Molecular Mass and Formula Unit Mass – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Define molecular mass. Show the step-by-step calculation for the molecular masses of H₂O and HNO₃. Why do we add atomic masses?
Answer:
- Molecular mass is the sum of atomic masses of all atoms in a molecule.
- We add atomic masses because a molecule is made of many atoms joined together.
- For H₂O: H = 1 u, O = 16 u. So, 2 × 1 + 1 × 16 = 18 u.
- For HNO₃: H = 1 u, N = 14 u, O = 16 u. So, 1 + 14 + 3 × 16 = 63 u.
- The unit used is u (atomic mass unit).
- This helps us compare how heavy molecules are and do chemical calculations.
Q2. Calculate the molecular masses of CH₄, NH₃, and CH₃OH. Explain what you notice about the role of hydrogen atoms.
Answer:
- For CH₄: C = 12 u, H = 1 u. So, 12 + 4 × 1 = 16 u.
- For NH₃: N = 14 u, H = 1 u. So, 14 + 3 × 1 = 17 u.
- For CH₃OH (methanol): C = 12, H = 1, O = 16. So, 12 + 4 × 1 + 16 = 32 u.
- Hydrogen adds smaller amounts because its atomic mass is 1 u.
- Even many H atoms add only a small increase compared to O or C.
- This shows heavy atoms like Oxygen (16 u) affect mass more than hydrogen.
Q3. What is formula unit mass? How is it different from molecular mass? Use CaCl₂ to explain with steps.
Answer:
- Formula unit mass is the sum of atomic masses in the simplest ratio of ions.
- It is used for ionic compounds like salts where molecules do not exist as single units.
- Molecular mass is for molecules (covalent compounds) like H₂O, CO₂, etc.
- For CaCl₂: Ca = 40 u, Cl = 35.5 u. So, 40 + 2 × 35.5 = 111 u.
- We call it “formula unit” because CaCl₂ is the simplest ratio (1 Ca : 2 Cl).
- Use formula unit mass for ionic solids; use molecular mass for covalent substances.
Q4. Find the formula unit masses of ZnO, Na₂O, and K₂CO₃. Explain how subscripts guide the calculation.
Answer:
- For ZnO: Zn = 65 u, O = 16 u. So, 65 + 16 = 81 u.
- For Na₂O: Na = 23 u, O = 16 u. So, 2 × 23 + 16 = 62 u.
- For K₂CO₃: K = 39 u, C = 12 u, O = 16 u. So, 2 × 39 + 12 + 3 × 16 = 138 u.
- The subscripts show how many atoms of each element are present.
- Multiply each atomic mass by its subscript before adding.
- This method avoids missing atoms and gives the correct mass.
Q5. How do you calculate the molecular mass of diatomic molecules like H₂, O₂, and Cl₂? Compare their values and explain the reason for differences.
Answer:
- Diatomic molecules have two atoms of the same element.
- For H₂: 2 × 1 = 2 u.
- For O₂: 2 × 16 = 32 u.
- For Cl₂: 2 × 35.5 = 71 u.
- The difference comes from different atomic masses of H, O, and Cl.
- Heavier atoms like Cl (35.5 u) make a much heavier molecule.
High Complexity (Analysis & Scenario-Based)
Q6. A student treats NaCl as a molecule and tries to find its “molecular mass.” Explain why this is wrong. Show the correct way and its impact on calculations.
Answer:
- NaCl is an ionic compound, not a molecule.
- It exists as a lattice of ions, so we use formula unit mass, not molecular mass.
- Correct mass: Na = 23 u, Cl = 35.5 u. So, 23 + 35.5 = 58.5 u.
- Thinking it is a molecule may lead to wrong concepts and wrong terms.
- In reactions and problems, using the wrong idea may cause calculation errors.
- Always check if a substance is ionic or molecular before finding its mass.
Q7. Two gases, CO₂ and N₂, are compared. Which has a higher molecular mass, and by how much? Explain your reasoning clearly.
Answer:
- For CO₂: C = 12 u, O = 16 u. So, 12 + 2 × 16 = 44 u.
- For N₂: N = 14 u. So, 2 × 14 = 28 u.
- CO₂ is heavier than N₂.
- Difference = 44 u − 28 u = 16 u.
- The extra mass comes from oxygen atoms in CO₂.
- So, per molecule, CO₂ has a higher molecular mass by 16 u.
Q8. A learner calculates sugar’s mass (C₁₂H₂₂O₁₁) as 12 + 22 + 11 + 16 = 61 u. Point out the mistakes. Show the correct method and rules to remember.
Answer:
- The mistake is ignoring the multiplication by subscripts.
- They also added only one oxygen mass (16) instead of 11 × 16.
- Correct method: C = 12 u, H = 1 u, O = 16 u.
- So, 12 × 12 + 22 × 1 + 11 × 16 = 144 + 22 + 176 = 342 u.
- Rule: multiply each atomic mass by its subscript first, then add.
- Always write the full expression to avoid missing factors.
Q9. You have 1 molecule of H₂O and 1 formula unit of CaCl₂. Find the total mass in u. Explain how you added masses from different types of compounds.
Answer:
- For H₂O: 2 × 1 + 16 = 18 u (molecular mass).
- For CaCl₂: 40 + 2 × 35.5 = 111 u (formula unit mass).
- Total mass = 18 u + 111 u = 129 u.
- We can add them because both are in atomic mass units (u).
- The type (molecular vs ionic) changes the term we use, not the unit.
- Be clear: use molecular mass for H₂O and formula unit mass for CaCl₂.
Q10. A compound X has the formula M₂O. The atomic mass of M is 23 u. Identify X by calculating its formula unit mass and comparing with a known compound.
Answer:
- For M₂O: mass = 2 × 23 + 16 = 62 u.
- Compare with Na₂O: Na = 23 u, O = 16 u. So, 2 × 23 + 16 = 62 u.
- The values match. So, M is likely Sodium (Na).
- Therefore, compound X = Na₂O.
- This method uses formula pattern and atomic mass to identify substances.
- It shows how mass calculation helps in recognition of compounds.