Atoms And Molecules - Class 9 Science (Compulsory Test) - Class 9 MCQ

Elements present in baking soda:
- Sodium (Na): Sodium is a chemical element with the symbol Na. It is a highly reactive metal that is essential for various biological processes.
- Carbon (C): Carbon is a chemical element with the symbol C. It is a nonmetallic element that is the basis of organic chemistry and is present in all living organisms.
- Hydrogen (H): Hydrogen is a chemical element with the symbol H. It is the lightest and most abundant element in the universe and is present in various compounds.
- Oxygen (O): Oxygen is a chemical element with the symbol O. It is a highly reactive nonmetal that is essential for combustion and respiration.
Explanation:
Baking soda, also known as sodium bicarbonate (NaHCO3), is a compound that consists of the elements sodium, carbon, hydrogen, and oxygen. The chemical formula of baking soda indicates the presence of these elements.
- Sodium (Na) is the metal element present in baking soda.
- Carbon (C) is the nonmetal element present in baking soda.
- Hydrogen (H) is the element present in baking soda.
- Oxygen (O) is also an element present in baking soda.
Therefore, the correct answer is option C: Sodium, carbon, hydrogen, and oxygen.

The first scientist to use symbols of elements was John Dalton.
Explanation:
- John Dalton was an English chemist, physicist, and meteorologist who is best known for his work on modern atomic theory.
- In 1803, Dalton proposed a system of symbols to represent elements and their atomic weights.
- He used circles with different markings to represent different elements.
- Dalton's symbol system was later modified and expanded by Jöns Jakob Berzelius, a Swedish chemist.
- Berzelius introduced the use of letters to represent elements and their atomic symbols.
- However, Dalton was the first scientist to use symbols specifically to represent elements.
- His work laid the foundation for the development of the modern periodic table and the use of chemical symbols in chemistry.
In conclusion, John Dalton was the first scientist to use symbols to represent elements.

The overall charge on an ionic compound is equal to zero.


The overall charge on an ionic compound is determined by the balance of positive and negative charges between the cations and anions present in the compound. Here's a detailed explanation of why the correct answer is B:
Explanation:


Ionic compounds are formed when atoms of different elements gain or lose electrons to form ions. The ions then combine to form a neutral compound. In an ionic compound, the cations are positively charged ions, and the anions are negatively charged ions.
Key points:
- The charge on a cation is always positive, as it has lost one or more electrons.
- The charge on an anion is always negative, as it has gained one or more electrons.
- In an ionic compound, the sum of the charges of the cations and anions must be zero in order to maintain overall neutrality.
Example:
Let's consider the ionic compound sodium chloride (NaCl) as an example:
- Sodium (Na) is a cation with a charge of +1.
- Chlorine (Cl) is an anion with a charge of -1.
- In order to balance the charges, one sodium ion combines with one chloride ion to form NaCl.
- The overall charge of NaCl is therefore 0, as the positive charge of the sodium ion is balanced by the negative charge of the chloride ion.
Conclusion:
The overall charge on an ionic compound is always zero, as the sum of the positive charges of the cations is equal to the sum of the negative charges of the anions.

The chemical formula of copper nitrate is Cu(NO3)2.
To determine this, we need to understand the rules for writing chemical formulas:
1. The symbol for the element copper is Cu.
2. The symbol for the nitrate ion is NO3.
Now, let's break down the options given:
A: Cu(NO3)2
- This option correctly represents the copper ion (Cu) bonded with two nitrate ions (NO3). Therefore, option A is the correct answer.
B: CuNO3
- This option only represents one nitrate ion bonded with copper, which is not correct.
C: Cu2(NO3)3
- This option represents two copper ions bonded with three nitrate ions, which is not correct.
D: Cu2NO3
- This option represents two copper ions bonded with one nitrate ion, which is not correct.
In conclusion, the correct chemical formula for copper nitrate is Cu(NO3)2, which can be found in option A.

