What will be the amount of the substance left after n half-lives? The half life period of a first order reaction is 60 minutes. What percentage will be left after 240 minutes?

Amount of the substance left after one half life = A₀ / 2
Amount of the substance left after two half lives = A₀ / 2²
Amount of the substance left after three half lives = A₀ / 2³

In general,
Amount of the substance left after n half life = A₀/2ⁿ
The half life period of a first order reaction is 60 minutes
Number of half lives = 240 / 60 = 4 i.e. n = 4
Amount of the substance left after 4 half lives = A₀ / 2 ⁴ = A₀ / 16
= 0.0625 of A₀ = 6.25 %