The half-life of a radio-active element is 8 years. If we start with 20 grams, how much is left after 16 years? 20 years?

5 g is left after 16 years

3.54 g is left after 20 years

Explanation:

Given that:

Half life = 8 years

Where, k is rate constant

So,  

The rate constant, k = 0.0866 year⁻¹

Using integrated rate law for first order kinetics as:

Where,  

is the concentration at time t

is the initial concentration  = 20 g

Time = 16 years

So,  

5 g is left after 16 years

Time = 20 years

So,  

3.54 g is left after 20 years

less than one you can't create energy