A chemical reaction takes place in a continuous stirred tank reactor (CSTR). The reaction rate is given by the equation:

r = k * C^2

where r is the reaction rate, k is the reaction rate constant, and C is the concentration of the reactant. The volume of the reactor is 10 L, and the flow rate of the reactant is 0.2 L/min. The initial concentration of the reactant is 0.5 mol/L, and the desired final concentration is 0.1 mol/L. Calculate the reaction rate constant, k, and the residence time of the reactor.