Hydrated Salts

In chemical formulas, the water of crystallisation is separated from the main salt formula by a central dot ($\cdot$). This notation indicates that the water molecules are part of the compound's structure but are not covalently bonded to the salt ions in the same way the ions are bonded to each other.

A general formula is written as $Salt \cdot xH_2O$, where $x$ represents the number of moles of water associated with one mole of the salt. For example, if a salt is a 'pentahydrate', $x$ would be 5, meaning five water molecules are present for every formula unit of the salt.

When calculating the relative formula mass ($M_r$) of a hydrated salt, the mass of the water molecules must be added to the mass of the anhydrous salt. It is a common error to multiply the masses; instead, one must sum the $M_r$ of the salt and $x$ times the $M_r$ of water ($18.0$).

Check the Molar Mass: When calculating moles of the salt, always use the $M_r$ of the anhydrous version. Using the $M_r$ of the hydrated version at this step is a logical error that will lead to an incorrect ratio.

Precision in Subtraction: Ensure you clearly identify which mass represents the 'hydrated salt', the 'anhydrous salt', and the 'water'. Small errors in subtraction at the start of a multi-step calculation will propagate and result in a non-integer value for $x$.

Rounding Ratios: If your calculated ratio for $x$ is very close to a whole number (e.g., $4.98$ or $5.02$), it is standard practice to round to the nearest integer. However, if the value is significantly different (e.g., $2.5$), check your initial mass calculations or consider if the salt was not heated to constant mass.

Incomplete Dehydration: A frequent mistake in practical work is failing to heat the sample for long enough. If some water remains, the calculated mass of water will be too low, leading to an underestimated value for the degree of hydration ($x$).

Thermal Decomposition: Some salts may decompose if heated too strongly, losing more than just water (e.g., releasing $CO_2$ or $SO_x$ gases). This would make the mass loss appear larger than it actually is, resulting in an artificially high value for $x$.

The 'Dot' Confusion: Students often mistake the dot in $CuSO_4 \cdot 5H_2O$ as a multiplication sign in a mathematical sense. In chemistry, it signifies a specific structural association, and for mass calculations, it implies addition of the constituent parts.