We know that for a first-order reaction, the rate of reaction is given by: $$ \begin{aligned} \frac{dx}{dt} &= kC \end{aligned} $$ Here, $$ \begin{aligned} \frac{dx}{dt} &= 2.2 \times 10^{-5} \text{ mol dm}^{-3} \text{ s}^{-1} \\ k &= 1.1 \times 10^{-4} \text{ s}^{-1} \end{aligned} $$ Rearranging the equation to find the concentration $C$: $$ \begin{aligned} C &= \frac{\frac{dx}{dt}}{k} \\ &= \frac{2.2 \times 10^{-5}}{1.1 \times 10^{-4}} \text{ mol dm}^{-3} \\ &= 0.2 \text{ mol dm}^{-3} \end{aligned} $$ Therefore, the concentration of the reactant is $0.2 \text{ mol dm}^{-3}$.