Question

The half-life of uranium-235 is 700 million years. If a rock that formed in the early Precambrian era (about 2100 million years ago) originally contained 20 grams of uranium-235, about how much uranium-235 would be left in that rock today?

Answer 1

The answer to your question is 2.5 g of Uranium-235 left

Step-by-step explanation:

Data

Half-life of Uranium-235 = 700 million years

Total time = 2100 million years

Initial mass = 20 grams

Final mass = ?

Process

1.- Compare the years with the mass left considering the half-life

2100 million years ago ------------------ 20 g left

After 700 million years

1400 million years ago ----------------- 10 g left

After 700 million years

700 million years ago ----------------- 5 g left

After 700 million years

0 million years ago ---------------- 2.5 g left