In a batch adsorption process, 5 g of fresh adsorbent is used to treat 1 liter of an aqueous phenol solution. The initial phenol concentration is 100 mg/liter. The equilibrium relation is given by q* = 1.3C where q* is the amount of phenol adsorbed in mg of phenol per gram of adsorbent; and C is the concentration of phenol in mg/liter in the aqueous solution. When equilibrium is attained between the adsorbent and the solution, the concentration of phenol in the solution, rounded to 1 decimal place, is