<prompt> True about pKa is:pH at which ionized fraction of drug equals to unionized fractionpH at which ionized fraction of drug is more than unionized fractionpH at which ionized fraction of drug is less than unionized fractionpH at which ionized fraction of drug is twice unionized fraction  <response>: Ans. a. pH at which ionized fraction of drug equals to unionized fractionRef: Goodman Gilman 12th EP 18pKa is the pH at which as drug is 50 ionized and 50 unionized, i.e. the fraction of ionized drug equals to the fraction of unionized drug.The effect of pH on drug ionization and unionization can be calculated for acidic and basic drug by Hasselbalch equations given below:Basic drug: Log10 UnionizeddrugIonizeddrug pK4 -PHAcidic drug: Log10 IonizeddrugIonizeddrug pKa -PHWhat is pKa ?If the ionized drugunionized drug, i.e. 50 each, then the equation becomes,Log10 1PKa -PHOr,0pKa -pHOr. pKa pHHencepKa is the pH at which the drug is 50 ionized and 50 unionized. 