Molecule and Molecular Mass - UNSOLVED PRACTICE SET

Q7. Define a molecule. Differentiate between a molecule of an element and a molecule of a compound with one example of each.

Q8. Calculate the molecular mass of the following: (i) NH₃, (ii) CO₂, (iii) H₂SO₄, (iv) CaCO₃.
(Atomic masses: N = 14, H = 1, C = 12, O = 16, S = 32, Ca = 40)

Q9. Your friend says that O₂ and O₃ are both molecules of oxygen. Is he correct? Explain the difference between O₂ and O₃, and identify what O₃ is called.

Q10. What is a polyatomic ion? Give two examples. How is the molecular mass of a compound containing polyatomic ions calculated?

Q11. Calculate the molecular mass of glucose (C₆H₁₂O₆). How many atoms of each element are present in one molecule of glucose?

Q12. Why do elements like helium, neon, and argon exist as monoatomic molecules while oxygen, nitrogen, and hydrogen exist as diatomic molecules?

Q13. Define a molecule and explain the different types of molecules — monoatomic, diatomic, and polyatomic. Give two examples of each type. Calculate the molecular mass of nitrogen gas (N₂), water (H₂O), and calcium carbonate (CaCO₃), showing all steps clearly.

Q14. A student is confused about the difference between atomic mass and molecular mass. Help the student by:

(a) Defining both terms clearly.

(b) Explaining how molecular mass is calculated from atomic masses.

(c) Calculating the molecular mass of ammonium sulphate ((NH₄)₂SO₄).

(d) Explaining why molecular mass is important in stoichiometric calculations.

Q15. "Understanding molecules and their masses is essential in fields ranging from medicine to environmental science." Discuss this statement by giving three examples: (i) how molecular mass helps determine the dosage of medicines, (ii) how it is used to calculate the amount of pollutants in air, and (iii) how it helps in understanding nutrition labels on food packets.

Q16. The molecular masses of several compounds are to be calculated for a chemistry project:

(a) Calculate the molecular mass of ethanol (C₂H₅OH).

(b) Calculate the molecular mass of baking soda (NaHCO₃).

(c) Calculate the molecular mass of washing soda (Na₂CO₃·10H₂O).

(d) A medicine contains 500 mg of paracetamol (C₈H₉NO₂) per tablet. Calculate the molecular mass of paracetamol and express the mass of one tablet in atomic mass units.

(Atomic masses: C = 12, H = 1, O = 16, N = 14, Na = 23, S = 32)

Q17. In a school laboratory, students are given four unknown compounds and asked to determine their molecular masses:

Compound X: Contains 2 atoms of A and 3 atoms of B; atomic mass of A = 12 u, B = 16 u

Compound Y: Contains 1 atom of C, 4 atoms of D, and 1 atom of E; atomic mass of C = 14 u, D = 1 u, E = 16 u

Compound Z: Contains 1 atom of F and 2 atoms of G; atomic mass of F = 23 u, G = 35.5 u

Compound W: Contains 2 atoms of H, 1 atom of I, and 4 atoms of J; atomic mass of H = 1 u, I = 14 u, J = 16 u

(a) Calculate the molecular mass of each compound.

(b) Identify which real compounds these might represent.

(c) If a student has 10 g of Compound X, how many molecules does it contain? (Use Avogadro's number = 6.022 × 10²³)

Q18. A pharmaceutical company needs to calculate the exact molecular masses of several vitamins for their product labels:

Vitamin C (Ascorbic acid): C₆H₈O₆

Vitamin B12 (Cyanocobalamin): C₆₃H₈₈CoN₁₄O₁₄P

Vitamin D (Cholecalciferol): C₂₇H₄₄O

(a) Calculate the molecular mass of Vitamin C.

(b) Calculate the molecular mass of Vitamin D.

(c) For Vitamin B12, the company states the molecular mass is approximately 1355 u. Verify this by performing the calculation. (Atomic mass of Co = 59, P = 31)

(d) If a daily recommended dose of Vitamin C is 75 mg, how many molecules of Vitamin C does this represent?

(e) Why is it important for pharmaceutical companies to calculate molecular mass accurately?