Question

In a first order reaction, the concentration of the reactant decreases from 0.8 M to 0.4 in 15 minutes. The time taken for the concentration to change from 0.1M to 0.025 M is:

• A. 30 min
• B. 40 min
• C. 35 min
• D. 25 min

Answer 1

The correct option is A 30 min

Order = 1 (given)

$\because t_{75}=2\times t_{50}$

$⟹2\times 15=30$

$k=\frac{0.693}{t_{50}}=\frac{0.693}{15}$

$a=0.1M$

$(a-x)=0.025M$

For first order reaction $k=\frac{2.303}{t}\log \left(\frac{a}{a-x}\right)$

$⟹\frac{2.303\times \log 2}{15}=\frac{2.303}{t}\log \left(\frac{0.1}{0.025}\right)$

$⟹t=30minutes$