Question

A \(2.634-\mathrm{g}\) sample containing \(\mathrm{CuCl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O}\) and other materials was heated. The sample mass after heating to drive off the water was \(2.125 \mathrm{g} .\) What was the mass percent of \(\mathrm{CuCl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O}\) in the original sample?

Short answer

The mass percent of \(\mathrm{CuCl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O}\) is approximately 19.32%.

Step-by-step solution

Identify the mass of water lost

Start by calculating the mass of the water driven off during heating. Subtract the final mass of the sample after heating from the original mass of the sample: \(2.634 \, \text{g} \) (initial mass) - \(2.125 \, \text{g} \) (mass after heating) = \(0.509 \, \text{g} \).

Calculate the mass of CuCl₂ · 2H₂O

Since water loss corresponds to the water in \(\mathrm{CuCl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O}\), the mass of \(\mathrm{CuCl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O}\) is the same as this water mass because heating has only removed the water of hydration: \(0.509 \, \text{g} \).

Determine the percentage of CuCl₂ · 2H₂O in the original sample

Find the mass percent of \(\mathrm{CuCl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O}\) in the original sample by using the formula: \[ \text{Mass percent} = \left( \frac{\text{mass of } \mathrm{CuCl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O}}{\text{mass of original sample}} \right) \times 100 \] Substituting the known values, we have \[ \text{Mass percent} = \left( \frac{0.509 \, \text{g}}{2.634 \, \text{g}} \right) \times 100 \approx 19.32\%. \]

Key concepts

Understanding Hydrated Compounds

Hydrated compounds are fascinating substances in chemistry that contain water molecules as part of their structure. These water molecules are not just randomly placed; they are integral parts of the compound's lattice structure, often referred to as "water of hydration."
In the formula for a hydrated compound such as \( \mathrm{CuCl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O} \), the dots symbolize the association of water molecules with the principal substance. The \( 2 \mathrm{H}_{2} \mathrm{O} \) notation indicates that there are two water molecules for each \( \mathrm{CuCl}_{2} \) formula unit. Understanding this concept is essential, as the water included can significantly affect the physical properties and reactions of the compound.
Key points about hydrated compounds include:

• They are common in nature and form through crystallization from aqueous solutions.
• The water of hydration can be easily removed by heating, which is important for various analytical chemistry tools.

The Process of Chemical Water Loss

When heating a hydrated compound like \( \mathrm{CuCl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O} \), you initiate a process called `chemical water loss`, where the water molecules are driven off due to heating. This process usually results in a change in the compound's physical state.
For \( \mathrm{CuCl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O} \) when heated, the structurally bound water evaporates or leaves the compound, causing a noticeable loss in mass. This is because the water is converted into water vapor, which escapes into the air.
Some important aspects to consider:

• The process is usually endothermic, meaning it requires energy input (heat).
• The removal of water can impact the compound's structure and properties, potentially converting it into a different form.
• This process is reversible, as rehydration can occur if exposed to moisture again.

Recognizing that mass change due to water loss is indicative of the water bound in the compound offers a good basis for calculations, like finding mass percent.

Conducting Mass Change Analysis

Mass change analysis is a critical technique used in chemistry to understand how mass is transferred or lost in reactions or processes. In the context of hydrated compounds, this analysis helps determine how much of the compound's mass is due to water.
In the exercise provided, we calculated that the mass lost upon heating was \(0.509 \, \text{g}\). This value represents the mass of water expelled from the \(\mathrm{CuCl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O}\). By knowing the mass of the original sample, we can calculate the mass percentage of the hydrated compound.
Steps involved in the process generally include:

• Accurately measuring the initial mass of the hydrated compound.
• Applying heat to drive off the water of hydration and measuring the mass again.
• Subtracting the two masses to identify the amount of water lost.

Such analyses are fundamental in laboratories across various scientific fields, as they help characterize substances and understand their compositions by simple and effective means.