- Keneitsino Lydia
- March 11, 2026
Best Radioactive Decay Time Problems MCQs Class 12 to Master Exam Concepts Fast
The study of nuclear decay focuses on how radioactive substances transform into more stable elements by emitting radiation such as alpha particles, beta particles, or gamma rays. These transformations occur spontaneously and follow a predictable mathematical pattern. Because radioactive decay Time problems MCQs class 12 follows first-order kinetics, the rate of decay depends directly on the amount of radioactive substance present at any given time. This fundamental idea is often explored through Radioactive Decay Time Problems MCQs Class 12, which help aspirants apply the mathematical relationships between decay rate and time.
The rate law for radioactive decay Time problems MCQs class 12 can be written using the first-order rate equation. This equation explains how the number of radioactive nuclei decreases exponentially with time. Understanding this concept allows aspirants to calculate the time required for a certain fraction of a radioactive sample to decay. Many exam questions framed as Radioactive Decay Time Problems MCQs Class 12 test this relationship by asking aspirants to determine the remaining mass or time required for decay.
A key concept in radioactive decay Time problems MCQs class 12 studies is half-life. The half-life of a radioactive substance is defined as the time required for half of the radioactive nuclei in a sample to decay. One important feature of half-life is that it remains constant regardless of the initial amount of the substance. Because of this predictable property, radioactive decay calculations become easier to perform. Many aspirants strengthen their understanding of this idea by solving Radioactive Decay Time Problems MCQs Class 12, where they practice determining remaining quantities after multiple half-lives.
Half-life calculations also help explain how radioactive materials disappear gradually over time. For example, after one half-life, half of the original sample remains. After two half-lives, only one-quarter remains, and after three half-lives, one-eighth remains. This exponential reduction continues until the material becomes almost negligible. These decay patterns are frequently tested through Radioactive Decay Time Problems MCQs Class 12, making them an important practice area for aspirants.
The mathematical expression for radioactive decay Time problems MCQs class 12 is closely related to the first-order integrated rate equation used in chemical kinetics. This equation helps determine the time required for a specific fraction of a radioactive sample to decay.
N=N0e−ktN = N_0 e^{-kt}
Here, N0N_0 represents the initial number of radioactive nuclei, NN is the number remaining after time tt, and kk is the decay constant. When aspirants solve Radioactive Decay Time Problems MCQs Class 12, they frequently apply this equation to determine unknown quantities such as decay constant, remaining mass, or decay time.
Another important parameter used in radioactive decay calculations is the decay constant. The decay constant represents the probability of a nucleus decaying per unit time. A higher decay constant indicates a faster decay process. Understanding how the decay constant relates to half-life is essential for solving numerical problems. Aspirants often practice these relationships through Radioactive Decay Time Problems MCQs Class 12, which include problems connecting half-life, decay constant, and remaining mass.
Radioactive decay time problems MCQs class 12 calculations are not only important for examinations but also have practical applications in many scientific fields. For instance, radiocarbon dating uses radioactive decay to determine the age of ancient biological materials. Nuclear medicine uses radioactive isotopes to diagnose and treat diseases. Similarly, nuclear power plants rely on controlled nuclear reactions to generate electricity. Understanding these applications becomes easier when aspirants regularly practice Radioactive Decay Time Problems MCQs Class 12, which reinforce the underlying scientific principles.
Another advantage of practicing Radioactive Decay Time Problems MCQs Class 12 is that it improves problem-solving speed and accuracy. Many exam questions require quick calculations involving exponential decay or repeated half-life reductions. Regular practice helps aspirants recognize patterns and solve these problems efficiently. By working through Radioactive Decay Time Problems MCQs Class 12, aspirants develop confidence in applying mathematical formulas to real nuclear chemistry scenarios.
In addition, conceptual clarity is greatly enhanced when aspirants repeatedly analyze problems involving radioactive decay. Questions framed as Radioactive Decay Time Problems MCQs Class 12 often test multiple ideas simultaneously, such as half-life, decay constant, and exponential decay behavior. Practicing these problems strengthens analytical thinking and ensures that aspirants understand the topic from both theoretical and numerical perspectives.
