Analyze the pKa of the acid. The pKa is the negative logarithm of the acid dissociation constant, which can help you to understand how strong or weak the acid is. The higher the value of the pKa, the weaker the acid is. The lower the value of the pKa, the stronger the acid is. For example, trichloroethanoic acid has a pKa of 0.65 and ethanoic acid has a pKa of 4.76. Therefore, trichloroethanoic acid is a stronger acid than ethanoic acid.
Examine the structure of the acidic compound. By drawing the structure of the compound, or looking it up online or in a chemistry book, you can better understand the acidity of the atoms. For example, the structure of ethanoic acid is CH3COOH. The carbon to the right is double bonded to one oxygen and single bonded to the other.
Draw the ion that results from the removal of the H+ atom. The structure of the ion can help you to analyze the stability of the resulting compound. For example, when ethanoic acid loses a proton, the resulting compound is CH3COO-. This can be represented with a resonance structure, which shows the carbon single bonded to one oxygen and double bonded to the other and with the carbon double bonded to one oxygen and single bonded to the other. This represents how the actual bond is an average of both those structures, meaning that the negative charge is delocalized over both of the oxygen atoms. The greater the amount of delocalization, the more acidic the atom in the acidic compound is.
Determine which functional groups are part of the acidic compound. Some functional groups, such as methyl groups, make the atom in the acidic compound less acidic. The reason for this is that the methyl, CH3, groups tend to push the electrons away, making it easier for the ion to reform with the hydrogen. Some functional groups, such as the halogens, pull the electrons, which increases the delocalization and therefore makes the atom more acidic.