A First-Order Reaction Has a Half-Life of 20

Multiple Choice

A first-order reaction has a half-life of 20.0 minutes. Starting with 1.00 × 10²⁰ molecules of reactant at time t = 0, how many molecules remain unreacted after 100.0 minutes?

A) 1.00 × 10 ⁴ molecules
B) 2.00 × 10 ¹⁹ molecules
C) 3.20 × 10 ¹⁶ molecules
D) 5.00 × 10 ²⁰ molecules
E) None of these choices are correct.