Atoms & Molecules - Practice Test, Class 9 Science - Class 9 MCQ

Triatomic Molecules
Triatomic molecules are molecules that consist of three atoms bonded together. One example of a triatomic molecule is ozone, which has the chemical formula O3. Ozone is composed of three oxygen atoms bonded together.
Explanation:
- Triatomic molecules have three atoms bonded together.
- Ozone is a triatomic molecule with the chemical formula O3.
- Ozone consists of three oxygen atoms bonded together.
- The oxygen atoms in ozone are arranged in a bent shape, with an angle of approximately 116 degrees between the oxygen-oxygen-oxygen bonds.
- Ozone is a pale blue gas that has a distinct odor.
- It is formed in the atmosphere through the reaction of oxygen molecules (O2) with ultraviolet radiation from the sun.
- Ozone plays a crucial role in the Earth's atmosphere as it absorbs most of the sun's harmful ultraviolet radiation, protecting life on Earth.
Conclusion:
An example of a triatomic molecule is ozone (O3), which consists of three oxygen atoms bonded together. Ozone is important for protecting life on Earth by absorbing harmful ultraviolet radiation from the sun.

The quantity of matter present in an object is called its Mass.
Explanation:
- Mass is a fundamental property of matter that measures the amount of substance an object contains.
- It is different from weight, which is the gravitational force acting on an object.
- Mass is an intrinsic property of an object and remains the same regardless of its location or the gravitational field it is in.
- The SI unit of mass is the kilogram (kg).
- Mass can be measured using a balance or a scale.
- It is a scalar quantity, meaning it only has magnitude and no direction.
- Mass is an extensive property, meaning it depends on the amount of substance present in an object.
- The more matter an object contains, the greater its mass will be.
- In everyday life, mass is commonly used interchangeably with weight, but they are distinct concepts in physics.
- The mass of an object determines its inertia, which is the resistance of an object to changes in its motion.
- Mass also plays a role in determining an object's gravitational attraction to other objects.
In summary, mass is the quantity of matter present in an object and is measured in kilograms (kg).

Indivisibility of an atom was proposed by Dalton.
Explanation:
- John Dalton was an English chemist who proposed the atomic theory in the early 19th century.
- The atomic theory suggested that all matter is made up of tiny, indivisible particles called atoms.
- Dalton's theory stated that atoms are the smallest unit of matter and cannot be divided further.
- He believed that atoms of the same element are identical in size, mass, and other properties, while atoms of different elements have different properties.
- Dalton's atomic theory also proposed that atoms combine in simple, whole number ratios to form compounds.
- This theory laid the foundation for our modern understanding of atoms and their behavior.
- Although Dalton's atomic theory has undergone modifications with the discovery of subatomic particles, the concept of atoms being indivisible still holds true in the sense that they are the fundamental building blocks of matter.

Law of Constant Proportion
The Law of Constant Proportion, also known as the Law of Definite Proportions, states that in a chemical compound, the elements are always present in definite proportions by mass. This means that the ratio of the masses of the elements in a compound is always the same, regardless of the amount or source of the compound.
Explanation:
- The given statement states that all samples of carbon dioxide (CO2) contain carbon and oxygen in the mass ratio of 3:8.
- This means that for every 3 parts of carbon, there are 8 parts of oxygen by mass in carbon dioxide.
- According to the Law of Constant Proportion, the ratio of carbon to oxygen in carbon dioxide will always be 3:8, regardless of the source or quantity of the compound.
- This law applies to all chemical compounds, and it helps to establish the fixed composition of substances.
- The Law of Constant Proportion is one of the fundamental principles of chemistry and is essential for stoichiometry calculations and understanding the behavior of substances.
- It provides a basis for determining the empirical formula of a compound and allows scientists to predict and analyze chemical reactions and reactions stoichiometry.
- In summary, the given mass ratio of carbon and oxygen in carbon dioxide is in agreement with the Law of Constant Proportion, which states that the elements in a compound are always present in fixed proportions by mass.

