Question

In a first order reaction the amount of reactant decayed in three half lives (let $a$ be is initial amount) would be:

• A. $7a/8$
• B. $a/8$
• C. $a/6$
• D. $5a/6$

Answer 1

The correct option is A $7a/8$

The amount of reactant left after n half lives for a 1st order reaction is $(\frac{1}{2}{)}^n$ times the Initial concentration.

Here n =3 . So Amount of Reactant left is $\frac{a}{8}$

So Amount of reactant decayed is $\frac{7a}{8}$