Atomic and Molecular Mass - UNSOLVED PRACTICE SET
Chapter: Some Basic Concepts of Chemistry | Topic: Atomic and Molecular Mass
ATOMIC AND MOLECULAR MASS - UNSOLVED PRACTICE SET
Topic: Atomic and Molecular Mass
Multiple Choice Questions
Q1. The atomic mass unit (amu) is defined as:
- 1/12th the mass of a hydrogen atom
- 1/12th the mass of a carbon-12 atom
- The mass of one proton
- The mass of one electron
Q2. The molecular mass of water (H₂O) is:
- 16 u
- 17 u
- 18 u
- 20 u
Q3. One atomic mass unit is equal to:
- 1.66 × 10⁻²⁴ g
- 1.66 × 10⁻²³ g
- 1.66 × 10⁻²² g
- 1.66 × 10⁻²¹ g
Q4. The molecular mass of CO₂ is:
- 28 u
- 32 u
- 44 u
- 48 u
Q5. Which of the following has the largest molecular mass?
- H₂O
- CO₂
- NH₃
- CH₄
Q6. The formula unit mass is used for:
- Molecular compounds only
- Ionic compounds
- Gaseous elements
- Liquid mixtures
Short Answer Questions
Q7. Define atomic mass. Why is it called relative atomic mass?
Q8. Differentiate between atomic mass and molecular mass with examples.
Q9. What is formula unit mass? Calculate the formula unit mass of NaCl. (Atomic masses: Na = 23 u, Cl = 35.5 u)
Q10. Why is carbon-12 chosen as the standard for atomic mass instead of hydrogen or oxygen?
Q11. Your mother buys 1 kg of sugar (sucrose, C₁₂H₂₂O₁₁) from the market. If you know the molecular mass of sucrose, can you calculate how many molecules of sugar are in that 1 kg packet? What additional concept would you need?
Q12. The atomic mass of chlorine is 35.5 u, but no single chlorine atom has this exact mass. Explain this apparent contradiction.
Long Answer Questions
Q13. Explain the concept of atomic mass and molecular mass. How is atomic mass different from mass number? Describe how the atomic mass of an element is calculated when it exists as a mixture of isotopes. Illustrate with the example of chlorine.
Q14. Define gram atomic mass and gram molecular mass. How are they related to the mole concept? Calculate the gram molecular mass of:
(a) Sulphuric acid (H₂SO₄)
(b) Calcium carbonate (CaCO₃)
(c) Glucose (C₆H₁₂O₆)
(Atomic masses: H = 1 u, C = 12 u, N = 14 u, O = 16 u, S = 32 u, Ca = 40 u)
Q15. A chemistry laboratory receives three samples:
Sample A: 10 g of sodium chloride (NaCl)
Sample B: 10 g of water (H₂O)
Sample C: 10 g of calcium carbonate (CaCO₃)
(a) Calculate the molecular/formula unit mass of each compound.
(b) Which sample contains the largest number of formula units? Explain your reasoning without calculating the exact number.
(c) If you had to prepare 1 mole of each compound in the lab, which would require the largest mass? Calculate the mass needed for each.
(d) Why is it important for chemists to know both molecular mass and molar mass?
Numerical / Application-Based Problems
Q16. Calculate the molecular masses of the following compounds:
(a) Ethanol (C₂H₅OH)
(b) Sulphuric acid (H₂SO₄)
(c) Urea (CO(NH₂)₂)
(Atomic masses: C = 12 u, H = 1 u, O = 16 u, N = 14 u, S = 32 u)
Q17. Chlorine exists as two isotopes: Cl-35 (75% abundance) and Cl-37 (25% abundance).
(a) Calculate the average atomic mass of chlorine.
(b) If a sample contains 1000 chlorine atoms, how many atoms of each isotope are present?
(c) Why does the average atomic mass (35.5 u) not match either isotope's mass?
Q18. A compound has the formula C₆H₁₂O₆ (glucose).
(a) Calculate its molecular mass.
(b) Calculate the percentage by mass of each element in glucose.
(c) If you have 180 g of glucose, how many moles is this? How many molecules does it contain? (Avogadro's number = 6.022 × 10²³)