Problem 75 - General Chemistry

Question

A liquid with a volume of \(8.5 \mathrm{~mL}\) has a mass of \(6.71 \mathrm{~g}\). The liquid is either octane, ethanol, or benzene, the densities of which are \(0.702 \mathrm{~g} / \mathrm{cm} 3,0.789 \mathrm{~g} / \mathrm{cm}^{3},\) and \(0.879 \mathrm{~g} / \mathrm{cm}^{3}\), respectively. What is the identity of the liquid?

Step-by-Step Solution

Answer

The liquid is ethanol.

Step 1: Calculate the Density

Density is defined as the mass divided by the volume. To find the density of the liquid, use the formula: \[ \text{Density} = \frac{\text{Mass}}{\text{Volume}} \]Given that the mass is 6.71 g and the volume is 8.5 mL, the density can be calculated as follows:\[ \text{Density} = \frac{6.71 \text{ g}}{8.5 \text{ mL}} \approx 0.789 \text{ g/cm}^3 \]

Step 2: Compare with Known Densities

Compare the calculated density with the densities of octane, ethanol, and benzene: - Octane: 0.702 g/cm³ - Ethanol: 0.789 g/cm³ - Benzene: 0.879 g/cm³ The calculated density of the liquid (0.789 g/cm³) matches exactly with the density of ethanol.

Key Concepts

Mass

In the world of chemistry and physics, **mass** is a fundamental concept used to describe the amount of matter in an object or substance. Mass is typically measured in grams (g) or kilograms (kg). It's a crucial property because it remains constant regardless of the substance's location or condition, unlike weight, which can change with gravity.
When working with liquids, determining mass is relatively straightforward using a balance. For example, in the problem provided, the liquid's mass is measured at 6.71 grams.

• Mass does not change with temperature or pressure.
• It is directly proportional to the number of particles in a given sample.

Understanding mass is essential for calculating other properties like density, which helps identify substances.

Volume

Volume measures the amount of space that a substance or object occupies. In liquids, it's typically measured in milliliters (mL) or liters (L). Volume is an important characteristic as it can affect the properties and behavior of substances.
The exercise specifies a volume of 8.5 mL for the liquid in question. Knowing the volume, along with mass, allows one to calculate the substance's density.

• Volume can change with temperature and pressure.
• It's crucial for determining concentration and other properties.

Remember that while mass remains constant, volume can be more variable, so always measure carefully in experiments.

Identification of Substances

The **identification of substances** relies heavily on their physical properties, such as density. Density is defined as mass divided by volume, and it provides a unique fingerprint for many substances. This characteristic is especially useful for identifying liquids, gases, and even solids.
In the given exercise, the liquid density is found by dividing its mass by its volume, resulting in a density of 0.789 g/cm³. This matches the known density of ethanol.

• Compare the calculated density with a list of known densities for identification.
• Common practice in labs to confirm the presence of a particular substance.

By utilizing these comparisons, one can efficiently identify the unknown substance.