Question

Without per frming detailed calculations, indicate which of the following hydrates has the greatest \% ${\mathrm{H}}_2\mathrm{O}$ by mass: ${\mathrm{CuSO}}_4\cdot 5{\mathrm{H}}_2\mathrm{O},{\mathrm{Cr}}_2{\left({\mathrm{SO}}_4\right)}_3\cdot 18{\mathrm{H}}_2\mathrm{O},{\mathrm{MgCl}}_2\cdot 6{\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{LiC}}_2{\mathrm{H}}_3{\mathrm{O}}_2\cdot 2{\mathrm{H}}_2\mathrm{O}$

Short answer

The hydrate with the greatest \% ${\mathrm{H}}_2\mathrm{O}$ by mass should be ${\text{Cr}}_2{\left({\text{SO}}_4\right)}_3·18{\mathrm{H}}_2\mathrm{O}$, as it contains the greatest number of water molecules per formula unit.

Step-by-step solution

Analyze the given hydrates

Here are the given hydrates xH2O with x being the number of of water molecules: ${\text{CuSO}}_4·5{\mathrm{H}}_2\mathrm{O}$, ${\text{Cr}}_2{\left({\text{SO}}_4\right)}_3·18{\mathrm{H}}_2\mathrm{O}$, ${\text{MgCl}}_2·6{\mathrm{H}}_2\mathrm{O}$ and ${\text{LiC}}_2{\text{H}}_3{\text{O}}_2·2{\mathrm{H}}_2\mathrm{O}$. \nWithout performing detailed calculations, we are focusing on the number of water molecules in each hydrate.

Compare the number of water molecules

By just looking at the number of water molecules associated with each compound, the hydrate with 18 water molecules, namely ${\text{Cr}}_2{\left({\text{SO}}_4\right)}_3·18{\mathrm{H}}_2\mathrm{O}$, stands out. We can assume that this compound has the highest proportion of ${\mathrm{H}}_2\mathrm{O}$ by mass because it contains the most water molecules per formula unit.

Make the assumption

Finally, suppose that the number of water molecules is the main factor that determines which hydrate has the greatest \% ${\mathrm{H}}_2\mathrm{O}$ by mass. It is a reasonable assumption to say that ${\text{Cr}}_2{\left({\text{SO}}_4\right)}_3·18{\mathrm{H}}_2\mathrm{O}$ should have the greatest percentage of ${\mathrm{H}}_2\mathrm{O}$ by mass.

Key concepts

Water of Crystallization

When talking about chemical hydrates, the term 'water of crystallization' often comes up. Water of crystallization refers to the fixed number of water molecules that are integrated into the crystalline structure of a compound. Hydrates contain specific water molecules that are essential to maintaining their structure.

These water molecules are part of the solid when it is formed and remain part of its structure under certain conditions. This is why, for example, - - - CuSO extsubscript{4} · 5H extsubscript{2}O is a common example of a hydrate.

In summary, water of crystallization plays a crucial role in stabilizing the hydrate form of a compound. It affects the physical and chemical properties and is essential for their crystalline integrity.

Percentage Composition

Percentage composition is a way to express the relative amounts of each component in a chemical compound. For hydrates, it helps understand the proportion of water within the compound.

The percentage composition of a hydrate is calculated by finding the mass of each element or molecule in one formula unit of the compound and then dividing it by the total molar mass of the hydrate.
- This gives you a measure of what fraction of the compound's mass comes from water molecules. - Through this method, you can compare how much of a compound is actually water across different hydrates.

Ultimately, understanding percentage composition assists in comparing and evaluating different hydrates based on the quantity of water they contain, as we did when determining which hydrate contains the highest % of H extsubscript{2}O in the given examples.

Chemical Hydrates

Chemical hydrates are complexes that include water molecules as a part of their structure. They are a fascinating intersection of organic and inorganic chemistry.

Hydrates can bond water molecules in very specific ratios, like: - - - CuSO extsubscript{4} · 5H extsubscript{2}O is composed of five water molecules for every copper sulfate unit.
- MgCl extsubscript{2} · 6H extsubscript{2}O contains six water molecules per magnesium chloride unit.
These associated water molecules are crucial in defining the properties of the hydrate, influencing solubility, reactions, and the appearance of the substance.

For students, one key point about chemical hydrates is that even though the water seems bound, it can often be driven off by heating, leaving the anhydrous form behind. This reversible process is one of many reasons hydrates are important in various chemical processes.

Mass Percentage Calculation

Mass percentage calculation is an essential tool in chemistry, especially in determining the makeup of compounds. When dealing with hydrates, this process involves calculating what percentage of the compound is made up of water.

To calculate the mass percentage of water in a hydrate, you: - Determine the total molar mass of the hydrate. - Calculate the mass of all the water molecules combined in the formula unit. - Divide the mass of the water by the total molar mass and multiply by 100 to get a percentage.

For example, by applying this calculation to compounds like Cr extsubscript{2}(SO extsubscript{4}) extsubscript{3} · 18H extsubscript{2}O, students can deduce it has a higher water proportion than other presented hydrates just by knowing that the associated water greatly exceeds that in others.

This method is essential not only for academic exercises but also for real-world applications wherever hydrates need to be quantified or utilized.