In the Lindemann theory of unimolecular reaction, it is shown that the apparent rate constant for such a reaction is $k_{app}=\frac{k_{1}C}{1+\alpha C}$  where C is the concentration of the reactant ${\mathrm{k}}_1$ and $\mathrm{\alpha }$ are constants. Calculate the value of C for which $k_{app}$ has 90% of its limiting value at C tending to infinitely large values, given $\alpha =9\times {10}^5$.