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WAEC Physics 2021 Paper

Question : 38

Total: 50

Solution:

Concept:

In one half-life, half of the radioactive material decays. The remaining mass after

n

half-lives is given by initial mass

\times \left(\frac{1}{2}\right)^n

Explanation:

From 6 a.m. to 9 a.m. is

3

hours.

The half-life is

1.0

hour, so the number of half-lives is:

n = \frac{3 \text{ hours}}{1.0 \text{ hour}} = 3

Remaining mass after 3 half-lives:

200.0 \times \left(\frac{1}{2}\right)^3 = 200.0 \times \frac{1}{8} = 25.0 \text{ g}

Mass decayed = initial mass - remaining mass:

200.0 - 25.0 = 175.0 \text{ g}

Answer:

175.0

g (Option A)

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