Atomic Mass and Atomic Mass Unit - UNSOLVED PRACTICE SET

Q7. Define atomic mass unit (amu or u). Why was carbon-12 chosen as the standard for defining atomic mass?

Q8. The atomic mass of sodium is 23 u. What does this tell you about the mass of a sodium atom relative to a carbon-12 atom?

Q9. Your teacher tells you that the mass of a single hydrogen atom is approximately 1.66 × 10⁻²⁴ g. Using this information, calculate how many hydrogen atoms would be present in 1 gram of hydrogen.

Q10. Differentiate between atomic mass and actual mass of an atom. Why do chemists prefer to use atomic mass rather than actual mass in calculations?

Q11. The atomic mass of nitrogen is 14 u and that of oxygen is 16 u. Compare the masses of one nitrogen atom and one oxygen atom. How many nitrogen atoms would have the same total mass as 8 oxygen atoms?

Q12. Why can't we use a balance to measure the mass of a single atom? How does the concept of atomic mass unit solve this problem?

Q13. Explain the concept of atomic mass and atomic mass unit in detail. Why was the carbon-12 scale adopted as the universal standard? Compare the atomic masses of hydrogen (1 u), carbon (12 u), and oxygen (16 u) and explain what these values tell us about the relative masses of their atoms.

Q14. A student reads that the atomic mass of chlorine is 35.5 u. He wonders why it is not a whole number like 35 or 36.

(a) Explain why the atomic mass of chlorine is 35.5 u.

(b) What does this tell us about the existence of isotopes?

(c) If chlorine has two isotopes with masses 35 u and 37 u, and the average atomic mass is 35.5 u, calculate the relative abundance of each isotope.

(d) Why is the average atomic mass more useful than the mass of any single isotope?

Q15. "The atomic mass unit is one of the most important concepts in chemistry because it allows us to work with incredibly small masses on a practical scale." Discuss this statement by explaining: (i) why actual atomic masses are too small for practical use, (ii) how the amu scale makes chemical calculations manageable, (iii) the relationship between amu and grams, and (iv) how this concept connects to the mole concept.

Q16. The atomic masses of some elements are given below:

Table

Element Atomic Mass (u)

Hydrogen 1

Carbon 12

Nitrogen 14

Oxygen 16

Sodium 23

Magnesium 24

Sulphur 32

Chlorine 35.5

Calcium 40

(a) Arrange these elements in order of increasing atomic mass.

(b) Calculate how many times heavier a calcium atom is than a hydrogen atom.

(c) If the mass of a carbon-12 atom is 1.9926 × 10⁻²³ g, calculate the mass of one atomic mass unit in grams.

(d) Using your answer from (c), calculate the actual mass of one sodium atom in grams.

Q17. Boron has two naturally occurring isotopes: B-10 (mass = 10 u, abundance = 20%) and B-11 (mass = 11 u, abundance = 80%).

(a) Calculate the average atomic mass of boron.

(b) Verify whether your calculated value matches the atomic mass of boron given in the periodic table (10.8 u).

(c) If a sample contains 1000 boron atoms, how many atoms of B-10 and B-11 are present?

(d) Calculate the total mass of these 1000 atoms in atomic mass units.

Q18. A science museum has a display showing the relative sizes and masses of atoms. The display states:

Mass of one hydrogen atom = 1.673 × 10⁻²⁴ g

Mass of one carbon-12 atom = 1.993 × 10⁻²³ g

Mass of one oxygen atom = 2.657 × 10⁻²³ g

(a) Calculate the mass of one hydrogen atom in atomic mass units.

(b) Calculate the mass of one oxygen atom in atomic mass units.

(c) Compare your calculated values with the standard atomic masses (H = 1 u, O = 16 u). Are they approximately the same?

(d) If the museum wants to show a model where 1 cm represents 1 u, what would be the diameter of a carbon atom model if the actual diameter of a carbon atom is 1.54 × 10⁻¹⁰ m? (Note: This is a conceptual question about scale, not a direct calculation)