Exams › NEET › Chemistry

One microgram of radioactive sodium 24Na11 with a half-life of 15 hours was injected into a living system for a bio-assay. How long will it take for the radioactive substance to fall up to 25% of the initial value?

1. 60 hours
2. 22.5 hours
3. 375 hours
4. 30 hours

Correct answer: 30 hours

Solution

The substance falls to 25% of its initial value in two half-lives because 50% remains after one half-life, and 25% remains after the second. Since the half-life is 15 hours, the total time is 15 × 2 = 30 hours.

Related NEET Chemistry questions

• Why is catalyst added while preparing oxygen from decomposition of \( K C l O_{3} ? \)
• In the Arrhenius equation \( k=A e^{E / R T} \) rate will be constant at:
• As the concentration of reactants increases:
• Assertion The rate of reaction can also increase w.r.t its product if one of the product act as catalyst. Reason A catalyst lowers the activation energy of reactions. Read the above assertion and reason and choose the correct option regarding ¡t.
• Assertion Chlorine reacts more rapidly with \( \boldsymbol{H}_{2}, \) in comparision to \( \boldsymbol{O}_{\mathbf{2}} \) Reason \( D-C l \) bond is stronger in comparison to \( H-C l \) bond
• In the Haber process for the industrial manufacturing of ammonia involving the reaction \( N_{2}(g)+3 H_{2}(g) \rightleftharpoons 2 N H_{3}(g) \) at 200atm pressure in the presence of a catalyst, a temperature of about \( 500^{\circ} \mathrm{C} \) is used. This is considered as optimum temperature for the process because:

⚔️ Practice NEET Chemistry free + battle 1v1 →