Q1: State the Law of Conservation of Mass.

Answer: The Law of Conservation of Mass, proposed by Antoine Lavoisier, states that in any chemical reaction, the total mass of the reactants is equal to the total mass of the products. Mass is neither created nor destroyed during a chemical reaction.

Q2: State the Law of Constant Proportions.

Answer: The Law of Constant Proportions, proposed by J.L. Proust, states that in a chemical compound, the constituent elements are always present in definite proportions by mass, regardless of the source or method of preparation. For example, in water (H₂O), the ratio of the mass of hydrogen to the mass of oxygen is always 1:8.

Q3: What is an atom? What determines the mass of an atom?

Answer: An atom is the smallest particle of an element that retains the chemical properties of that element. The mass of an atom is primarily concentrated in its nucleus, which contains protons and neutrons. The atomic mass is approximately the sum of the number of protons and neutrons.

Q4: What is meant by atomic mass unit (u)?

Answer: The atomic mass unit (u), also known as the unified atomic mass unit or Dalton (Da), is a standard unit used to express atomic and molecular masses. Currently, 1 u is defined as one-twelfth (1/12) the mass of a single neutral atom of Carbon-12 (¹²C) in its ground state. 1 u ≈ 1.660539 × 10⁻²⁷ kg.

Q5: Define valency.

Answer: Valency is the combining capacity of an element or a radical. It represents the number of electrons an atom can donate, accept, or share to form chemical bonds with other atoms. It is typically represented as a positive integer.

Q6: What are molecules?

Answer: Molecules are the smallest particles of an element or a compound that can exist independently under ordinary conditions and show all the properties of that substance. They are formed when two or more atoms bond together chemically (e.g., O₂, H₂O, CO₂).

Q7: Define molecular mass.

Answer: The molecular mass of a substance is the sum of the atomic masses of all the atoms present in one molecule of that substance. It is expressed in atomic mass units (u).

Q8: How do you calculate the molecular mass of a compound? Give an example for water (H₂O).

Answer: To calculate the molecular mass, you multiply the atomic mass of each element by the number of its atoms in the molecule and then add these values together.
Example for Water (H₂O):
Atomic mass of Hydrogen (H) = 1 u
Atomic mass of Oxygen (O) = 16 u
Molecular mass of H₂O = (Number of H atoms × Atomic mass of H) + (Number of O atoms × Atomic mass of O)
= (2 × 1 u) + (1 × 16 u)
= 2 u + 16 u
= 18 u

Q9: What is the mole concept?

Answer: The mole concept relates the mass of a substance to the number of particles (atoms, molecules, ions) it contains. A mole is a unit used to measure the amount of substance.

Q10: Define one mole.

Answer: One mole (mol) of any substance is defined as the amount of that substance which contains the same number of particles (atoms, molecules, ions, etc.) as there are atoms in exactly 12 grams of Carbon-12 (¹²C). This number is known as Avogadro's number.

Q11: What is Avogadro's number? What is its value?

Answer: Avogadro's number (N<0xE2><0x82><0x90>) is the number of constituent particles (usually atoms or molecules) that are contained in one mole of a substance. Its accepted value is approximately 6.022 × 10²³ particles per mole.

Q12: How are moles, mass, and number of particles related?

Answer:

1. Moles and Mass: The mass of one mole of a substance in grams is numerically equal to its atomic mass (for elements) or molecular mass (for compounds). This is called the molar mass (grams/mol).
   Number of moles (n) = Given mass (m) / Molar mass (M)
2. Moles and Number of Particles: One mole of any substance contains Avogadro's number (N<0xE2><0x82><0x90>) of particles.
   Number of particles = Number of moles (n) × Avogadro's number (N<0xE2><0x82><0x90>)

Q13: Calculate the number of moles in 36 g of water (H₂O).

Answer:
First, find the molar mass of water (H₂O).
Molecular mass of H₂O = (2 × 1) + 16 = 18 u.
Therefore, the molar mass of H₂O = 18 g/mol.
Given mass (m) = 36 g.
Number of moles (n) = Given mass (m) / Molar mass (M)
n = 36 g / 18 g/mol = 2 mol.
There are 2 moles in 36 g of water.

Q14: How many molecules are present in 66 g of CO₂?

Answer:
First, find the molar mass of CO₂.
Atomic mass of C = 12 u, Atomic mass of O = 16 u.
Molecular mass of CO₂ = 12 + (2 × 16) = 12 + 32 = 44 u.
Molar mass of CO₂ = 44 g/mol.
Number of moles (n) = Given mass / Molar mass = 66 g / 44 g/mol = 1.5 mol.
Number of molecules = Number of moles × Avogadro's number (N<0xE2><0x82><0x90>)
= 1.5 × 6.022 × 10²³
= 9.033 × 10²³ molecules.
There are 9.033 × 10²³ molecules in 66 g of CO₂.

Q15: What are radicals (ions)?

Answer: Radicals, also known as ions, are atoms or groups of atoms that carry a positive or negative charge due to the loss or gain of electrons. Positively charged ions are called cations, and negatively charged ions are called anions.

Q16: Differentiate between simple radicals and compound radicals.

Answer:

• Simple Radicals (Monoatomic ions): These consist of a single atom carrying a charge (e.g., Na⁺, Mg²⁺, Cl⁻, O²⁻).
• Compound Radicals (Polyatomic ions): These consist of a group of two or more atoms bonded together that collectively carry a charge (e.g., SO₄²⁻ (sulfate), NH₄⁺ (ammonium), OH⁻ (hydroxide)).

