Flexi Says: The acid ionization represents the fraction of the original acid that has been ionized in solution. Therefore, the numerical value of Ka is a reflection of the strength of the acid. pKa is negative of the logarithm of Ka. Thus, the higher the value of pKa, the weaker the acid is. Weak acids with relatively higher Ka or lower pKa values are stronger than acids with relatively lower Ka or higher pKa values. Because strong acids are essentially 100% ionized, the concentration of the acid in the denominator is nearly zero and the Ka value approaches infinity. For this reason, Ka values are generally reported for weak acids only.