Which shows the formula for an organic acid

            The formula of Organic acid is as follow,

                                                 R-COOH

Explanation:
                   The class of organic acids is called Carboxylic Acids. In above general structure, R is alkyl group and can vary. While -COOH is the functional group. 
                    Carboxylic Acids has the tendency to loose protons and their pKa value depends upon the alkyl group. For example the pKa value of Acetic acid (R = -CH₃) is 4.7. The driving force for this acidity is the stability of carboxylate (conjugate base) due resonance. i.e

                               RCOOH     ⇄    RCOO⁻  +  H⁺
Where;
            RCOO⁻  =  Carboxylate Ion (Conjugate base)