Question

What is the formula for magnesium sulfate heptahydrate?

Answer 1

A chemical formula of any compound represents the exact number of atoms present in that compound. A formula is written in such a way as to balance the charges of each atom. Water of hydration is the extra moles of water which is attached with any salt.

Complete answer:
The chemical or molecular formula of any compound is the formula that consists of the symbols of the elements involved, written with their respective quantities in that compound. A chemical formula is written in such a way that cations are written first and then the anions.
We have been given magnesium sulfate heptahydrate that consists of magnesium, sulfate and water as water of hydration. This compound contains magnesium as the cation which contains 2 electrons in its valence shell that are donated to sulfate ion that is the anion, so, magnesium has a charge of $M{{g}^{2+}}$ and sulfate has a charge of $S{{O}_{4}}^{2-}$ , so the molecule of magnesium sulfate will have the formula, $MgS{{O}_{4}}$.
Now, heptahydrate is the water molecule. As we know, penta denotes 5, hexa denotes 6, so, hepta denotes 7. Therefore 7 moles of water are present as water of hydration. So the formula will become, $MgS{{O}_{4}}.7{{H}_{2}}O$.
Hence, the formula for magnesium sulfate heptahydrate is $MgS{{O}_{4}}.7{{H}_{2}}O$.

Note:
Water of hydration is also called as water of crystallization. It does not alter the chemical composition of any salt. But it is present as water molecules in the crystal of that salt. This is involved, when the crystals of any salt is formed by its aqueous solution. This water of hydration can be removed by heating the compound.