To determine the number of carbon atoms in 1g of CaCO₃, we need to calculate the number of moles of CaCO₃ and then multiply it by Avogadro's number, which represents the number of atoms in one mole of a substance.
Here's the step-by-step calculation:
1. Calculate the molar mass of CaCO₃:
- The molar mass of Ca is 40.08 g/mol.
- The molar mass of C is 12.01 g/mol.
- The molar mass of O is 16.00 g/mol.
- Therefore, the molar mass of CaCO₃ is 40.08 + (12.01 x 1) + (16.00 x 3) = 100.09 g/mol.
2. Calculate the number of moles of CaCO₃ in 1g:
- Number of moles = mass / molar mass
- Number of moles = 1g / 100.09 g/mol = 0.00999 mol.
3. Calculate the number of carbon atoms:
- Since there is one carbon atom in each molecule of CaCO₃, the number of carbon atoms is equal to the number of moles of CaCO₃.
4. Multiply the number of moles of CaCO₃ by Avogadro's number:
- Avogadro's number is 6.022 x 10²³ mol^-1.
- Number of carbon atoms = 0.00999 mol x (6.022 x 10²³ mol^-1) = 6.022 x 10²¹ atoms.
Therefore, the number of carbon atoms in 1g of CaCO₃ is 6.022 x 10²¹ (option B).

To determine the mass of a single atom of carbon, we need to refer to the periodic table. The atomic mass of carbon is 12.01 amu (atomic mass units).
1. The atomic mass of carbon is given in amu (atomic mass units), not in grams. So we need to convert it to grams.
2. 1 amu is equal to 1.66 x 10^-24 grams.
3. To convert the atomic mass of carbon to grams, we multiply it by the conversion factor:
12.01 amu * 1.66 x 10^-24 g/amu = 1.99 x 10^-23 grams.
Therefore, the mass of a single atom of carbon is 1.99 x 10^-23 grams.
Answer: C. 1.99 × 10^-23 g

To determine the number of molecules present in 9g of water, we can use the concept of molar mass and Avogadro's number.
1. Calculate the molar mass of water (H2O):
- The molar mass of hydrogen (H) is approximately 1 g/mol.
- The molar mass of oxygen (O) is approximately 16 g/mol.
- Since there are two hydrogen atoms and one oxygen atom in each water molecule, the molar mass of water is 18 g/mol.
2. Determine the number of moles of water present in 9g:
- Divide the given mass (9g) by the molar mass of water (18 g/mol).
- 9g ÷ 18 g/mol = 0.5 mol
3. Use Avogadro's number to calculate the number of molecules:
- Avogadro's number, represented by "NA," is approximately 6.022 × 10^23 molecules/mol.
- Multiply the number of moles (0.5 mol) by Avogadro's number:
0.5 mol × 6.022 × 10^23 molecules/mol = 3.01 × 10^23 molecules
Therefore, there are approximately 3.01 × 10^23 molecules present in 9g of water.
Hence, the correct answer is A: 3.01 × 10^23.

Formula of Sulphuric Acid:
The formula of sulphuric acid is H₂SO₄.
Explanation:
Sulphuric acid is a strong acid that is commonly used in various industrial processes. It is composed of hydrogen, sulphur, and oxygen atoms. Let's break down the formula and understand it in detail:
- H: Represents the hydrogen atom, which is a monovalent element.
- 2: Indicates that there are two hydrogen atoms present in the molecule.
- S: Represents the sulphur atom, which is a tetravalent element.
- O: Represents the oxygen atom, which is a divalent element.
- 4: Indicates that there are four oxygen atoms present in the molecule.
Therefore, the formula of sulphuric acid is H₂SO₄.
To summarize, the correct answer is B: H₂SO₄.

