The molecular weight of Oxalic acid dihydrate [HOOCCOOH.2H₂O] is 126.0308.

To calculate molecular weight of any compound, the first step is to know the constituent atoms and their number in that particular compound. Then calculate the total weight of each atom by multiplying its atomic weight by its number. The sum of total weight of all constituent atoms will be the molecular weight of the compound. Note that the value of atomic weight may differ slightly from different sources.

CALCULATION PROCEDURE: Oxalic acid dihydrate [HOOCCOOH.2H₂O] Molecular Weight Calculation

Step 1: Find out the chemical formula and determine constituent atoms and their number in a Oxalic acid molecule.
From the chemical formula, you will know different atoms and their number in a Oxalic acid molecule. Chemical formula of Oxalic acid is C₂H₂O₄. From the chemical formula of Oxalic acid, you can find that one molecule of Oxalic acid has two Carbon (C) atoms, two Hydrogen (H) atoms and four Oxygen (O) atoms. Dihydrate form of oxalic acid also contains two molecules of water. 

Step 2: Find out atomic weights of each atom (from periodic table).

Atomic weight of Carbon (C): 12.0107 (Ref: Jlab-ele006)
Atomic weight of Hydrogen (H) : 1.008 (Ref: Lanl-1)
Atomic weight of Oxygen (O) : 15.9994 (Ref: Jlab-ele008)

Step 3: Calculate the total weight of each atom present in a Oxalic acid molecule by multiplying its atomic weight by its number.

Number of Carbon atoms in Oxalic acid: 2
Atomic weight of Carbon: 12.0107
Total weight of Carbon atoms in Oxalic acid: 12.0107 x 2 = 24.0214

Number of Hydrogen atoms in Oxalic acid: 2
Atomic weight of Hydrogen: 1.008
Total weight of Hydrogen atoms in Oxalic acid: 1.008 x 2 = 2.016

Number of Oxygen atoms in Oxalic acid: 4
Atomic weight of Oxygen: 15.9994
Total weight of Oxygen atoms in Oxalic acid: 15.9994 x 4 = 63.9976

Step 4: Calculate the molecular weight of Oxalic acid by adding up the total weight of all atoms.

Molecular weight of Oxalic acid: 24.0214 (Carbon) + 2.016 (Hydrogen) + 63.9976 (Oxygen) = 90.035

So the molecular weight of Oxalic acid is 90.035.

Step 5: Add the weight of water molecules in oxalic acid molecular weight
Since dihydrate form contains two molecules of water, the molecular weight of oxalic acid dihydrate will be equal to: molecular weight of oxalic acid + 2 x molecular weight of water 

Molecular weight of water: 18.0154

Molecular weight of two molecules of water: 2 x 18.0154 = 36.0308

Molecular weight of oxalic acid dihydrate: 90.035 (oxalic acid) + 36.0308 (2x water)

So the molecular weight of Oxalic acid dihydrate is 126.0308.

Oxalic acid dihydrate [HOOCCOOH.2H₂O] Molecular Weight Calculation

REFERENCES:

• Lanl-1: https://periodic.lanl.gov/1.shtml 
• Pubchem-Hydrogen : https://pubchem.ncbi.nlm.nih.gov/element/Hydrogen 
• Jlab-ele006; https://education.jlab.org/itselemental/ele006.html
• Pubchem-6: https://pubchem.ncbi.nlm.nih.gov/element/6
• Jlab-ele008: https://education.jlab.org/itselemental/ele008.html
• Pubchem-8: https://pubchem.ncbi.nlm.nih.gov/element/8

See also:

• Apiose (C5H10O5) Molecular Weight Calculation