Answer:
268 mg
Step-by-step explanation:
Let A₀ = the original amount of caffeine
The amount remaining after one half-life is ½A₀.
After two half-lives, the amount remaining is ½ ×½A₀ = (½)²A₀.
After three half-lives, the amount remaining is ½ ×(½)²A₀ = (½)³A₀.
We can write a general formula for the amount remaining:
A =A₀(½)ⁿ
where n is the number of half-lives .
n = t/t_½
Data:
A₀ = 800 mg
t₁ = 10 a.m.
t₂ = 7 p.m.
t_½ = 5.7 h
Calculations:
(a) Calculate t
t = t₂ - t₁ = 7 p.m. - 10 a.m. = 19:00 - 10:00 = 9:00 = 9.00 h
(b) Calculate n
n = 9.00/5.7 = 1.58
(b) Calculate A
A = 800 × (½)^1.58 = 800 × 0.334 = 268 mg
You will still have 268 mg of caffeine in your body at 10 p.m.