Potassium chlorate (KClO₃) is a compound that has important properties and characteristics. Let's examine each statement to determine which one is true.
A: It gives oxygen gas on strong heating
This statement is incorrect. Potassium chlorate decomposes upon strong heating to form potassium chloride (KCl) and oxygen gas (O₂). The reaction can be represented as follows:
2KClO₃ → 2KCl + 3O₂
B: Its molecular mass is 122.5 kg/mol
This statement is incorrect. The molecular mass of potassium chlorate (KClO₃) can be calculated by adding the atomic masses of potassium (K), chlorine (Cl), and oxygen (O). The atomic masses are:
Potassium (K) = 39.1 g/mol
Chlorine (Cl) = 35.5 g/mol
Oxygen (O) = 16.0 g/mol
Therefore, the molecular mass of potassium chlorate is:
39.1 g/mol + 35.5 g/mol + (3 * 16.0 g/mol) = 122.6 g/mol, not 122.5 kg/mol.
C: 122.5g of it contain oxygen atoms three times the Avogadro number
This statement is correct. The Avogadro number represents the number of atoms in one mole of a substance, which is approximately 6.022 x 10^23 atoms/mol.
The molar mass of oxygen (O) is 16.0 g/mol. Therefore, if we have 122.5 g of potassium chlorate (KClO₃), the number of moles can be calculated as follows:
Number of moles = mass / molar mass = 122.5 g / 122.6 g/mol ≈ 1 mol
Since one mole of KClO₃ contains three moles of oxygen atoms, the number of oxygen atoms in 122.5 g of KClO₃ is:
3 moles x (6.022 x 10^23 atoms/mol) = 1.8066 x 10^24 atoms
This value is greater than three times the Avogadro number, so the statement is true.
D: Its molecular formula is KClO₄
This statement is incorrect. The correct molecular formula for potassium chlorate is KClO₃, not KClO₄.
Therefore, the correct answer is C: 122.5g of it contain oxygen atoms three times the Avogadro number.

The Law of Multiple Proportions
The law of multiple proportions was proposed by John Dalton, an English chemist, physicist, and meteorologist. Dalton is known for his contributions to atomic theory and his studies on the relationship between the masses of elements in chemical compounds.
Explanation of the Law
The law of multiple proportions states that when two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in ratios of small whole numbers. This means that the same elements can combine in different ratios to form different compounds.
Example
To understand the law, let's consider the combination of oxygen (O) and carbon (C) to form carbon monoxide (CO) and carbon dioxide (CO2):
- In carbon monoxide (CO), 12 grams of carbon combine with 16 grams of oxygen.
- In carbon dioxide (CO2), 12 grams of carbon combine with 32 grams of oxygen.
The ratio of oxygen to carbon in carbon monoxide is 16:12, which simplifies to 4:3. In carbon dioxide, the ratio is 32:12, which simplifies to 8:3. These ratios are small whole numbers, demonstrating the law of multiple proportions.
Significance of the Law
The law of multiple proportions provided evidence for the existence of atoms and their combination in fixed ratios to form compounds. It supported Dalton's atomic theory, which states that atoms are indivisible and combine in simple, whole-number ratios to form compounds. This law also laid the foundation for further advancements in chemistry and our understanding of the composition of matter.
In conclusion, John Dalton proposed the law of multiple proportions, which states that the masses of elements that combine to form compounds are in ratios of small whole numbers. This law played a crucial role in the development of atomic theory and our understanding of chemical compounds.

Explanation:
To solve this problem, we need to understand the concept of equivalent weight and the ratio between the weights of two elements that combine with each other. Here's a detailed explanation:
Atomic heights:
- Atomic height is not a parameter that determines the ratio of weights between two elements.
Atomic volumes:
- Atomic volume is also not a parameter that determines the ratio of weights between two elements.
Equivalent weight:
- The equivalent weight of an element is the weight of the element that combines with or displaces 1 gram-equivalent of hydrogen or any other element.
- The ratio of weights between two elements that combine with each other is equal to the ratio of their equivalent weights.
- Therefore, the weight of two elements which combine with each other is in the ratio of their equivalent weights.
Molecular weights:
- Molecular weight is the sum of the atomic weights of all the atoms in a molecule.
- The molecular weight of a compound is not directly related to the ratio of weights between two elements that combine with each other.
Conclusion:
- The weight of two elements which combine with each other is in the ratio of their equivalent weights (Option C).

Question:
180 grams of water contains ________ moles.

To find the number of moles in 180 grams of water, we need to use the molar mass of water and the formula:
Molar mass of water:
The molar mass of water (H2O) is calculated as the sum of the molar masses of its constituent elements:
- Hydrogen (H): 1.01 g/mol
- Oxygen (O): 16.00 g/mol
Therefore, the molar mass of water (H2O) is 18.02 g/mol.
Now we can calculate the number of moles in 180 grams of water using the formula:
Number of moles = Mass / Molar mass
- Mass: 180 grams
- Molar mass of water (H2O): 18.02 g/mol
Substituting the values into the formula:
Number of moles = 180 g / 18.02 g/mol
Calculating the number of moles:
Number of moles = 9.99 moles (rounded to two decimal places)
Conclusion:
Therefore, 180 grams of water contains approximately 10 moles of water.
Answer:
B: 10

Problem:
What is the weight of 3 gram atoms of sulphur?

