Question

The half-life period of a order reaction is 60 minutes What percentage will be left over after 240 minutes ?

• A. $6.25\%$
• B. $4.25\%$
• C. $5\%$
• D. $6\%$

Answer 1

Answer: (A)

$6.25\%$

Answer 2

Given, $t_{1 / 2}=60$ min

Number of half-lives (n),

$T=n \times t_{1 / 2}$
$\therefore \, n=\frac{T}{t_{1 / 2}}=\frac{240}{60}=4$

Let the initial amount be 100

Then amount left (a-x) after four $(n=4)$

half-lives $= \frac{100}{2^{n}}$
$(a-x)=\frac{100}{c^{4}}=\frac{100}{16}$
$(a-x)=6.25 \%$