The molecular weight of Phaunouxite [Ca_{3}(AsO_{4})_{2.}11H_{2}O] is 596.24179.

Calcium arsenate [Ca_{3}(AsO_{4})_{2}] is an inorganic compound of three elements: Calcium (Ca), Arsenic (As), and Oxygen (O). The undecahydrate form of Calcium arsenate, Phaunouxite [Ca_{3}(AsO_{4})_{2.}11H_{2}O], also contains eleven water (H_{2}O) molecules. The molecular weight of Phaunouxite is 596.24179 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: Phaunouxite [Ca**_{3}(AsO_{4})_{2.}11H_{2}O] Molecular Weight Calculation

_{3}(AsO

_{4})

_{2.}11H

_{2}O] Molecular Weight Calculation

**Step 1: Find out the chemical formula and determine constituent atoms and their number in a Phaunouxite molecule.**You will know different atoms and their number in a Phaunouxite molecule from the chemical formula. The chemical formula of Phaunouxite is Ca

_{3}(AsO

_{4})

_{2.}11H

_{2}O. From the chemical formula, you can find that one molecule of Phaunouxite has three Calcium (Ca) atoms, two Arsenic (As) atoms, eight Oxygen (O) atoms, and eleven water molecules.

**Step 2: Find out the atomic weights of each atom (from the periodic table). **Atomic weight of Calcium (Ca): 40.078 (Ref: PubChem-20, Ciaaw-calcium)

Atomic weight of Arsenic (As): 74.921595 (Ref: Jlab-ele033, ciaaw-arsenic)

Atomic weight of Oxygen (O): 15.9994 (Ref: Jlab-ele008)

Molecular weight of Water [H

_{2}O]: 18.0154

**Step 3: Calculate the total weight of each atom in a Phaunouxite molecule by multiplying its atomic weight by its number.**

Number of Calcium atoms in Phaunouxite: 3

Atomic weight of Calcium (Ca): 40.078

Total weight of Calcium atoms in Phaunouxite: 40.078 x 3 = 120.234

Number of Arsenic atoms in Phaunouxite: 2

Atomic weight of Arsenic (As): 74.921595

Total weight of Arsenic atoms in Phaunouxite: 74.921595 x 2 = 149.84319

Number of Oxygen atoms in Phaunouxite: 8

Atomic weight of Oxygen (O): 15.9994

Total weight of Oxygen atoms in Phaunouxite: 15.9994 x 8 = 127.9952

Number of water (H_{2}O) molecules in Calcium chloride monohydrate: 11

Molecular weight of water: 18.0154

Total weight of water in Calcium chloride monohydrate: 18.0154 x 11 = 198.1694

**Step 4: Calculate the molecular weight of Phaunouxite by adding up the total weight of all atoms. **

Molecular weight of Phaunouxite: 120.234 (Calcium) + 149.84319 (Arsenic) + 127.9952 (Oxygen) + 198.1694 (water)= 596.24179

So the molecular weight of Phaunouxite is 596.24179.

**Phaunouxite [Ca _{3}(AsO_{4})_{2.}11H_{2}O] Molecular Weight Calculation**

| |||

Constituent atoms | Number of each atom | Atomic weight | Total weight |

Calcium (Ca) | 3 | 40.078 | 120.234 |

Arsenic (As) | 2 | 74.921595 | 74.921595 |

Oxygen (O) | 8 | 15.9994 | 127.9952 |

Water (H_{2}O) | 11 | 18.0154 | 198.1694 |

Molecular weight of Phaunouxite [Ca_{3}(AsO_{4})_{2.}11H_{2}O]: | 596.24179 |

## REFERENCES:

- PubChem-20: https://pubchem.ncbi.nlm.nih.gov/element/20
- Ciaaw-calcium: https://www.ciaaw.org/calcium.htm
- Jlab-ele008: https://education.jlab.org/itselemental/ele008.html
- Jlab-ele033: https://education.jlab.org/itselemental/ele033.html
- ciaaw-arsenic: https://www.ciaaw.org/arsenic.htm

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