As Drew said these are not the same but related. Pelargonic is a common name for the compound while nonanoic acid is the IUPAC name (a technical naming system used by chemists which helps to indicate the molecular structure of the compound). This is the fatty acid itself, unaltered. It means it has a chain of carbon atoms (in this case 9 "nona") - which is the "fatty" part, and ending with a chemical group called a carboxylic acid. https://en.wikipedia.org/wiki/Fatty_acid If the acid encounters a base or alkali (which are opposite of acid on the pH scale) then the acid reacts with the base to make, in most cases, water and a salt. Hydrochloric acid (HCL) to take a simple example of an acid reacts with sodium hydroxide (NaOH a base) to make water and table salt: H-Cl + Na-O-H becomes H-O-H (water H2O) and Na-Cl (table salt). If the pelargonic acid reacts with potassium hydroxide (another base) it would make water and potassium pelargonate (the potassium salt of the fatty acid). The salt and the acid have different properties - as Drew said the salt has some water solubility while the water solubility of the acid is very low. Also the salt is a neutral compound and the acid is, well, acid.
Additionally, the phrase you are using "potassium salts of fatty acids" implies that it is the potassium salt of more than one fatty acid - not just a nine carbon acid, but mixture of different carbon chain lengths. In nature plants and animals do not usually just make one fatty acid - and when fats or oils (triglyercides) from animals or plants are extracted they usually contain a mixture of fatty (carbon) chains. The plant or animal makes triglycerides - a glycerin with 3 fatty acids attached. In soap making when the base (NaOH or KOH) is added, the bonds between the glycerin and the fatty acids are broken and then the acid is neutralized and becomes the salt. As Drew said the salt of a fatty acid is a soap. This website gives a diagram of the reaction although the triglyceride they show is made of 3 fatty acid chains that are all the same length (16 carbons) so all the soap made is all sodium salt of palmitate (palmitate = 16, IUPAC hexadecanoic). http://www.chem.latech.edu/~deddy/chem122m/L06U00Soap122.htm In real life this would be rare - most triglyerides would have chains of different lengths in random arrangements (10,8,16 or 10,14,10 or 18,8,12 - you get the idea). Therefore, if this natural mixture is reacted with a base (like potassium hydroxide) what will result is a mixture of fatty acid salts.
Drew's answer is the short version - I didn't know if you would understand the chemical terms he used.
Karen
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