Atomic and Molecular Masses, Define 1 amu: Q&A, Examples, Formulas / JEE & NEET CBSE

ANAND CLASSES Study Material and Notes to learn Class 11 Chemistry topic “Atomic and Molecular Masses” in a detailed question-answer format. Includes formulas, solved problems, and conceptual clarity for JEE, NEET, and CBSE.

🧪 Atomic and Molecular Masses – Q&A

🔹 Atomic Mass

Q1. What is atomic mass? Explain with the modern definition.

Ans:
Atomic mass is the mass of a single atom of an element, expressed in atomic mass units (u). It is extremely small because atoms themselves are very small.

In the modern definition, atomic mass is:

The mass of one atom expressed relative to one-twelfth of the mass of one carbon-12 (¹²C) atom.

Mathematically, $$1\,\text{u} = \frac{1}{12} \times \text{mass of one ¹²C atom} = 1.66056 \times 10^{-24} \, \text{g}$$

Q2. What was the older method of determining atomic mass?

Ans:
Earlier, hydrogen was considered the lightest atom and was arbitrarily assigned a mass of 1. Other atomic masses were then determined relatively, compared to hydrogen.

However, this method was replaced in 1961 with the carbon-12 scale.

Q3. Why is carbon-12 used as the standard reference for atomic masses?

Ans:
Carbon-12 is:

• A stable isotope of carbon.
• Universally available and measurable.
• Easy to use as a standard reference.

So, one atom of ¹²C is assigned exactly 12 u, and other atomic masses are calculated relative to it.

Q4. What is the atomic mass of hydrogen in amu? Show the calculation.

Ans:
Mass of hydrogen atom = $1.6736 \times 10^{-24} g$
1 atomic mass unit (u) = $1.66056 \times 10^{-24} g $

$$\text{Atomic mass of hydrogen} = \frac{1.6736 \times 10^{-24}}{1.66056 \times 10^{-24}} = 1.0078 \, \text{u} \approx \boxed{1.008 \, \text{u}}$$

🔹 Average Atomic Mass

Q5. Why do we use average atomic mass instead of a fixed atomic mass for elements?

Ans:
Most naturally occurring elements exist as mixtures of isotopes (atoms with the same atomic number but different mass numbers).

Hence, the atomic mass shown in the periodic table is the weighted average of all isotopes based on their abundance.

Q6. How is average atomic mass calculated? Give an example.

Ans:
Formula: $$\text{Average Atomic Mass} = \sum (\text{Isotopic Mass} \times \text{Fractional Abundance})$$

Example – Carbon Isotopes:

$$\text{Average atomic mass} = $$

$$=(0.98892 \times 12) + (0.01108 \times 13.00335) + (2 \times 10^{-12} \times 14.00317) $$

$$= 11.86704 + 0.14416 + \text{(negligible)} = \boxed{12.011 \, \text{u}}$$

🔹 Molecular Mass

Q7. What is molecular mass? How is it calculated?

Ans:
Molecular mass is the sum of atomic masses of all atoms in a molecule. $$\text{Molecular mass} = \sum (\text{Atomic mass of element} \times \text{Number of atoms})$$

Q8. Calculate the molecular mass of methane (CH₄).

Ans:

• Atomic mass of C = 12.011 u
• Atomic mass of H = 1.008 u

$$\text{Molecular mass of CH₄} =$$

$$= 1 \times 12.011 + 4 \times 1.008 \\ = 12.011 + 4.032 = \boxed{16.043 \, \text{u}}$$

Q9. Calculate the molecular mass of water (H₂O).

Ans:

• H = 1.008 u
• O = 16.00 u

Molecular mass=2×1.008+1×16.00=2.016+16.00=18.016 u

Q10. Problem: Calculate the molecular mass of glucose (C₆H₁₂O₆).

Ans:

• C = 12.011 u
• H = 1.008 u
• O = 16.00 u

Molecular Mass = 6×12.011+12×1.008+6×16.00=72.066+12.096+96.00=180.162 u

🔹 Formula Mass

Q11. What is formula mass and how is it different from molecular mass?

Ans:
Formula mass is used for ionic compounds which do not exist as molecules. Instead, they exist as 3D lattices of ions (e.g., NaCl).

Since these don’t have discrete molecules, we calculate mass using the simplest ratio of ions in the formula unit.

Q12. Calculate the formula mass of sodium chloride (NaCl).

Ans:

• Na = 23.0 u
• Cl = 35.5 u

Formula mass of NaCl=23.0+35.5=58.5 u

🧠 Do You Know?

• The term “amu” (atomic mass unit) is now replaced with “u” (unified mass unit).
• Elements with only one isotope (e.g., fluorine) have atomic mass equal to isotopic mass.
• ¹²C was chosen as a standard instead of hydrogen to avoid inaccuracies in gaseous measurements.

📘 FAQs – Quick Recap

🔹 MCQs on Atomic and Molecular Masses

Q1. Which of the following is the correct value of 1 atomic mass unit (amu) in grams?

