Is it more useful to have the percent by mass of water in a hydrate or the percentage composition assuming you know the formula for the associated anhydrous ionic compound?
The Correct Answer and Explanation is:
The more useful value depends on the specific purpose, but generally, the percent by mass of water in a hydrate is more practical when working with hydrates, especially in laboratory settings.
If you already know the formula for the anhydrous ionic compound, the percent by mass of water in the hydrate directly tells you how much water is present in the compound. This is particularly helpful when determining how much of a hydrate you need to weigh to obtain a specific amount of the anhydrous substance, or when calculating how much water will be lost upon heating.
For example, in a hydrate like copper(II) sulfate pentahydrate (CuSO₄·5H₂O), knowing that approximately 36 percent of its mass is water provides immediate insight for experiments where water removal or accurate mass measurements are required. This information is crucial when heating the hydrate to drive off water, as you can predict the mass change and confirm complete dehydration.
On the other hand, percentage composition, which refers to the percent by mass of each element in a compound, is more general and useful when the formula is unknown or when determining the empirical formula. However, if the formula for the anhydrous ionic compound is already known, this detailed elemental breakdown adds less practical value when dealing with hydrates specifically.
In summary, for laboratory work, mass calculations, or reactions involving hydrates where the anhydrous formula is already known, the percent by mass of water in the hydrate is more useful. It simplifies stoichiometric calculations, helps monitor dehydration processes, and aids in preparing precise quantities of compounds. Percentage composition is better suited for identifying unknowns or confirming empirical formulas rather than for routine practical applications involving known hydrates.
