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# It take 40min for concentration of reactant in first order to drop from 0.4M to 0.3M at $25^oC$.How long it will take for reaction to be 90% complete?

$\begin{array}{1 1}(a)\;330.15min\\(b)\;300.15min\\(c)\;310.15min\\(d)\;320.15min\end{array}$

For first order
$K\times t=ln \large\frac{a}{a-x}$
$K\times 40=ln\large\frac{0.4}{0.3}$
$K=\large\frac{1}{40}$$ln \large\frac{4}{3} K\times t=ln\large\frac{1}{1-x/a} \large\frac{x}{a}$$=0.90$
$Kt=ln\large\frac{1}{1-0.9}$
$\large\frac{1}{40}$$ln\large\frac{4}{3}$$t=ln 10$
$t=320.15min$
edited Jul 27, 2014