to prepare a dog for surgery, about 35 mg/(kg body mass) of phenobarbital must be administered intravenously. the anesthetic is metabolized with first-order kinetics and a half-life of 4 h. after about 2 h, the drug begins to lose its effect in a 18-kg dog. what mass of phenobarbital must be re-injected to restore the original level of anesthetic in the dog?