The molecular weight of 7H₂O is 126 g/mol. Here's how we calculate that:

* H₂O (water) has a molecular weight of 18 g/mol:

* Hydrogen (H) has a molecular weight of 1 g/mol.

* Oxygen (O) has a molecular weight of 16 g/mol.

* Therefore, H₂O = (2 * 1) + 16 = 18 g/mol

* Multiply the molecular weight of water by 7:

* 18 g/mol * 7 = 126 g/mol

Therefore, the molecular weight of 7H₂O is 126 g/mol.