To find the weight of 3 gram atoms of sulphur, we can follow these steps:
Step 1: Determine the atomic mass of sulphur
- The atomic mass of sulphur is 32.06 grams per mole.
Step 2: Convert grams to moles
- We are given 3 gram atoms of sulphur.
- Since 1 mole of any element contains Avogadro's number of atoms (6.022 x 10^23), 1 gram atom is equal to 1 mole.
- Therefore, 3 gram atoms of sulphur is equal to 3 moles.
Step 3: Calculate the weight
- Multiply the number of moles (3) by the atomic mass of sulphur (32.06 grams/mole).
- The weight of 3 gram atoms of sulphur is 3 moles x 32.06 grams/mole = 96.18 grams.
Answer:
The weight of 3 gram atoms of sulphur is 96 grams (rounded to two decimal places).
Explanation:
- The atomic mass of sulphur is an essential factor in calculating the weight of 3 gram atoms.
- By converting the given amount from gram atoms to moles, we can use the atomic mass to determine the weight accurately.
- The weight is calculated by multiplying the number of moles by the atomic mass of sulphur.
- The final weight of 96 grams is obtained by performing the calculation.

To determine the number of gram atoms present in 144 g of magnesium, we need to use the concept of molar mass and Avogadro's number.
1. Find the molar mass of magnesium:
- The atomic mass of magnesium (Mg) is 24.31 g/mol.
2. Use the molar mass to convert grams to moles:
- Divide the given mass (144 g) by the molar mass of magnesium:
144 g / 24.31 g/mol = 5.93 mol
3. Determine the number of atoms in the given moles:
- Avogadro's number states that there are 6.022 × 10^23 atoms in one mole of a substance.
- Multiply the number of moles (5.93 mol) by Avogadro's number:
5.93 mol × 6.022 × 10^23 atoms/mol = 3.57 × 10^24 atoms
4. Convert the number of atoms to gram atoms:
- One gram atom is equal to one mole of atoms.
- Divide the number of atoms (3.57 × 10^24 atoms) by Avogadro's number:
3.57 × 10^24 atoms / 6.022 × 10^23 atoms/mol = 5.93 mol
Therefore, there are 6 gram atoms present in 144 g of magnesium.

To determine the number of moles of oxygen atoms present in one mole of acetic acid (CH3COOH), we need to examine the chemical formula of acetic acid and count the number of oxygen atoms.
The chemical formula of acetic acid is CH3COOH. Breaking down the formula, we can see that it consists of the following atoms:
- Carbon (C) - 2 atoms
- Hydrogen (H) - 4 atoms
- Oxygen (O) - 2 atoms
Therefore, in one molecule of acetic acid, there are 2 oxygen atoms. Since one mole of a substance contains Avogadro's number of particles (6.022 x 10^23), we can conclude that:
- In one mole of acetic acid, there are 2 moles of oxygen atoms.
Hence, the correct answer is option C: 2 moles.

Explanation:
To determine the number of particles in one mole of a substance, we need to understand Avogadro's number. Avogadro's number is defined as the number of atoms, molecules, or ions in one mole of a substance. It is approximately equal to 6.023 × 10^23.
Given:
Number of particles in one mole of a substance = ?

To find the number of particles in one mole of a substance, we use Avogadro's number, which is 6.023 × 10^23. Therefore, the answer is A: 6.023 × 10^23.
Summary:
The number of particles in one mole of a substance is 6.023 × 10^23.

To determine the number of atoms and gram atoms in 10 grams of calcium, we need to use Avogadro's number and the molar mass of calcium.
1. Calculate the number of gram atoms:
- The molar mass of calcium is 40.08 g/mol.
- Divide the given mass (10 grams) by the molar mass to find the number of gram atoms:
10 g / 40.08 g/mol = 0.2498 mol
- Therefore, there are approximately 0.25 gram atoms of calcium.
2. Calculate the number of atoms:
- Avogadro's number states that one mole of any substance contains 6.023 × 10^23 particles (atoms, molecules, etc.).
- Multiply the number of gram atoms by Avogadro's number to find the number of atoms:
0.25 mol × 6.023 × 10^23 atoms/mol = 1.50 × 10^23 atoms
Therefore, there are approximately 0.25 gram atoms and 1.50 × 10^23 atoms in 10 grams of calcium. Therefore, the correct answer is B.

