Rates and Lives Quiz 10

1
Rates and ½ Lives Quiz 10 To prepare a dog for
surgery, 30.0 mg per kg of body mass of
phenobarbital must be administered
intravenously. The anesthetic is metabolized
with first-order kinetics and a half life of 4.50
hours. After about 2 hours the drug begins to
lose its effect in a 15.0 kg dog. What mass of
phenobarbital must be re-injected after 2.00
hours to restore the original level of anesthetic
in the 15.0 kg dog? (This problem is taken from
Physical Chemistry for the Life Sciences by
Atkins and Paula)
2
milligrams of phenobarbital administered to a 15
kg dog initially (15.0 kg ) (30.0 mg / kg)
450 mg 1st order rate constant k ln (2) /
t1/2 ln (2) / 4.50 hours 0.154 hours -1
milligrams of phenobarbital remaining after
2.00 hours m mo e k t
(450 mg) e (0.154 / hours) (2.00 hours) 331
mg milligrams of phenobarbital that must be
administered after 2.00 hours to bring the dose
back to its original value 450 mg - 331 mg
119 mg