A large value of Ka means the acid is a strong acid, and a small value of Ka means the acid is a weaker acid. If the Ka is greater than 10, the acid will be completely dissociated in water 3.3B ACIDITY AND pKa Chemists usually express the acidity constant, Ka, as its negative logarithm, pKa pKa=-logKa For acetic acid the pKa is 4.75 pKa=4og(1.76×1035)=-(-4.76)=476 Notice that there is an inverse relationship between the magnitude of the pKa and the strength of the acid3.3B ACIDITY AND pKa A large value of Ka means the acid is a strong acid, and a small value of Ka means the acid is a weaker acid. If the Ka is greater than 10, the acid will be completely dissociated in water. Chemists usually express the acidity constant, Ka, as its negative logarithm, pKa. pKa = - logKa For acetic acid the pKa is 4.75: Notice that there is an inverse relationship between the magnitude of the pKa and the strength of the acid: pKa = -log(1.76 ×10 -5 ) = - (-4.76) = 4.76