<prompt> Which of the following best describes the drug phenomenon at pKapH?Drug exists as 50 ionized and 50 unionizedDrug exists as 25 ionized and 75 unionizedDrug exists as 75 ionized and 25 unionizedDrug is completely 100 Ionized  <response>: pka is numerically equal to the pH at which a drug is 50 ionized. The pKa of a drug is the hydrogen ion concentration pH at which 50 of the drug exists in its ionized hydrophilic form i.e., in equilibrium with its un-ionized lipophilic form pKa is a different term than pH pH is simply a measure of the concentration in a given solution pKa is defined as the pH were a drug exists as 50 ionized and 50 unionized If pKa-pH 0, then 50 of drug is ionized and 50 is unionized Ref: KDT 7th edition Pgno: 11,12 