Question

Write formulas for magnesium sulfate heptahydrate and disodium hydrogen phosphate dodecahydrate.

Short answer

Formulas are MgSO₄·7H₂O and Na₂HPO₄·12H₂O.

Step-by-step solution

Identify the Cations and Anions for Magnesium Sulfate Heptahydrate

Magnesium sulfate heptahydrate consists of magnesium ions (Mg²⁺) and sulfate ions (SO₄²⁻) combined with seven water molecules (heptahydrate).

Write the Basic Formula for Magnesium Sulfate

Combine the magnesium ion (Mg²⁺) with the sulfate ion (SO₄²⁻) to get the basic formula for magnesium sulfate: MgSO₄.

Add the Water of Hydration

Indicate the seven water molecules (heptahydrate) in the formula: MgSO₄·7H₂O.

Identify the Cations and Anions for Disodium Hydrogen Phosphate Dodecahydrate

Disodium hydrogen phosphate dodecahydrate consists of two sodium ions (Na⁺), one hydrogen phosphate ion (HPO₄²⁻), and twelve water molecules (dodecahydrate).

Write the Basic Formula for Disodium Hydrogen Phosphate

Combine the sodium ions (Na⁺) with the hydrogen phosphate ion (HPO₄²⁻) to get the basic formula for disodium hydrogen phosphate: Na₂HPO₄.

Add the Water of Hydration

Indicate the twelve water molecules (dodecahydrate) in the formula: Na₂HPO₄·12H₂O.

Key concepts

Magnesium Sulfate Heptahydrate

Magnesium sulfate heptahydrate is a common hydrate in chemistry, often referred to as Epsom salt. To write its chemical formula, you start by identifying the cation and anion involved.
The magnesium ion (Mg²⁺) and the sulfate ion (SO₄²⁻) combine to form the basic compound magnesium sulfate, whose formula is MgSO₄.
Since it is a hydrate, this compound is associated with water molecules. Specifically, *heptahydrate* indicates there are *seven* water molecules.
The chemical formula is thus written as MgSO₄·7H₂O.
Understanding hydrates is essential because these water molecules affect the properties and reactions of the compound.

Disodium Hydrogen Phosphate Dodecahydrate

Disodium hydrogen phosphate dodecahydrate involves a different hydrate where the compound is combined with *twelve* water molecules.
Firstly, let's identify the ions: *two* sodium ions (Na⁺) and a hydrogen phosphate ion (HPO₄²⁻) form the base compound, *disodium hydrogen phosphate*.
Combining these ions gives you the basic formula: Na₂HPO₄.
Adding the hydration aspect, the compound incorporates twelve water molecules.
Thus, we write it as: Na₂HPO₄·12H₂O.
Understanding such complex hydrates is important as you will encounter them in various applications, from laboratory work to biological systems.

Writing Chemical Formulas of Hydrates

Writing chemical formulas for hydrates involves a straightforward process once you understand the basics.
Firstly, you identify the cation (positive ion) and the anion (negative ion) of the compound.
From these ions, you write the basic formula by combining them appropriately according to their charges.
If the charges are equal and opposite, they cancel each other out; if not, you balance their stoichiometry.
For hydrates, you then add the water of hydration.
The number of water molecules is indicated with a numerical prefix (like 'mono-', 'di-', 'tri-', 'hepta-', 'dodeca-', etc.) and written as '·nH₂O'.
Remembering these steps helps ensure you write accurate and meaningful chemical formulas.