To calculate the weight of 0.1 mole of sodium carbonate, we need to use the formula:
Weight = Number of moles × Molar mass
Step 1: Determine the molar mass of sodium carbonate (Na2CO3)
The molar mass of sodium (Na) is 22.99 g/mol, carbon (C) is 12.01 g/mol, and oxygen (O) is 16.00 g/mol.
So, the molar mass of Na2CO3 can be calculated as follows:
Molar mass of Na2CO3 = (2 × Molar mass of Na) + Molar mass of C + (3 × Molar mass of O)
= (2 × 22.99 g/mol) + 12.01 g/mol + (3 × 16.00 g/mol)
= 45.98 g/mol + 12.01 g/mol + 48.00 g/mol
= 105.99 g/mol
Step 2: Calculate the weight of 0.1 mole of sodium carbonate
Weight = Number of moles × Molar mass
= 0.1 mol × 105.99 g/mol
= 10.599 g
Step 3: Round off the weight to the nearest tenth
Rounding off 10.599 g to the nearest tenth gives 10.6 g.
So, the weight of 0.1 mole of sodium carbonate is 10.6 g.
Therefore, the correct answer is C. 10.6 g.

To determine the number of moles present in 540 g of glucose, we need to use the formula:
Number of moles = Mass of substance / molar mass
1. Find the molar mass of glucose:
- Glucose formula: C6H12O6
- Molar mass of carbon (C): 12.01 g/mol (from periodic table)
- Molar mass of hydrogen (H): 1.008 g/mol (from periodic table)
- Molar mass of oxygen (O): 16.00 g/mol (from periodic table)
- Multiply the molar masses of each element by the number of atoms in glucose and sum them up:
- Molar mass of carbon = 12.01 g/mol x 6 = 72.06 g/mol
- Molar mass of hydrogen = 1.008 g/mol x 12 = 12.096 g/mol
- Molar mass of oxygen = 16.00 g/mol x 6 = 96.00 g/mol
- Total molar mass of glucose = 72.06 g/mol + 12.096 g/mol + 96.00 g/mol = 180.156 g/mol
2. Calculate the number of moles:
- Number of moles = Mass of glucose / Molar mass of glucose
- Number of moles = 540 g / 180.156 g/mol = 2.998 moles (approximately)
3. Rounding the answer:
- Since the answer needs to be rounded to the nearest whole number, we round 2.998 to 3.
Therefore, the number of moles present in 540 g of glucose is 3 moles.
So, the correct answer is B: 3 mole.

Explanation:
To solve this problem, we need to use the concept of molar mass and Avogadro's number.
Avogadro's number (represented by the symbol "N") is the number of atoms or molecules in one mole of a substance. It is equal to 6.022 × 10^23.
Molar mass is the mass of one mole of a substance. It is expressed in grams per mole (g/mol).
We are given that the mass of one Avogadro's number of O atoms is required.
To find the mass, we need to determine the molar mass of oxygen (O).
The atomic mass of oxygen is 16 amu (atomic mass units).
To convert the atomic mass to molar mass, we use the conversion factor: 1 amu = 1 g/mol.
Therefore, the molar mass of oxygen is also 16 g/mol.
Since Avogadro's number (N) represents the number of atoms or molecules in one mole, if we have one mole of oxygen atoms, the mass will be equal to the molar mass of oxygen.
Therefore, the mass of one Avogadro's number of O atoms is 16 grams (option B).

To determine the ratio between the masses of hydrogen and oxygen in water, we need to consider the molar ratio of the elements in the compound.
Molar ratio of hydrogen and oxygen in water:
- Water (H2O) consists of 2 hydrogen atoms and 1 oxygen atom.
- The molar mass of hydrogen (H) is approximately 1 g/mol.
- The molar mass of oxygen (O) is approximately 16 g/mol.
Calculating the masses:
- The mass of hydrogen in water is 2 x 1 g/mol = 2 g
- The mass of oxygen in water is 1 x 16 g/mol = 16 g
Ratio between the masses of hydrogen and oxygen:
- The ratio is obtained by dividing the mass of hydrogen by the mass of oxygen.
- Therefore, the ratio is 2 g : 16 g, which simplifies to 1 : 8.
Conclusion:
The ratio between the masses of hydrogen and oxygen in water is 1 : 8. Therefore, the correct answer is option A.