30 Radioactive Decay Time Problems MCQs Class 12:
Q1.
The rate equation for a reaction A→BA \rightarrow B is r=k[A]0r = k[A]^0. If the initial concentration of the reactant is a mol dm−3a\; mol\,dm^{-3}, the half-life period of the reaction is:
A. a/2ka/2k
B. aa
C. a/ka/k
D. 2a2a
Answer: A
Q2.
Half-lives of a first order and a zero order reactions are same. Then the ratio of the initial rate of first order reaction to that of zero order reaction is:
A. 1/0.6931/0.693
B. 2×0.6932 \times 0.693
C. 0.6930.693
D. 22
Answer: B
Q3.
The rate of a first order reaction is 1.5×10−2 mol L−1 min−11.5 \times 10^{-2}\; mol\,L^{-1}\,min^{-1} at 0.5 M concentration of the reactant. The half-life of the reaction is:
A. 0.383 min
B. 23.1 min
C. 8.73 min
D. 75.3 min
Answer: B
Q4.
Which condition among the following holds true at the stage of half-completion for the reaction A⇌BA \rightleftharpoons B?
A. ΔG°<0ΔG° < 0
B. ΔG°>0ΔG° > 0
C. ΔG°=0ΔG° = 0
D. ΔG°≠0ΔG° ≠ 0
Answer: B
Q5.
A and B decompose via first order kinetics with half-lives 54 min and 18 min respectively. Starting from an equimolar mixture, the time taken for the concentration of A to become 16 times that of B is:
A. 4.5 min
B. 2.25 min
C. 100 min
D. 108 min
Answer: D
Q6.
If the rate constant for a first order reaction is kk, then the time required for completion of 80% of the reaction is:
A. 3.2/k3.2/k
B. 1.6/k1.6/k
C. 4.8/k4.8/k
D. 0.8/k0.8/k
Answer: B
Q7.
For a reaction, the initial rate is R0=k[A0]2[B0]R_0 = k[A_0]^2[B_0]. By what factor will the rate increase if A becomes 1.5 times and B becomes 3 times?
A. 4.5
B. 3
C. 6.75
D. 2
Answer: C
Q8.
The half-life of a first order reaction is 60 minutes. If initial reactant amount is 50 g, the amount left after 4 hours is:
A. 6.25 g
B. 12.5 g
C. 3.125 g
D. 1.25 g
Answer: C
Q9.
For the elementary reaction X→Y+ZX \rightarrow Y + Z, the half-life is 10 minutes. Time required for concentration to reduce to 10% is:
A. 20 min
B. 33.2 min
C. 15 min
D. 25.2 min
Answer: B
Q10.
Sucrose hydrolyses in acid solution following first order kinetics with half-life 3.33 h. After 9 h, value of log10(1/f)log_{10}(1/f) is:
A. 81 ×10⁻²
B. 90 ×10⁻²
C. 9 ×10⁻²
D. 8 ×10⁻²
Answer: A
Q11.
A reaction has half-life of 1 minute. Time required for 99.9% completion is:
A. 10 min
B. 11 min
C. 13 min
D. 12 min
Answer: A
Q12.
For the first order reaction A→2BA \rightarrow 2B, 1 mole of A gives 0.2 mole of B after 100 min. The half-life is:
A. 693 min
B. 700 min
C. 793 min
D. 600 min
Answer: A
Q13.
Half-life of a substance is 36 minutes. If initial amount is 10 g, amount left after 2 hours is:
A. 1 g
B. 2 g
C. 3 g
D. 4 g
Answer: A
Q14.
Time taken for 12.8 g of radioactive substance to decay to 0.4 g (half-life = 138 s) is:
A. 720 s
B. 690 s
C. 245 s
D. 69 s
Answer: B
Q15.
For a first order reaction, the rate decreases from 0.04 to 0.01 between 10 min and 30 min. The half-life is:
A. 4 min
B. 8 min
C. 6 min
D. 2 min
Answer: C
Q16.
