Question

You find a rock that is about 20 grams large. This rock contains about 10 grams of Uranium-235 and 10 grams of the element it decays into (Thorium-231). The half life of Uranium-235 is 700 million years. How old is this rock?

Answer 1

The age of the given rock is 700 million years.

The given parameters;

• original mass of the rock, = 20 g
• mass of the rock remaining after decay, = 10 g
• half life of Uranium, $t_{1/2}$ = 700 years

age of the rock ----------------------------- mass remaining

0 $(t_0)$  -------------------------------------------  20 g

1 half life ($1 \ t_{1/2}$ ) ------------------------------ 10 g

Age of the rock $= t_{1/2} - t_0 = 700 \ - 0 = 700 \ million \ years$

Thus, the age of the given rock is 700 million years.

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