To determine which substance has the highest intermolecular forces of attraction, we need to consider the type of intermolecular forces present in each substance.
Liquid water:
- Water molecules are held together by hydrogen bonding, which is a strong intermolecular force.
- Hydrogen bonding occurs when a hydrogen atom in one molecule is attracted to the electronegative oxygen atom of another water molecule.
- This results in strong attractions between water molecules.
Liquid ethyl alcohol:
- Ethyl alcohol also exhibits hydrogen bonding due to the presence of the hydroxyl (-OH) group.
- Like water, the hydrogen bonding in ethyl alcohol contributes to its relatively high boiling point and viscosity.
Gaseous CO₂:
- Carbon dioxide (CO₂) is a nonpolar molecule and does not exhibit hydrogen bonding.
- The intermolecular forces of attraction in gaseous CO₂ are weak London dispersion forces.
- London dispersion forces arise from temporary fluctuations in electron distribution, causing temporary dipoles in the molecules.
- These forces are relatively weak compared to hydrogen bonding.
Solid CO₂:
- Solid CO₂, also known as dry ice, consists of CO₂ molecules held together by intermolecular forces.
- In the solid state, CO₂ molecules are arranged in a crystal lattice structure.
- The intermolecular forces in solid CO₂ are stronger than in gaseous CO₂ because the molecules are held in fixed positions within the lattice.
Conclusion:
- Among the given options, solid CO₂ (dry ice) has the highest intermolecular forces of attraction.
- This is because solid CO₂ exhibits stronger intermolecular forces due to the arrangement of molecules in a crystal lattice structure.

Gases have high intermolecular forces of attraction is not correct regarding gases.

Explanation:
Gases are a state of matter that have unique properties. Let's discuss each of the given options to understand why option D is incorrect.
1. Gases exert pressure: Gases consist of molecules that are in constant motion. As they collide with the walls of their container, they exert pressure. This is known as gas pressure.
2. Gases have the same intermolecular space: Gases are highly compressible, meaning their volume can change easily. This indicates that the molecules in a gas have a significant amount of free space between them, allowing them to move around freely.
3. Gases have a tendency to diffuse: Diffusion is the process by which gases mix and spread out evenly in a given space. Gases have high kinetic energy, which enables their molecules to move rapidly and spread out from areas of high concentration to areas of low concentration.
4. Gases have high intermolecular forces of attraction: This statement is incorrect. Gases have negligible intermolecular forces of attraction. Unlike liquids and solids, where the intermolecular forces hold the molecules together in a close arrangement, gases have weak forces of attraction between their molecules.
In conclusion, option D is incorrect because gases have low intermolecular forces of attraction, unlike liquids and solids.

Avogadro's Number and the Number of Atoms
Avogadro's Number:
Avogadro's number, denoted as "N_A", represents the number of atoms or molecules in one mole of a substance. It is approximately equal to 6.022 x 10^23. Avogadro's number allows us to relate the mass of a substance to the number of atoms or molecules it contains.
Calculating the Number of Atoms:
To calculate the number of atoms in a given mass of a substance, we can use the formula:
Number of atoms = (Mass of substance / Molar mass) x Avogadro's number
Applying the Formula:
In this specific case, we are given the mass of different substances, and we need to determine the substance that corresponds to Avogadro's number.
1. 12g of C¹²:
- The molar mass of C¹² is 12 g/mol.
- Using the formula, the number of atoms = (12 g / 12 g/mol) x (6.022 x 10^23)
- Simplifying the equation, we get 6.022 x 10^23, which is equal to Avogadro's number.
2. 320g of sulphur:
- The molar mass of sulfur is 32 g/mol.
- Using the formula, the number of atoms = (320 g / 32 g/mol) x (6.022 x 10^23)
- Simplifying the equation, we get 6.022 x 10^24, which is not equal to Avogadro's number.
3. 32g of oxygen:
- The molar mass of oxygen is 16 g/mol.
- Using the formula, the number of atoms = (32 g / 16 g/mol) x (6.022 x 10^23)
- Simplifying the equation, we get 1.2044 x 10^24, which is not equal to Avogadro's number.
4. 12.7 g of iodine:
- The molar mass of iodine is 127 g/mol.
- Using the formula, the number of atoms = (12.7 g / 127 g/mol) x (6.022 x 10^23)
- Simplifying the equation, we get 6.022 x 10^22, which is not equal to Avogadro's number.
Conclusion:
Based on the calculations, the number of atoms in 12g of C¹² is equal to Avogadro's number. Therefore, option A is the correct answer.

