Question

In a first-order reaction, the concentration of the reactant, decreases from $0.8M$ to $0.4M$ in $15\text{minutes}$ half life. The time taken for the concentration to charge from $0.1M$ to $0.025M$ is:

• A. $30minutes$
• B. $15minutes$
• C. $7.5minutes$
• D. $60minutes$

Answer 1

The correct option is A $30minutes$

Give that the concentration of the reactant decreases from $0.8M$ to $0.4M$ in $15$ minutes.
$t_{1/2}$ = $15$ minutes.
So, the concentration of reactant will fall from $0.1M$ to $0.025M$ in two half lives.

i.e., $2t_{1/2}=2\times 15=30$ minutes

Hence option (a) is the answer.