Question

what fraction of reactant in first order reaction is left after 40 minutes if T half is 20 minute​

Answer 1

Answer: $\frac{1}{4}$

Explanation: Now, to calculate the amount left , we use the formula:

$a=\frac{a_o}{2^n}$

where,

a = amount of reactant left after n-half lives = ?

$a_o$ = Initial amount of the reactant = $a_0$

n = number of half lives  =$\frac{\text {Given time}}{\text [half life}}=\frac{40}{20}= 2$

Putting values in above equation, we get:

$a=\frac{a_0}{2^2}$

$a=\frac{a_0]{4}$

Thus the amount of reactant left after 40 minutes is $\frac{1}{4}$ of original concentration.