If a first order reaction is 80% complete in 60 minutes, the half-life is:
A. 16 min
B. 42 min
C. 25.85 min
D. 14.28 min
Answer: C
Q17.
In a concentration vs time graph for reaction A→BA → B, the point where curves intersect represents:
A. t1/2t_{1/2}
B. t1/4t_{1/4}
C. t2/3t_{2/3}
D. None
Answer: A
Q18.
When initial concentration doubles, the half-life of a zero-order reaction:
A. Is halved
B. Is doubled
C. Remains same
D. Tripled
Answer: B
Q19.
For first order decomposition of CH₃CHO, if initial pressure is 80 mm Hg and total pressure after 20 min is 120 mm Hg, half-life is:
A. 80 min
B. 20 min
C. 40 min
D. 10 min
Answer: B
Q20.
20% of a first order reaction completed at 10 AM and 20% remaining at 11:30 AM. Half-life is:
A. 90 min
B. 60 min
C. 45 min
D. 30 min
Answer: C
Q21.
Half-life of radioactive substance is 15 min. Amount decayed from 50 g after 1 hour is:
A. 37.5 g
B. 28 g
C. 46.875 g
D. 25 g
Answer: C
Q22.
The two-third life of first order reaction with k=5.48×10−14s−1k = 5.48 ×10^{-14} s^{-1} is:
A. 2.3035.48×10−14log3\frac{2.303}{5.48×10^{-14}} \log3
B. 2.3035.48×10−14log2\frac{2.303}{5.48×10^{-14}} \log2
C. 2.3035.48×10−14log(1/3)\frac{2.303}{5.48×10^{-14}} \log(1/3)
D. 2.3035.48×10−14log(1/2)\frac{2.303}{5.48×10^{-14}} \log(1/2)
Answer: A
Q23.
For first order reaction with rate constant 7×10−4s−17×10^{-4}s^{-1}, the half-life is:
A. 990 s
B. 79.2 s
C. 1237 s
D. 1.01×1051.01×10^5 s
Answer: A
Q24.
For reaction A→ProductsA → Products, half-life doubles when concentration halves. Order is:
A. Zero
B. One
C. Two
D. 0.5
Answer: C
Q25.
Rate constant = 4×10−3molL−1min−14×10^{-3} mol L^{-1} min^{-1}. Initial concentration = 2×10−22×10^{-2}. Half-life (seconds) is:
A. 300
B. 150
C. 180
D. 240
Answer: B
Q26.
Half-life of first order reaction = 60 min. Percentage left after 240 min is:
A. 6.25%
B. 4.25%
C. 5%
D. 6%
Answer: A
Q27.
Relation between half-life and rate constant for first order reaction is:
A. t1/2=0.693/kt_{1/2} = 0.693/k
B. t1/2=0.693kt_{1/2} = 0.693k
C. t1/2=kt_{1/2} = k
D. None
Answer: A
Q28.
A first order reaction is one-fifth completed in 40 minutes. Time for 100% completion is:
A. 100 min
B. 200 min
C. 350 min
D. Infinity
Answer: D
Q29.
If 90% completion time = t, then time for 99% completion is:
A. 2t
B. 3t
C. 4t
D. t
Answer: A
Q30.
Half-life of first order reaction = 10 min. Initial concentration = 12 M. Rate after 20 min is:
A. 0.0693 M min⁻¹
B. 0.693 M min⁻¹
C. 0.0693×30.0693 × 3 M min⁻¹
D. 0.0693×40.0693 × 4 M min⁻¹
Answer: C
Conclusion on Radioactive Decay Time Problems MCQs Class 12
In conclusion, radioactive decay is a fundamental concept that combines principles of nuclear chemistry and chemical kinetics. Mastering decay equations, half-life calculations, and decay constants is essential for success in chemistry examinations. Regular practice through Radioactive Decay Time Problems MCQs Class 12 enables aspirants to build strong conceptual understanding and improve numerical problem-solving skills. By consistently working on Radioactive Decay Time Problems MCQs Class 12, aspirants can develop confidence in solving complex decay calculations and perform well in both board and competitive examinations.