A) 1.0078 × 10⁻²⁴ g
B) 1.6736 × 10⁻²⁴ g
C) 1.66056 × 10⁻²⁴ g
D) 12.0000 × 10⁻²⁴ g

Correct Answer: C
Explanation:
By definition, 1 atomic mass unit (amu or u) = 1/12th the mass of a carbon-12 atom
So, 1 u = 1.66056 × 10⁻²⁴ g

Q2. The average atomic mass of chlorine is 35.5 u. Which of the following statements is correct?

A) All chlorine atoms have a mass of 35.5 u
B) Chlorine has isotopes with atomic masses 35 u and 36 u
C) Chlorine has two main isotopes, and 35.5 u is their average
D) Chlorine exists only as ³⁵Cl isotope

Correct Answer: C
Explanation:
Chlorine has two isotopes:

• ³⁵Cl (abundance ~75%)
• ³⁷Cl (abundance ~25%)

So, average atomic mass = (0.75 × 35) + (0.25 × 37) = 35.5 u

Q3. What is the molecular mass of CO₂? (Atomic mass: C = 12.01 u, O = 16.00 u)

A) 44.00 u
B) 28.01 u
C) 32.00 u
D) 45.99 u

Correct Answer: A
Explanation:
CO₂ has 1 carbon and 2 oxygen atoms.
Molecular mass = (1 × 12.01) + (2 × 16.00)
= 12.01 + 32.00 = 44.01 u (approx 44.00 u)

Q4. Which of the following substances is best described using formula mass instead of molecular mass?

A) Water (H₂O)
B) Oxygen (O₂)
C) Sodium chloride (NaCl)
D) Methane (CH₄)

Correct Answer: C
Explanation:
NaCl is an ionic compound, not a discrete molecule. Hence, we calculate its formula mass, not molecular mass.
Formula mass of NaCl = 23.0 + 35.5 = 58.5 u

Q5. Which isotope is used as a standard for defining the atomic mass unit?

A) ¹H
B) ¹⁴N
C) ¹²C
D) ³⁵Cl

Correct Answer: C
Explanation:
The modern atomic mass scale is based on the carbon-12 (¹²C) isotope.
1 atomic mass unit (u) is defined as 1/12th the mass of one carbon-12 atom.

Q6. Which of the following is the correct molecular mass of ammonia (NH₃)?

(Atomic mass: N = 14.01 u, H = 1.008 u)

A) 15.024 u
B) 16.032 u
C) 17.034 u
D) 18.038 u

Correct Answer: C
Explanation:
Molecular mass of NH₃ = (1 × 14.01) + (3 × 1.008)
= 14.01 + 3.024 = 17.034 u

Q7. An element has two isotopes: ²⁰X (abundance = 75%) and ²²X (abundance = 25%). What is the average atomic mass of element X?

A) 21.0 u
B) 20.5 u
C) 22.0 u
D) 20.0 u

Correct Answer: A
Explanation:
Average atomic mass = (0.75 × 20) + (0.25 × 22)
= 15.0 + 5.5 = 20.5 u

Q8. Which of the following correctly represents the definition of 1 u (atomic mass unit)?

A) Mass of 1 atom of hydrogen
B) 1/16th the mass of an oxygen atom
C) 1/12th the mass of a hydrogen atom
D) 1/12th the mass of a carbon-12 atom

Correct Answer: D
Explanation:
By modern definition, 1 u = 1/12th the mass of one atom of carbon-12 (¹²C).

Q9. Which of the following compounds has the highest molecular mass?

A) Water (H₂O)
B) Methane (CH₄)
C) Ammonia (NH₃)
D) Glucose (C₆H₁₂O₆)

Correct Answer: D
Explanation:

• H₂O = 18.016 u
• CH₄ = 16.043 u
• NH₃ = 17.034 u
• C₆H₁₂O₆ = 180.162 u → Highest

Q10. The molecular mass of calcium hydroxide Ca(OH)₂ is:

(Atomic mass: Ca = 40.08 u, O = 16.00 u, H = 1.008 u)

A) 56.00 u
B) 58.10 u
C) 74.096 u
D) 80.08 u

Correct Answer: C
Explanation:
Molecular mass = 1 × 40.08 + 2 × (16.00 + 1.008)
= 40.08 + 2 × 17.008
= 40.08 + 34.016 = 74.096 u

Q11. Which of the following terms is not associated with molecular compounds?

A) Molecular mass
B) Molecule
C) Formula unit
D) Atomic mass

Correct Answer: C
Explanation:
Formula unit is used for ionic compounds, not molecular compounds. Molecular compounds use terms like molecular mass and molecule.

Q12. What is the formula mass of magnesium chloride (MgCl₂)?

(Atomic mass: Mg = 24.31 u, Cl = 35.45 u)

A) 59.76 u
B) 70.21 u
C) 95.21 u
D) 97.31 u

Correct Answer: C
Explanation:
Formula mass = 24.31 + 2 × 35.45 = 24.31 + 70.90 = 95.21 u