Density is defined as the mass of a substance in air at a specific temperature (typically 25°) per unit volume of that substance at the same temperature.

#### Density may be calculated with the following equation:

Density = (mass of substance/volume of substance) at a particular temperature and pressure

Specific gravity (SG) is the unitless ratio of the density of a substance to the density of water at 4°, or [(g of substance/mL)/1.00 g/mL]. Alternatively, SG can be calculated at a particular temperature in some common units of density from density of substance per density of water.

#### Specific gravity (SG) may be calculated with the following equation:

SG = (weight of the substance)/(weight of an equal volume of water)

#### Density calculation

Calculate 2.3 g of activated charcoal powder occupies a bulk volume of 5.2 mL at 20° and 1 atm.

The density of activated charcoal powder can be calculated as follows:

Density = 2.3 g/5.2 mL = 0.44 g/mL

#### Specific gravity (SG) calculation

Calculate 125 g of glycerin occupies a volume of 99 mL at 25°. [Note—The density of water at 25° is 0.997 g/mL.]

The SG of glycerin can be calculated as follows:

SG = (125 g/99 mL)/(0.997 g/mL) = 1.266

#### Relation between density and specific gravity formula

Specific gravity (SG) is the unitless ratio of the density of a substance to the density of water at 4°, or [(g of substance/mL)/1.00 g/mL]. As water has a density of 1 gram/cm3 (at standard temperature and pressure), and since all of the units cancel, specific gravity is usually very close to the same value as density (but without any units).

**Reference:** USP [1160]

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