Question

A 60-g sample of radioactive isotope is measured after two half-lives have passed. What pair of percentages describes the sample after two half-lives? 50% unchanged and 50% stable 25% unchanged and 75% stable 12. 5% unchanged and 87. 5% stable 75% unchanged and 25% stable.

Answer 1

The half-life gives the time  after which a give unstable substance decays

to half the initial quantity.

• The pair of percentages that describes the sample after two half-lives is; 25% unchanged and 75% stable.

Reasons:

The mass of the sample of the radioactive isotope, m = 60-g

The number of half lives that passes = 2 half lives

Required:

The pair of percentages that describes the sample.

Solution:

The half-life is the time half of an unstable radioactive substance decays into a stable substance.

Therefore, after one half life, the pair of percentages we have are;

50% unchanged and 50% stable

After the second half life, the 50% of the sample decays further by half to give; 25%  stable substance and 25% unchanged substance

Therefore, after two half-lives, we get;

(50% + 25%) stable substance and 25% unchanged sample

75% stable substance and 25% unchanged

The correct option is; 25% unchanged and 75% stable.

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