In the DebyeHckel theory, the counter charge in a spherical shell of radius r and thickness dr

In the Debye–Hückel theory, the counter charge in a spherical shell of radius r and thickness dr around the central ion of charge +Q is given by −Qκ²re^−κr dr. Calculate the radius at which the counter charge has its maximum value, r _max , from this expression. Evaluate r _max for a 0.090 m aqueous solution of Na₃ O₄ at 298 K.