<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>: pKa is the pH at which ionized fraction50 of drug equals to unionized fraction50. i.e. 50 is lipid soluble and 50 is water soluble. It is the pH at which 50 of drug can cross the membrane and 50 of drug cannot cross the membrane. 