CAPF AC 2020 Q24: Science & Technology – Chemistry
Which one of the following is the average atomic mass of chlorine atom?
- (a) 35.9 u
- (b) 35.5 u
- (c) 35.0 u
- (d) 37.0 u
Answer: (b) 35.5 u
Explanation
Average atomic mass of chlorine
How to think: chlorine has two isotopes.
The average must lie between 35 and 37, and closer to the one that is more common.
| Isotope | Mass | Share in nature |
|---|---|---|
| Chlorine-35 | 35 u | About 75% (3 parts) |
| Chlorine-37 | 37 u | About 25% (1 part) |
Working:
| Step | Calculation |
|---|---|
| Chlorine-35 part | 35 × 75/100 = 26.25 |
| Chlorine-37 part | 37 × 25/100 = 9.25 |
| Average | 26.25 + 9.25 = 35.5 u |
Educated guess: 35.0 and 37.0 are the single isotopes, so they cannot be the average.
35.9 is too far from 35 for a 3 : 1 mix.
Answer: (b) 35.5 u.
Why each option is right or wrong
- (a) Incorrect. 35.9 u would need almost equal amounts of the two isotopes.
- (b) CORRECT. A 3 : 1 mix of chlorine-35 and chlorine-37 gives 35.5 u.
- (c) Incorrect. 35.0 u is the mass of one isotope, not the average.
- (d) Incorrect. 37.0 u is the mass of the heavier, less common isotope.
Key point
Chlorine: isotopes 35 and 37 in a 3 : 1 ratio; average atomic mass = (35 × 3 + 37 × 1) / 4 = 35.5 u.
Remember
Chlorine = 35.5 u, from chlorine-35 and chlorine-37 in 3 : 1.
Common mistakes
- Taking a simple average of 35 and 37 and looking for 36.
- Forgetting that atomic mass is a weighted average of the isotopes.
- Thinking a fractional mass means an atom has a fraction of a neutron.
Exam tip
Weighted average sits nearer the common isotope. With 75% of 35, the answer must be near 35.
Sources
- NCERT Class IX Science — Structure of the Atom (isotopes)
- NCERT Class XI Chemistry — Some Basic Concepts of Chemistry (average atomic mass)