Molecules and Ions – Long Answer Questions
Medium Level (Application & Explanation)
Q1. What is a molecule? Distinguish molecules of elements and molecules of compounds. Explain the term atomicity with examples.
Answer:
- A molecule is a group of two or more atoms that are chemically bonded.
- It is the smallest part of a substance that can exist independently.
- A molecule of an element has same kind of atoms. Example: O₂, N₂, Cl₂.
- A molecule of a compound has different kinds of atoms. Example: H₂O, CO₂, NH₃.
- Atomicity is the number of atoms in a molecule. Example: He is monoatomic, O₂ is diatomic, O₃ is triatomic.
- Phosphorus (P₄) is tetra-atomic and sulphur (S₈) is poly-atomic.
- Thus, molecules show all properties of the substance they belong to.
Q2. Why are many non-metals diatomic while noble gases are monoatomic? Explain using oxygen and ozone as examples.
Answer:
- Noble gases like He and Ar are monoatomic because their atoms are already stable.
- They do not need to share or gain electrons. So they exist as single atoms.
- Many non-metals like O₂, N₂, and Cl₂ are diatomic.
- Their atoms share electrons to become stable, so they form diatomic molecules.
- Oxygen commonly exists as O₂ (a diatomic molecule).
- Some oxygen also exists as ozone (O₃), which is triatomic and has different properties.
- Both O₂ and O₃ are molecules of the same element, but their atomicity and properties differ.
Q3. Using the ratio by mass for water (H₂O), show step-by-step how to find the ratio by number of atoms.
Answer:
- Given: In water, H:O = 1:8 by mass.
- Take atomic masses: H = 1, O = 16.
- Divide each mass share by its atomic mass.
- For H: 1 ÷ 1 = 1.
- For O: 8 ÷ 16 = 1/2.
- Now compare the numbers: 1 : 1/2 equals 2 : 1 after multiplying by 2.
- So ratio by number of atoms is H:O = 2:1, which matches H₂O.
Q4. Convert the given ratios by mass into ratios by number of atoms for ammonia and carbon dioxide. Justify their formulas.
Answer:
- For ammonia (NH₃): Given N:H = 14:3 by mass.
- Atomic masses: N = 14, H = 1.
- Divide: N → 14 ÷ 14 = 1; H → 3 ÷ 1 = 3.
- Ratio by number is N:H = 1:3. So the formula is NH₃.
- For carbon dioxide (CO₂): Given C:O = 3:8 by mass.
- Atomic masses: C = 12, O = 16.
- Divide: C → 3 ÷ 12 = 0.25; O → 8 ÷ 16 = 0.5.
- Ratio by number is 0.25:0.5 = 1:2. So the formula is CO₂.
Q5. What is an ion? Differentiate between cation, anion, and polyatomic ion with an example of sodium chloride.
Answer:
- An ion is an atom or a group of atoms that carries a charge.
- A cation is a positively charged ion. It forms when an atom loses electrons.
- An anion is a negatively charged ion. It forms when an atom gains electrons.
- In sodium chloride (NaCl), Na loses one electron to form Na⁺ (cation).
- Cl gains one electron to form Cl⁻ (anion).
- A polyatomic ion is a group of atoms with a net charge (for example, ammonium in higher classes).
- Ionic compounds contain cations and anions arranged in a fixed ratio.
High Complexity (Analysis & Scenario-Based)
Q6. You have two cylinders: one with oxygen (O₂) and another with ozone (O₃). Compare their molecular composition, atomicity, and properties. Explain why they differ though both are forms of oxygen.
Answer:
- Oxygen gas has molecules of O₂. Its atomicity is 2 (diatomic).
- Ozone has molecules of O₃. Its atomicity is 3 (triatomic).
- Both are molecules of the same element (oxygen).
- They differ because their structures are different.
- O₂ supports breathing and burning. It is more stable.
- O₃ has a sharp smell and is a strong oxidizing agent.
- Thus, different atomicity and arrangement cause different properties.
Q7. A white solid shows a constant mass ratio of calcium to oxygen as 5:2. Use atomic masses to deduce its simplest formula. Is it calcium oxide?
Answer:
- Given Ca:O = 5:2 by mass.
- Atomic masses: Ca = 40, O = 16.
- Divide each mass share by its atomic mass.
- Ca: 5 ÷ 40 = 0.125.
- O: 2 ÷ 16 = 0.125.
- The numbers are equal, so the atom ratio is Ca:O = 1:1.
- The simplest formula is CaO. Yes, the solid is calcium oxide.
Q8. A student dissolves sodium chloride in water and connects a bulb. It glows. Explain this observation using the concept of ions and their movement.
Answer:
- Sodium chloride (NaCl) is an ionic compound.
- In water, it dissociates into Na⁺ and Cl⁻ ions.
- These free ions can move in the solution.
- Moving charges carry electric current.
- So the circuit is completed, and the bulb glows.
- In solid NaCl, ions are fixed in a lattice, so it does not conduct.
- In solution or molten state, ions are free, so it conducts.
Q9. A sample labeled magnesium sulphide shows a mass ratio Mg:S = 3:4. Use atomic masses to find the simplest ratio by number of atoms and the formula. Comment on bonding.
Answer:
- Given Mg:S = 3:4 by mass.
- Atomic masses: Mg = 24, S = 32.
- Divide: Mg → 3 ÷ 24 = 0.125; S → 4 ÷ 32 = 0.125.
- The atom ratio is Mg:S = 1:1.
- The formula is MgS.
- It is an ionic compound made of Mg²⁺ and S²⁻ (charge idea at basic level).
- It forms by electron transfer from Mg to S.
Q10. A mixture contains hydrogen and oxygen in the mass ratio 1:8 and is allowed to react. Predict the compound formed and justify its formula using number ratios.
Answer:
- Given H:O = 1:8 by mass in the mixture.
- This matches the mass ratio in water (H₂O).
- Convert mass ratio to number ratio using atomic masses H = 1, O = 16.
- H: 1 ÷ 1 = 1; O: 8 ÷ 16 = 1/2.
- Number ratio becomes 1 : 1/2 = 2 : 1.
- Thus, atoms combine as 2 H to 1 O, giving H₂O.
- So the product is water, which has the formula H₂O.