Q17: What are acidic and basic radicals?

Answer: In the context of salts formed from acids and bases:

• Acidic Radicals (Anions): These are negatively charged ions derived from acids (e.g., Cl⁻ from HCl, SO₄²⁻ from H₂SO₄, NO₃⁻ from HNO₃).
• Basic Radicals (Cations): These are positively charged ions derived from bases (e.g., Na⁺ from NaOH, Ca²⁺ from Ca(OH)₂, NH₄⁺ from NH₄OH).

Q18: What is variable valency? Give an example.

Answer: Some elements exhibit more than one valency in different compounds. This property is called variable valency. Examples include:

• Iron (Fe): Shows valency 2 (Fe²⁺, ferrous) and 3 (Fe³⁺, ferric).
• Copper (Cu): Shows valency 1 (Cu⁺, cuprous) and 2 (Cu²⁺, cupric).
• Mercury (Hg): Shows valency 1 (Hg₂²⁺, mercurous) and 2 (Hg²⁺, mercuric).

Q19: What are the steps to write the chemical formula of a compound?

Answer:

1. Step 1: Write the symbols of the constituent radicals (cation first, then anion).
2. Step 2: Write the valency below each respective radical.
3. Step 3: Cross-multiply the valencies to get the number of atoms/radicals. Write these numbers as subscripts. (Ignore charges during cross-multiplication). Simplify the subscripts to the simplest whole number ratio if possible.
4. Step 4: Write the final chemical formula. If a subscript is 1, it is omitted. If a compound radical needs a subscript greater than 1, enclose the radical in parentheses before writing the subscript.

Q20: Write the chemical formula for Sodium Sulfate.

Answer:
1. Symbols: Na (Sodium), SO₄ (Sulfate)
2. Valencies: Na (1), SO₄ (2)
3. Cross-multiply: Na gets 2, SO₄ gets 1
4. Formula: Na₂SO₄

Q21: Write the chemical formula for Aluminum Hydroxide.

Answer:
1. Symbols: Al (Aluminum), OH (Hydroxide)
2. Valencies: Al (3), OH (1)
3. Cross-multiply: Al gets 1, OH gets 3.
4. Formula: Al(OH)₃ (Parentheses needed for the compound radical OH).

Q22: Write the chemical formula for Ferric Oxide.

Answer:
Ferric indicates Iron (Fe) with valency 3.
Oxide indicates Oxygen (O) with valency 2.
1. Symbols: Fe, O
2. Valencies: Fe (3), O (2)
3. Cross-multiply: Fe gets 2, O gets 3.
4. Formula: Fe₂O₃

Q23: Calculate the molecular mass of Sulphuric Acid (H₂SO₄).

Answer:
Atomic mass H = 1 u, S = 32 u, O = 16 u.
Formula: H₂SO₄
Molecular mass = (2 × Atomic mass of H) + (1 × Atomic mass of S) + (4 × Atomic mass of O)
= (2 × 1 u) + (1 × 32 u) + (4 × 16 u)
= 2 u + 32 u + 64 u
= 98 u

Q24: Calculate the molecular mass of Calcium Chloride (CaCl₂).

Answer:
Atomic mass Ca = 40 u, Cl = 35.5 u.
Formula: CaCl₂
Molecular mass = (1 × Atomic mass of Ca) + (2 × Atomic mass of Cl)
= (1 × 40 u) + (2 × 35.5 u)
= 40 u + 71 u
= 111 u

Q25: What is the mass of 0.5 moles of Nitrogen gas (N₂)?

Answer:
First, find the molar mass of Nitrogen gas (N₂).
Atomic mass of N = 14 u.
Molecular mass of N₂ = 2 × 14 u = 28 u.
Molar mass of N₂ = 28 g/mol.
Number of moles (n) = 0.5 mol.
Mass (m) = Number of moles (n) × Molar mass (M)
m = 0.5 mol × 28 g/mol
m = 14 g

Q26: How many atoms are there in 1 mole of Sodium (Na)?

Answer: One mole of any element contains Avogadro's number of atoms. Therefore, 1 mole of Sodium (Na) contains 6.022 × 10²³ atoms.

Q27: Name the cation and anion in the compound MgCl₂.

Answer:
Cation (positive ion): Mg²⁺ (Magnesium ion)
Anion (negative ion): Cl⁻ (Chloride ion)

Q28: Deduce the valency of Cu in CuO and Cu₂O.

Answer:
Oxygen (O) typically has a valency of 2 (O²⁻).
* In CuO: Let the valency of Cu be x. Total charge = 0. x + (-2) = 0 => x = +2. Valency of Cu is 2 (Cupric).
* In Cu₂O: Let the valency of Cu be y. (2 × y) + (-2) = 0 => 2y = 2 => y = +1. Valency of Cu is 1 (Cuprous).

Q29: What information is conveyed by the chemical formula of a compound?

Answer: The chemical formula of a compound conveys:

1. The types of elements present in the compound.
2. The number of atoms of each element present in one molecule (or formula unit) of the compound.
3. The ratio in which atoms of different elements combine.
4. It allows calculation of the molecular mass (or formula mass) of the compound.

Q30: Why is it important to know the valencies of elements and radicals?

Answer: Knowing the valencies is crucial for correctly writing the chemical formulae of compounds. The valencies determine the ratio in which atoms or radicals combine to form electrically neutral compounds, following the principle of charge balance (total positive valency = total negative valency).