCO3
- Ca: atomic mass = 40
- C: atomic mass = 12
- O: atomic mass = 16 (3 oxygen atoms present)
Total molecular mass = 40 + 12 + (16 * 3) = 40 + 12 + 48 = 100
B: SO3
- S: atomic mass = 32
- O: atomic mass = 16 (3 oxygen atoms present)
Total molecular mass = 32 + (16 * 3) = 32 + 48 = 80
C: Na2CO3
- Na: atomic mass = 23 (2 sodium atoms present)
- C: atomic mass = 12
- O: atomic mass = 16 (3 oxygen atoms present)
Total molecular mass = (23 * 2) + 12 + (16 * 3) = 46 + 12 + 48 = 106
D: NaCl
- Na: atomic mass = 23
- Cl: atomic mass = 35.5
Total molecular mass = 23 + 35.5 = 58.5
From the calculations, we can see that the molecular mass of compound C (Na2CO3) is the closest to 10^6. Therefore, the answer is C: Na2CO3.

Dalton's Atomic Theory:
- Postulate A: Atoms cannot be created or destroyed.
- This postulate states that atoms are indivisible and cannot be created or destroyed in a chemical reaction.
- It implies that the total mass of the reactants must be equal to the total mass of the products in a chemical reaction.
- Postulate B: Atoms of different elements have different sizes, masses, and chemical properties.
- This postulate suggests that each element is composed of unique atoms that have distinct sizes, masses, and chemical properties.
- It explains why different elements exhibit different behaviors and react differently with other substances.
- Postulate C: Atoms of the same element can combine in only one ratio to produce more than one compound.
- This postulate is not a part of Dalton's atomic theory.
- It contradicts Dalton's theory as it suggests that atoms of the same element can combine in multiple ratios to form different compounds.
- Postulate D: Atoms are very tiny particles that cannot be divided further.
- This postulate states that atoms are the smallest indivisible particles and cannot be further divided into smaller particles.
- It aligns with the concept of atom as the fundamental building block of matter.
Conclusion:
Among the given options, postulate C - "Atoms of the same element can combine in only one ratio to produce more than one compound" - is not a postulate of Dalton's atomic theory.

To determine the wrong pairs/combination, we need to compare the given statements and identify any inconsistencies or incorrect information. Let's analyze each option:
A: 6.022 × 10²³ molecules of oxygen = 32 g of oxygen
- This statement is correct. The Avogadro's number (6.022 × 10²³) represents the number of particles (atoms, molecules, or ions) in one mole of a substance. The molar mass of oxygen is 32 g/mol, so 6.022 × 10²³ molecules of oxygen would indeed weigh 32 g.
B: 6.022 × 10²³ ions of sodium = 23 g of Na
- This statement is correct as well. Similar to the previous case, the Avogadro's number represents the number of particles in one mole. The molar mass of sodium is 23 g/mol, so 6.022 × 10²³ ions of sodium would weigh 23 g.
C: 6.022 × 10²³ atoms of C = 24 g of carbon
- This statement is incorrect. The molar mass of carbon is 12 g/mol, not 24 g/mol. Therefore, 6.022 × 10²³ atoms of carbon would weigh 12 g, not 24 g.
D: 6.022 × 10²³ atoms of H = 1 g of H atoms
- This statement is correct. The molar mass of hydrogen is 1 g/mol, so 6.022 × 10²³ atoms of hydrogen would weigh 1 g.
Therefore, the wrong pair/combination is C: 6.022 × 10²³ atoms of C = 24 g of carbon. The correct statement should be 6.022 × 10²³ atoms of C = 12 g of carbon.