Question

A reaction that is of the first order with respect to reactant A has a rate constant $6\min^{- 1}.$ If we start with $\lbrack A\rbrack = 0.5moll^{- 1}$, when would $\lbrack A\rbrack$ reach the value 0.05mol $l^{- 1}$.

• A. 0.384 min
• B. 0.15 min
• C. 3 min
• D. 3.84 min

Answer 1

Answer: (A)

0.384 min

Answer 2

We know that for first order kinetics

$k = \frac{2.303}{t}\log\frac{a}{a - x}$

$(a - x) = 0.05moll^{- 1}$,$6 = \frac{2.303}{t}\log\frac{0.5}{0.05}$

or $t = \frac{2.303}{6}\log\frac{0.5}{0.05} = \frac{2.303}{6} = 0.384\min$