To find the number of moles in 46g of sodium, we can use the formula:
Number of moles = Mass of substance / Molar mass
Given that the atomic mass of sodium is 23, the molar mass of sodium is also 23 g/mol.
Using the formula:
Number of moles = 46g / 23 g/mol = 2 mol
Therefore, the number of moles in 46g of sodium is 2 moles.
Answer: A: 2

Explanation:
The Latin name for an element is often derived from its chemical symbol. In the case of potassium, the chemical symbol is "K". The Latin name for potassium is "Kalium". Here is a detailed explanation:
Potassium:
- Potassium is a chemical element with the symbol K and atomic number 19.
- It is a soft, silvery-white alkali metal that is highly reactive.
- Potassium is an essential mineral that plays a vital role in various bodily functions.
- It is important for muscle contractions, nerve function, and maintaining a healthy balance of fluids in the body.
- Potassium is found in many foods, including fruits, vegetables, and whole grains.
Kalium:
- Kalium is the Latin name for potassium.
- The Latin name is derived from the chemical symbol "K" for potassium.
- The use of Latin names for elements is common in scientific and chemical contexts.
- It helps to avoid confusion and provides a standardized way of referring to elements.
Conclusion:
- In conclusion, the Latin name "Kalium" refers to the chemical element potassium, which has the symbol "K".
- This Latin name is commonly used in scientific and chemical literature.

Answer:
The smallest particle of a substance that is capable of independent existence is a molecule.
Explanation:
- A molecule is made up of two or more atoms bonded together.
- Atoms are the basic units of matter, but they cannot exist independently as they are constantly interacting and bonding with other atoms.
- When atoms combine chemically, they form molecules.
- Molecules can be made up of atoms of the same element (e.g., oxygen gas - O2) or different elements (e.g., water - H2O).
- Molecules have their own unique properties and can exist independently.
- Molecules can further combine to form more complex structures like cells, tissues, and organisms.
Therefore, the correct answer is B: Molecule.

The number 16 in 16O8 represents the atomic mass of the isotope Oxygen-16 (O-16).
Explanation:
- The atomic mass of an element is the weighted average mass of all the isotopes of that element, taking into account their relative abundance.
- Oxygen has several isotopes, including Oxygen-16, Oxygen-17, and Oxygen-18, with atomic masses of 16, 17, and 18 respectively.
- The number 16 in 16O8 specifically refers to Oxygen-16, which is the most abundant and stable isotope of oxygen.
- The atomic mass is expressed in atomic mass units (u) or Dalton (Da), where 1 atomic mass unit is defined as 1/12th the mass of a carbon-12 atom.
- The atomic mass is determined by the number of protons and neutrons in the nucleus of an atom.
- In the case of Oxygen-16, it has 8 protons and 8 neutrons, giving it an atomic mass of 16.
- The number 8 in 16O8 represents the number of electrons in an oxygen atom, as the number of electrons is equal to the number of protons in a neutral atom.
Therefore, in the compound 16O8, the number 16 stands for the atomic mass of Oxygen-16.

The scientists had taken a standard unit to calculate the mass of atom. So they have taken the carbon 12 atoms as a unit to find the atomic mass. But it was very difficult to write always 1/12TH CARBON 12 ATOM.So instead they write it as amu(atomic mass unit) which was taken as 12 units also any mass of any
atom is always x times greater than the amu. So in this case the answer is 15amu.

The molecular mass of ammonia is 17 grams.
Explanation:
Ammonia is a compound that consists of one nitrogen atom and three hydrogen atoms. To calculate the molecular mass of ammonia, we need to find the sum of the atomic masses of each atom in the molecule.
1. Nitrogen atomic mass:
- Nitrogen (N) has an atomic mass of 14 grams per mole.
2. Hydrogen atomic mass:
- Hydrogen (H) has an atomic mass of 1 gram per mole.
3. Molecular mass calculation:
- Ammonia (NH₃) contains one nitrogen atom and three hydrogen atoms.
- Multiply the atomic mass of nitrogen (14 grams/mole) by its number of atoms (1).
- Multiply the atomic mass of hydrogen (1 gram/mole) by its number of atoms (3).
- Add the results together: (1 * 14) + (3 * 1) = 14 + 3 = 17 grams/mole.
Therefore, the molecular mass of ammonia is 17 grams.

