Oxalic acid is a weak acid because it is an organic acid.

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Oxalic acid is classified as a weak acid.   It is weaker than  H3O+ ion (water).  But it is stronger than Acetic acid, Sulfurous acid, Nitrous acid, Benzoic acid etc. 

Firstly it is an organic compound and generally organic compounds are not strong acids.  
Oxalic acid is also called as dicarboxylic acid.  As it is an organic compound, it is called organic oxalic acid.

Oxalic acid = H2 C2 O4  or  HOOC - COOH.
       Disociates into Hydrogen oxalate ions HOOC-COO-  :   Ka = 5.4 * 10^-2 =======
    Strong acids are those with Ka activity constant  much more than 1.
  Acids with Ka less than 1 are weak acids.

 Useful  information:

     Oxalic acid is a colorless crystalline sold and it dissolves well to a colorless solution in water.  Oxalic acid is produced in our bodies by the metabolic processes. Organic oxalic acid combines with Calcium (organic in the body) to help digestive assimilation of Calcium and also in the contractions and relaxations that take place in the muscles of digestive tract. When organic oxalic acid is boiled or processed, it combines with Calcium in a harmful way and causes deficiency of Calcium.

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