Study Notes on Hydrated Salts and Their Analysis

Hydrated Salts

Definition and Characteristics

Hydrated salts are compounds occurring naturally or obtained from commercial sources that contain water molecules chemically bound to their ionic components within a crystalline structure. These bound water molecules are referred to as waters of crystallization. The number of these water molecules per mole of salt is usually a definite and constant quantity.

For example, iron(III) chloride is represented as FeCl₃+6H₂O, indicating that it includes 6 moles of water for each mole of FeCl₃. Similarly, copper(II) sulfate is denoted as CuSO₄*5H₂O, which signifies the binding of 5 moles of water to each mole of CuSO₄.

Anhydrous Salts

An anhydrous salt is one that has had its water molecules removed. This moisture removal can happen through the application of heat, as some salts have water molecules that are so weakly held that heating can drive them off, resulting in the formation of the anhydrous form of the salt.

In contrast, there are salts where the water molecules are tightly bound to the ionic part; thus, even intense heating will not result in their dehydrated (anhydrous) state.

Efflorescent and Deliquescent Salts

Hydrated salts exhibit different behaviors with regard to moisture absorption:

• Efflorescent salts are those that spontaneously lose their bound water to the atmosphere without the application of heat.

• Deliquescent salts readily absorb moisture from the atmosphere, thereby dissolving in the absorbed water and forming a solution.

For instance, Epsom salt (magnesium sulfate heptahydrate) with the formula MgSO₄·7H₂O illustrates these concepts in action. Under gentle heating, the anhydrous form, magnesium sulfate (MgSO₄), can be obtained from it.

Example Calculation: Mass Percent of Water in Epsom Salt

In Epsom salt, each mole of magnesium sulfate consists of 7 moles of water, equating to a mass of 126.1 g of H₂O bound to 120.4 g of magnesium sulfate. The formula for calculating the percent by mass of water in the salt is:

$\text{Percent by mass of water} = \frac{126.1 \, \text{g H}<em>2O}{126.1 \, \text{g H}</em>2O + 120.4 \, \text{g salt}} \times 100$

Calculating this gives:
$\text{Percent by mass of water} = \frac{126.1}{246.5} \times 100 = 51.16\% \text{ H}_2O$ (Eq. 4.2)

Gravimetric Analysis

The experiment discussed typically employs a method known as gravimetric analysis, which is an analytical approach that relies predominantly on mass measurements for substance analysis.

• In this process, the mass of the hydrated salt is initially weighed, followed by heating to eliminate the water content (waters of crystallization).

• After the heating process, the remaining dry sample's mass is measured post-heating, allowing for the calculation of the mass percent of water content in the hydrated salt using the aforementioned mass percent formula.