Atomic mass of calcium:
- The atomic mass of calcium is given as 40.
- This means that one mole of calcium atoms has a mass of 40 grams.
Number of gram atoms:
- The question states that we have 2.5 gram atoms of calcium.
- A gram atom is defined as the amount of an element that has a mass in grams equal to its atomic mass.
Calculating the mass:
- To find the mass of 2.5 gram atoms of calcium, we need to multiply the atomic mass of calcium by 2.5.
- Therefore, the mass is calculated as 40 grams (atomic mass of calcium) multiplied by 2.5.
- This gives us a total mass of 100 grams.
Final answer:
- The mass of 2.5 gram atoms of calcium is 100 grams.
- Therefore, the correct answer is option C: 100 g.

Explanation:
The correct answer is D. Atomicity.
Definition:
Atomicity refers to the number of atoms present in a molecule of an elementary substance. It indicates the number of atoms that are chemically bonded together to form a molecule.
Key Points:
- Atomic number: The atomic number represents the number of protons in the nucleus of an atom. It is used to identify and classify elements on the periodic table.
- Avogadro number: The Avogadro number (6.022 x 10^23) represents the number of particles (atoms, molecules, ions) in one mole of a substance. It is a constant used in chemistry calculations.
- Atomic mass: The atomic mass represents the average mass of an atom of an element, taking into account the different isotopes and their relative abundance.
- Atomicity: Atomicity specifically refers to the number of atoms present in a molecule of an elementary substance.
In summary, while the atomic number, Avogadro number, and atomic mass are all important concepts in chemistry, none of them specifically refer to the number of atoms in a molecule of an elementary substance. The correct term for this is atomicity.

Calculation:
To determine the number of moles in 5 grams of calcium, we need to use the formula:
Number of moles = Mass of substance / Molar mass
Step 1: Calculate the molar mass of calcium.
The molar mass of calcium (Ca) is 40.08 g/mol.
Step 2: Substitute the values into the formula.
Number of moles = 5 g / 40.08 g/mol
Step 3: Solve the equation.
Number of moles = 0.1247512448 mol
Answer:
The number of moles in 5 grams of calcium is approximately 0.125 mole.

We are given that the sample contains 22 g of carbon dioxide and we need to determine what this quantity is equal to.
To solve this problem, we can use the concept of molar mass and Avogadro's number.
1. Calculate the molar mass of carbon dioxide:
- Carbon has a molar mass of 12.01 g/mol.
- Oxygen has a molar mass of 16.00 g/mol.
- Since carbon dioxide has one carbon atom and two oxygen atoms, we can calculate its molar mass as follows:
Molar mass of carbon dioxide = (1 * molar mass of carbon) + (2 * molar mass of oxygen)
Molar mass of carbon dioxide = (1 * 12.01 g/mol) + (2 * 16.00 g/mol)
Molar mass of carbon dioxide = 12.01 g/mol + 32.00 g/mol
Molar mass of carbon dioxide = 44.01 g/mol
2. Calculate the number of moles:
- We can use the formula: Moles = Mass / Molar mass
- Moles of carbon dioxide = 22 g / 44.01 g/mol
- Moles of carbon dioxide ≈ 0.499 moles
Therefore, the sample of 22 g of carbon dioxide is approximately equal to half a mole of carbon dioxide (Option C).

Molecules without a charged valency are contained by only non-metals.
There are certain key points to consider when determining which type of elements contain molecules without a charged valency:
1. Charged Valency:
- Valency refers to the combining capacity of an element, which determines the number of bonds it can form.
- A charged valency means that the element has gained or lost electrons, resulting in a positive or negative charge.
2. Metals vs. Non-metals:
- Metals are generally characterized by having a positive charge and losing electrons to form cations.
- Non-metals, on the other hand, tend to gain or share electrons to achieve a stable electron configuration.
3. Molecules without Charged Valency:
- Molecules without a charged valency are formed by non-metals.
- Non-metals can share electrons with other non-metals to form covalent bonds, resulting in a stable electron configuration.
- These covalent bonds allow non-metals to form molecules without gaining or losing electrons and without acquiring a charge.
4. Examples:
- Some examples of non-metals that form molecules without a charged valency include hydrogen (H2), oxygen (O2), nitrogen (N2), and carbon dioxide (CO2).
Therefore, based on the understanding that molecules without a charged valency are formed by non-metals, the correct answer is B: non-metals.

