A $2,000m^3$ water tower has been taken out of operation to install a chlorine monitor. When the water tower was put out of operation, its chlorine concentration was $2\mathrm{.}0m\frac{g}{L}.$ After ten hours, the chlorine content was determined to be $1\mathrm{.}2m\frac{g}{L}.$ Considering that the tank is a completely mixed batch reactor and that there is a first order decay.

a$)$ Calculate the reaction decay rate in day ${?}^{-1}.$

b$)$ What mass of chlorine $($in $kg)$ must be added to the tank to raise the chlorine level back to its original concentration?