Question

A substance $A$ undergoes decomposition in solution following the first order kinetics. Flask $I$ contains 1 litre of 1 M solution of $A$ and flask $II$ contains $100$ $ml$ of 0.6 M solution. After 8 hours, if the concentration of $A$ in flask $I$ became 0.25 M, what will be the time required in hours for the concentration of $A$ in flask $II$ to become 0.3 M?

• A. 0.4
• B. 2.4
• C. 4
• D. Unpredictable as rate constant is not given

Answer 1

The correct option is C 4

Initial concentration of $A$= $1M$

Concentration after $2$ hours= $0.25M$

Since, the reaction follows first order kinetics; so, time taken to have concentration $0.5M=4$ hours

This is half life $t_{1/2}$ of the reaction.

$\Rightarrow$ $t_{1/2}=4$ hours

Now in Flask II

Initial concentration of $A=0.6M$

Final concentration of $A=0.3M$

Since, the concentration is reduced to half of its initial value so time required for it to happen will be $t_{1/2}$ i.e. $4$ hours.