The molecular weight of Sodium phosphate dibasic heptahydrate [Na₂HPO₄.7H₂O] is 268.0667.

Sodium phosphate dibasic (Na₂HPO₄), also called Disodium phosphate (DSP) or disodium hydrogen phosphate, is an inorganic compound of four elements: Sodium, Hydrogen, Phosphorus, and Oxygen. The heptahydrate form of Sodium phosphate dibasic (Na₂HPO₄.7H₂O) also contains 7 water molecules. The molecular weight of Sodium phosphate dibasic heptahydrate is 268.0667 which can be calculated by adding up the total weight (atomic weight multiplied by their number) of all its elements and water molecules.

CALCULATION PROCEDURE: Sodium phosphate dibasic heptahydrate  [Na₂HPO₄.7H₂O] Molecular Weight Calculation

Step 1: Find out the chemical formula and determine constituent atoms and their number in a Sodium phosphate dibasic heptahydrate molecule.
From the chemical formula, you will know different atoms and their number in a Sodium phosphate dibasic heptahydrate molecule. The chemical formula of Sodium phosphate dibasic heptahydrate is Na₂HPO₄.7H₂O. From the chemical formula, you can find that one molecule of Sodium phosphate dibasic heptahydrate consists of two Sodium (C) atoms, one Hydrogen (H) atom, one Phosphorus (P) atom, four Oxygen atoms, and seven water molecules.

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

Atomic weight of Sodium (Na): 22.98976928 (Ref: Ciaaw-sodium)
Atomic weight of Hydrogen (H): 1.008 (Ref: Lanl-1)
Atomic weight of Oxygen (O): 15.9994 (Ref: Jlab-ele008)
Atomic weight of Phosphorus (P): 30.973761998 (Ref: Jlab-ele015)
Molecular weight of Water [H2O]: 18.0154

Step 3: Calculate the total weight of each atom in a Sodium phosphate dibasic heptahydrate molecule by multiplying its atomic weight by its number.

Number of Sodium atoms in Sodium phosphate dibasic heptahydrate: 2
Atomic weight of Sodium (Na): 22.98976928
Total weight of Sodium atoms in Sodium phosphate dibasic heptahydrate: 22.98976928 x 2 = 45.97953856

Number of Hydrogen atoms in Sodium phosphate dibasic heptahydrate: 1
Atomic weight of Hydrogen (H): 1.008
Total weight of Hydrogen atoms in Sodium phosphate dibasic heptahydrate: 1.008 x 1 = 1.008

Number of Phosphorus atoms in Sodium phosphate dibasic heptahydrate: 1
Atomic weight of Phosphorus (P): 30.973761998
Total weight of Phosphorus atoms in Sodium phosphate dibasic heptahydrate: 30.973761998 x 1 = 30.973761998

Number of Oxygen atoms in Sodium phosphate dibasic heptahydrate: 4
Atomic weight of Oxygen (O): 15.9994
Total weight of Oxygen atoms in Sodium phosphate dibasic heptahydrate: 15.9994 x 4 = 63.9976

Number of water (H₂O) molecules in Sodium phosphate dibasic heptahydrate: 7
Molecular weight of water: 18.0154
Total weight of water in Sodium phosphate dibasic heptahydrate: 18.0154 x 7 = 126.1078

Step 4: Calculate the molecular weight of Sodium phosphate dibasic heptahydrate by adding up the total weight of all atoms.

Molecular weight of Sodium phosphate dibasic heptahydrate: 45.97953856 (Sodium) + 1.008 (Hydrogen) + 30.973761998 (Phosphorus) + 63.9976 (Oxygen) + 126.1078 (Water) = 268.0667

So the molecular weight of Sodium phosphate dibasic heptahydrate is 268.0667.

Sodium phosphate dibasic heptahydrate [Na₂HPO₄.7H₂O] Molecular Weight Calculation

REFERENCES:

• Lanl-1: https://periodic.lanl.gov/1.shtml 
• Ciaaw-sodium: https://www.ciaaw.org/sodium.htm
• Jlab-ele008: https://education.jlab.org/itselemental/ele008.html
•  Jlab-ele015: https://education.jlab.org/itselemental/ele016.html