Half-life of a radioactive isotope is 5 years. What percentage remains after 15 years? MCQ with Answer and Explanation

Half-life of a radioactive isotope is 5 years. What percentage remains after 15 years?
A. 6.25%
B. 50%
C. 12.5%
D. 25%
Answer: Option C
Solution (By JKSSB Mock Tests)
15/5 = 3 half-lives. Remaining = (1/2)³ = 1/8 = 12.5%. Each half-life reduces the amount by half.

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Question #1
The acid used in the pickling of metals to remove oxide layers is:
A. Hydrochloric acid
B. Nitric acid
C. Carbonic acid
D. Acetic acid

Correct Answer: Option A


Explanation:
Hydrochloric acid (HCl) is commonly used for pickling (cleaning) steel to remove rust and scale (iron oxides) before further processing like galvanizing or plating. Reaction: Fe₂O₃ + 6HCl → 2FeCl₃ + 3H₂O. Sulfuric acid is also used. Nitric acid is an oxidizer and would passivate some metals. Acetic acid is too weak.

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Question #2
The process of converting unsaturated hydrocarbons into saturated hydrocarbons by adding hydrogen is called:
A. Polymerization
B. Hydrogenation
C. Substitution
D. Halogenation

Correct Answer: Option B


Explanation:
Hydrogenation is the addition of hydrogen to unsaturated compounds (like alkenes or alkynes) in the presence of a catalyst (like Ni, Pd, or Pt) to form saturated compounds (alkanes). For example, ethene + H₂ → ethane. This process is widely used in the food industry to solidify vegetable oils.

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Question #3
Which acid is used in the manufacture of explosives like TNT and nitroglycerin?
A. HCl
B. H₂SO₄
C. CH₃COOH
D. HNO₃

Correct Answer: Option D


Explanation:
Nitric acid (HNO₃) is used for nitration reactions to produce explosives such as TNT (trinitrotoluene), nitroglycerin, and dynamite. Sulfuric acid acts as a catalyst/dehydrating agent in nitration mixtures (mixed acid). HCl is used for cleaning and chemical synthesis. Acetic acid is organic acid. Nitric acid provides nitro groups (-NO₂).

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