Molecular Mass of Ozone
To determine the molecular mass of ozone (O3), we need to calculate the sum of the atomic masses of its constituent atoms.
- Ozone is composed of three oxygen atoms (O).
- The atomic mass of oxygen is 16 atomic mass units (u).
Calculation:
1. Multiply the atomic mass of oxygen by 3 (since there are three oxygen atoms in ozone):
- 16 u * 3 = 48 u
Therefore, the molecular mass of ozone (O3) is 48 atomic mass units (u).
Answer: C. 48 u

Dalton's Atomic Theory:
According to Dalton's atomic theory, there are several key principles that describe the nature of atoms and their behavior in chemical reactions. Let's examine each statement and determine which one is not correct.
A: Matter is made up of atoms
- This statement is correct according to Dalton's atomic theory. Atoms are the building blocks of all matter.
B: Atoms of all substances are identical in all respects
- This statement is not correct according to Dalton's atomic theory. Dalton believed that atoms of the same element are identical in all respects, but atoms of different elements have different properties.
C: Atoms combine in a simple whole number ratio
- This statement is correct according to Dalton's atomic theory. Atoms combine with each other to form compounds in specific ratios, and these ratios are usually simple whole numbers.
D: Atoms of two elements can combine to form more than one compound.
- This statement is correct according to Dalton's atomic theory. Atoms of different elements can combine in different ratios to form multiple compounds. For example, carbon and oxygen can combine to form both carbon dioxide (CO2) and carbon monoxide (CO).
Conclusion:
Based on Dalton's atomic theory, the statement that is not correct is B: Atoms of all substances are identical in all respects. Dalton believed that atoms of the same element are identical, but atoms of different elements have different properties.

To calculate the amount of lime obtained by burning 400 g of limestone, we need to understand the chemical reaction involved.
1. The chemical formula for limestone is CaCO3, which means it contains calcium carbonate.
2. When limestone is burned, it undergoes a chemical reaction known as thermal decomposition, where calcium oxide (lime) and carbon dioxide are produced.
3. The balanced chemical equation for this reaction is: CaCO3 → CaO + CO2
Now, let's calculate the amount of lime obtained:
1. Calculate the molar mass of calcium carbonate (CaCO3):
- Atomic mass of calcium (Ca) = 40.08 g/mol
- Atomic mass of carbon (C) = 12.01 g/mol
- Atomic mass of oxygen (O) = 16.00 g/mol
- Molar mass of CaCO3 = (40.08 g/mol) + (12.01 g/mol) + (3 * 16.00 g/mol) = 100.09 g/mol
2. Determine the molar mass of lime (CaO):
- Atomic mass of calcium (Ca) = 40.08 g/mol
- Atomic mass of oxygen (O) = 16.00 g/mol
- Molar mass of CaO = (40.08 g/mol) + (16.00 g/mol) = 56.08 g/mol
3. Calculate the number of moles of limestone:
- Moles of limestone = (mass of limestone) / (molar mass of CaCO3)
- Moles of limestone = 400 g / 100.09 g/mol = 3.997 moles (approximately)
4. According to the balanced chemical equation, 1 mole of limestone produces 1 mole of lime.
Therefore, the number of moles of lime obtained is also 3.997 moles (approximately).
5. Calculate the mass of lime obtained:
- Mass of lime = (moles of lime) * (molar mass of CaO)
- Mass of lime = 3.997 moles * 56.08 g/mol = 224.31 g (approximately)
From the calculation, we can conclude that approximately 224 g of lime can be obtained by burning 400 g of limestone. Therefore, the correct answer is A.