## General Chemistry

### Radioactivity

**5 out of 11**

Substance X has a radioactive half-life of 12 years. How much time must have elapsed if only 9 grams is left from an original sample of 150 grams?

A | 12 years |

B | 24 years |

C | 36 years |

D | 48 years |

**Ans. D**This question tests your knowledge of the concept of half-life. Half-life is defined as the amount of time for a radioactive substance to decay to half its initial quantity. Since the half-life of substance X is 12 years, it takes twelve years for substance X to decay to half its initial quantity. We have 9 grams of substance X present. The amount of substance X that was originally present is 150 grams. So it must have undergone radioactive decay for at least four half-lives. Write the decay mode, starting from the original amount present.

Each arrow indicates one half-life, for a total of four half-lives. Since one half-life is 12 years, the total time elapsed is 12 x 4 = 48 years. The exact answer is a little above 48 years. But the approximation is correct since the choices are far apart and no values higher than 48 are given.

##
Radioactivity Flashcard List

**11**flashcards

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