The amount of time it will take to have 125 grams of erbium-165 left
in the sample is; C: 31.2 hours
How to calculate decay time?
The formula for amount of substance after decay is;
N(t) = N₀(¹/₂)^(t/t_¹/₂)
We are given;
Half life; t_¹/₂ = 10.4 hours
Initial amount; N₀ = 1000 g
Amount left; N(t) = 125 g
Thus;
125 = 1000 * (¹/₂)^(t/10.4)
125/1000 = (¹/₂)^(t/10.4)
In 0.125 = (t/10.4) In 0.5
-2.08 = -0.693(t/10.4)
t/10.4 = -2.08/-0.693
t = 10.4 * 3
t = 31.2 hours
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