Question

The half-life of Pa-218 is 3 minutes. What mass of a 16g sample of Pa-218 will remain after 15 minutes?

• A. 3.2g
• B. 2.0g
• C. 1.6g
• D. 0.5g

Answer 1

Answer: (D)

0.5g

$t_{1/2}=3minutes$

$K=\frac{2.303}{t}log\frac{A_o}{A}$

Also, $t_{1/2}=\frac{0.693}{K}$

$\Rightarrow K=\frac{0.693}{3}=0.231$

After $15minutes$

$K=\frac{2.303}{15}log\frac{15}{x}$

$\Rightarrow 0.231=\frac{2.303}{15}log\frac{15}{x}$

$\Rightarrow x=0.5g$ [Remaining concentration]