Density Concepts and Calculations in GOB Chemistry

Density

Definition and Formula

Density is a fundamental property in chemistry that describes how much mass is contained within a given volume of a substance. It is commonly used to characterize solids, liquids, and gases.

• Density represents the amount of mass per unit of volume.

• The general formula for density is:

$\text{Density} = \frac{\text{mass}}{\text{volume}}$

• For solids and liquids, typical units are g/mL or g/cm3.

• For gases, which are less dense, units are often g/dm3 or g/L.

Example: Density Unit Conversion

Unit conversion is a common task in chemistry, especially when comparing densities expressed in different units.

• Example: If the density of an unknown metal is 2.14 g/cm3, express its density in lb/ft3.

• Step 1: Convert grams to pounds (1 lb = 453.592 g).

• Step 2: Convert cubic centimeters to cubic feet (1 ft3 = 28,316.8 cm3).

• Calculation:

$2.14\ \text{g/cm}^3 \times \frac{1\ \text{lb}}{453.592\ \text{g}} \times 28,316.8\ \text{cm}^3/\text{ft}^3 = 134.103\ \text{lb/ft}^3$

• Result: 2.14 g/cm3 = 134.103 lb/ft3

Practice Problem: Density and Parts Per Million (ppm)

Parts per million (ppm) is a unit of concentration often used in environmental and biological chemistry. It is useful for expressing very dilute concentrations of substances.

• Practice: When lead levels in blood exceed 0.80 ppm, the level is considered dangerous. 0.80 ppm means that 1 million g of blood would contain 0.80 g of Pb.

• Given the density of blood is 1.060 kg/L, how many grams of Pb would be found in 400.0 mL of blood at 0.620 ppm?

• Step 1: Convert 400.0 mL to L: $400.0\ \text{mL} = 0.400\ \text{L}$

• Step 2: Calculate mass of blood: $0.400\ \text{L} \times 1.060\ \text{kg/L} = 0.424\ \text{kg} = 424\ \text{g}$

• Step 3: Calculate mass of Pb: $0.620\ \text{ppm} = 0.620\ \text{g Pb}/1,000,000\ \text{g blood}$

• Step 4: $424\ \text{g blood} \times \frac{0.620\ \text{g Pb}}{1,000,000\ \text{g blood}} = 0.000263\ \text{g Pb}$

• Result: Approximately 0.000263 g of Pb would be found in 400.0 mL of blood at 0.620 ppm.

Summary Table: Common Density Units

The following table summarizes common units used for density in chemistry:

State of Matter	Common Density Units
Solids	g/cm3, kg/m3
Liquids	g/mL, g/cm3
Gases	g/L, g/dm3

Additional info: The notes also introduce the concept of ppm (parts per million) as a unit of concentration, which is relevant for toxicology and environmental chemistry.