Question

$Oxygen-15$ 's half -life is $9.98$ minutes. Find the remaining amount of $20.0g$ sample of $oxygen-15$ after $60.0$ minutes?

• A. $0.156g$
• B. $0.312g$
• C. $0.625g$
• D. $1.25g$
• E. $2.50g$

Answer 1

Answer: (B)

$0.312g$

Half life period is 9.98 minutes. This roughly corresponds to 10 minutes.
60 minutes corresponds to $\frac{60}{10}=6$ half life periods.
The amount remaining (out of 20 g) after 6 half life periods is $\frac{20}{2^6}=0.312g$.