(a) A sample of tetrachloroethylene, a liquid used in dry...

(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 usupercritical fluid" that is a much safer dry-cleaning agent than tetrachlorosthvlene, At a certain pressure, the density of super critical $\mathrm{CO}_{2}$ is 0.469 $\mathrm{g} / \mathrm{cm}^{3} .$ What is the mass of a 25.0 $\mathrm{-mL}$ sample of supercritical $\mathrm{CO}_{2}$ at this pressure?

Key Concepts

Supercritical Fluids

Supercritical fluids occur when a substance is subjected to temperatures and pressures above its critical point, where it exhibits properties of both gases and liquids. This state of matter features unique characteristics, such as enhanced solvation power and tunable density, making supercritical fluids useful in various industrial processes, including environmentally friendly cleaning methods.

Density

Density is defined as the mass of a substance divided by its volume. It is a fundamental property used to describe how compact a material is. Calculating density involves dividing the mass by the volume, yielding a value that can help determine how substances compare in terms of compactness or heaviness per unit volume.

Buoyancy and Relative Density

Buoyancy is the force that causes objects to float or sink when placed in a fluid, and it is determined by comparing the density of the object to the density of the fluid. An object that is less dense than the fluid will float, whereas an object with a greater density will sink. This concept is crucial for understanding the behavior of materials in different liquids.

Transcript

00:01 The first part of this question asks us to look at tetrachlorathylene and basically ask us to calculate the density.

00:11 So we should know that density is a measure of mass over volume.

00:19 And they actually give us the mass of the sample here as 40 .55 grams and the volume as 25 .0 milliliters.

00:31 So when we divide this out, we get 1 .622 grams per milliliter for the density of the tetrachloriaethylene.

00:44 Notice we just have three significant figures that were limited to because of the volume measurement.

00:52 There are four significant figures in the mass, but only three significant figures in the volume.

00:57 So we would have to round this answer then to three significant figures at 1 .62 figures.

01:04 Grams per milliliter and that would be the density then of the tetrachlorathylene.

01:11 Another part of the question had asked us will it float on water? so the density of water is 1 .0 grams per milliliter and in order for something to float in water it has to have a lower density than water.

01:30 Well tetrachlorathylene has a higher density at 1 .62 grams per millimeter higher than the 1 .0 grams per milliliter of water.

01:40 So the answer that would be no, it does not float on water because its density is higher than water.

01:54 Now the second part of the question deals with the new substance, a super critical form of carbon dioxide.

02:01 And they give us the density of the carbon dioxide as 0 .469 grams per cubic centimeter...

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