Problem 29 - Chemistry: The Central Science

Question

(a) A sample of tetrachloroethylene, a liquid used in dry cleaning that is being phased out because of its potential to cause cancer, has a mass of \(40.55 \mathrm{~g}\) and a volume of \(25.0 \mathrm{~mL}\) at \(25^{\circ} \mathrm{C}\). What is its density at this temperature? Will tetrachloroethylene float on water? (Materials that are less dense than water will float.) (b) Carbon dioxide \(\left(\mathrm{CO}_{2}\right)\) is a gas at room temperature and pressure. However, carbon dioxide can be put under pressure to become a "supercritical fluid" that is a much safer dry-cleaning agent than tetrachloroethylene. At a certain pressure, the density of supercritical \(\mathrm{CO}_{2}\) is \(0.469 \mathrm{~g} / \mathrm{cm}^{3}\). What is the mass of a \(25.0\)-mL sample of supercritical \(\mathrm{CO}_{2}\) at this pressure?

Step-by-Step Solution

Answer

The density of tetrachloroethylene is \(1.622 \mathrm{~g/mL}\) at 25°C, and it will not float on water. The mass of a 25.0-mL sample of supercritical CO2 at this pressure is \(11.725 \mathrm{~g}\).

Step 1: Finding the density of tetrachloroethylene

To find the density, use the formula: \(density = \frac{mass}{volume}\). The mass of tetrachloroethylene is given as 40.55 g and its volume is 25.0 mL. The density of tetrachloroethylene can be calculated as: \(density = \frac{40.55 \mathrm{~g}}{25.0 \mathrm{~mL}}\)

Step 2: Calculate the density of tetrachloroethylene

Now, divide the mass by the volume to find the density: \(density = \frac{40.55}{25.0} = 1.622 \mathrm{~g/mL}\) The density of tetrachloroethylene is \(1.622 \mathrm{~g/mL}\) at 25°C.

Step 3: Determine if tetrachloroethylene will float on water

To determine if tetrachloroethylene will float on water, compare its density to that of water. The density of water is approximately \(1.00 \mathrm{~g/mL}\). Since the density of tetrachloroethylene is greater than the density of water, it will not float on water. Now, let's move to the second question (b).

Step 4: Use the given density of supercritical CO2 to find its mass

We are given the density of supercritical CO2 as \(0.469 \mathrm{~g/cm^3}\) and asked to find the mass of a 25.0 mL sample. First, notice that mL is equivalent to cm3. Begin by using the density formula and rearranging for mass: \(mass = density \times volume\)

Step 5: Calculate the mass of supercritical CO2

Now, substitute the given values to find the mass: \(mass = 0.469 \mathrm{~g/cm^3} \times 25.0 \mathrm{~cm^3}\) \(mass = 11.725 \mathrm{~g}\) Therefore, the mass of a 25.0-mL sample of supercritical CO2 at this pressure is \(11.725 \mathrm{~g}\).

Key Concepts

Tetrachloroethylene

Tetrachloroethylene is a clear, colorless liquid primarily used in dry cleaning and metal degreasing applications. Known for its strong solvent properties, it can dissolve a wide array of organic materials. However, tetrachloroethylene is gradually being phased out due to its potential carcinogenic effects, posing environmental and health risks.
Its chemical formula is C₂Cl₄, and it has a significant density of 1.622 g/mL at 25°C, as determined by dividing its mass by its volume. This value is greater than the density of water (1.00 g/mL), so tetrachloroethylene will not float on water. Despite its sturdy industrial use, its future relies on safer alternatives to minimize harmful impacts, particularly in dry cleaning practices.

Supercritical Fluid

A supercritical fluid represents a state of matter where it manifests both liquid and gas characteristics. This state is achieved when a substance is subjected to specific temperature and pressure conditions surpassing its critical point. In this unique state, molecules experience enhanced solubility and diffusion, making supercritical fluids excellent solvents for industrial processes.
Supercritical carbon dioxide (CO₂) is widely recognized as an effective substitute for traditional solvents like tetrachloroethylene due to its lower toxicity and environmental impact. This form of CO₂ can effectively clean fabrics without the health hazards associated with traditional solvents. Its density can be controlled by pressure adjustments, offering flexibility across different applications.

• Eco-friendly solvent
• Efficient at dissolving substances
• Tunable properties by adjusting conditions

Molecular Density

Molecular density refers to the mass of molecules within a particular volume. In the context of substances like tetrachloroethylene and supercritical carbon dioxide, understanding molecular density is essential for practical applications like material separation and solvent efficacy.
For tetrachloroethylene, which is denser than water, it will sink when mixed in an aqueous environment, reflecting its high molecular density. This property directly impacts its behavior in various physical processes and the environment. On the other hand, supercritical CO₂ has a lower molecular density at specific conditions, making it suitable as a gentle and precise cleaner. The ability to calculate density helps in determining the mass of substances, which is crucial for processing and safety considerations in industries. Studying molecular density can demystify how different substances interact under various circumstances.

Chemical Properties

Chemical properties describe a substance's ability to undergo chemical changes or reactions due to its composition. These intrinsic characteristics include reactivity, toxicity, pH, and more.
Tetrachloroethylene features chemical stability, meaning it doesn't easily react with other chemicals, attributed mainly to its strong carbon-chlorine bonds. However, it's also hazardous as it can release toxic compounds when broken down or heated. These properties have led to concerns over its long-term environmental presence and impact on health.
Conversely, supercritical carbon dioxide offers an attractive balance of low chemical reactivity and efficient solvent properties. It is seen as a greener substitute, favoring environments where reducing toxic emissions is critical. Understanding these chemical properties is crucial when selecting appropriate substances for industrial applications, especially where safety and environmental health are priorities.