Explanation:
Definition:
- An atomic mass unit (amu) is a unit of mass used to express atomic and molecular weights.
- It is defined as 1/12th of the mass of a carbon-12 atom.
Given:
- One atomic mass unit is equal to the weight of:
A. One atom of hydrogen
B. 1/16th of an oxygen atom
C. 1/12th of a natural carbon atom
D. 1/12th of the C-12 isotope of carbon

- To determine which option is correct, we need to understand the definitions and relationships between the given options.
Option A: One atom of hydrogen

- The atomic mass of hydrogen is approximately 1.008 amu, which is not equal to 1 amu.
Option B: 1/16th of an oxygen atom
- The atomic mass of oxygen is approximately 16 amu.
- 1/16th of the atomic mass of oxygen would be approximately 1 amu.
- This means that one atomic mass unit could be equal to 1/16th of an oxygen atom, but it is not the standard definition.
Option C: 1/12th of a natural carbon atom
- The atomic mass of carbon is approximately 12 amu.
- 1/12th of the atomic mass of carbon would be approximately 1 amu.
- This means that one atomic mass unit could be equal to 1/12th of a natural carbon atom, but it is not the standard definition.
Option D: 1/12th of the C-12 isotope of carbon
- The atomic mass of the C-12 isotope of carbon is exactly 12 amu.
- 1/12th of the atomic mass of the C-12 isotope of carbon is exactly 1 amu.
- This matches the standard definition of an atomic mass unit.
Conclusion:
- Based on the definitions and relationships between the given options, the correct answer is D. One atomic mass unit is equal to 1/12th of the C-12 isotope of carbon.

Explanation:
When a chemical reaction takes place, there are certain changes that occur. However, one of the options listed does not change during a chemical reaction. Let's break down each option and explain why it does or does not change:
A: Volume
- Volume can change during a chemical reaction. For example, if a gas is produced, the volume of the system will increase.
- Therefore, volume can change during a chemical reaction.
B: Mass
- According to the law of conservation of mass, mass is conserved during a chemical reaction.
- This means that the total mass of the reactants is equal to the total mass of the products.
- Therefore, mass does not change during a chemical reaction.
C: Physical properties
- Physical properties refer to characteristics such as color, texture, density, and boiling point.
- In a chemical reaction, new substances are formed with different physical properties.
- Therefore, physical properties can change during a chemical reaction.
D: Chemical properties
- Chemical properties refer to the ability of a substance to undergo a chemical change.
- During a chemical reaction, substances react to form new substances with different chemical properties.
- Therefore, chemical properties can change during a chemical reaction.
Answer: B
- Mass does not change during a chemical reaction according to the law of conservation of mass.
- The total mass of the reactants is equal to the total mass of the products.
- Therefore, mass is the option that does not change when a chemical reaction takes place.

Formula Mass and Its Use
Formula mass, also known as molecular mass or molar mass, is the sum of the atomic masses of all the atoms in a chemical formula. It is used to calculate the mass of a substance in grams per mole.
Calculating Formula Mass
To calculate the formula mass of a compound, you need to know the atomic masses of the elements present and the number of atoms of each element in the compound. The atomic masses can be found on the periodic table.
Formula Mass and Compound Types
Formula mass is commonly used for compounds such as salts, acids, and covalent compounds. However, there are certain cases where formula mass is not used, such as:
Case A: NaCl
- Sodium chloride (NaCl) is a salt composed of sodium ions (Na+) and chloride ions (Cl-).
- Formula mass is used for NaCl because it is a compound made up of discrete ions.
Case B: MgCl2
- Magnesium chloride (MgCl2) is also a salt, but it has a higher formula mass compared to NaCl.
- Formula mass is used for MgCl2 because it is a compound made up of discrete ions.
Case C: CCl4
- Carbon tetrachloride (CCl4) is a covalent compound composed of carbon and chlorine atoms.
- Formula mass is used for CCl4 because it is a compound made up of discrete molecules.
Case D: CaO
- Calcium oxide (CaO) is an ionic compound, similar to NaCl.
- Formula mass is used for CaO because it is a compound made up of discrete ions.
Conclusion
In summary, formula mass is generally used for compounds such as salts, acids, and covalent compounds. It is not used based on the type of compound, but rather on the nature of the compound's constituent particles.

Mas of oxygen O₁ is 16 and the molar mass of O₂ will be 16×2=32gm