Definition
Mass is a physical property of matter. It is one of the basic quantities of the International System of Units (SI). The kilogram (kg) is used as the unit of mass. The mass of an object is equal to the sum of the atomic masses of all the atoms it contains.
Kilogram and gram
The following relationships apply:
- 1 kilogram (kg) = 1000 grams (g).
- 1 gram (g) = 1/1000 kg (one thousandth of a kilogram).
- 1 milligram (mg) = 1/1000 g (one thousandth of a gram).
- 1 microgram (µg, mcg) = 1’000’000 g (one millionth of a gram).
Mass and weight
In colloquial language, the term “weight” is often used for mass. However, in physics, these two quantities are not identical. The weight is one with the unit Newton (N), which depends on the mass and the gravitational acceleration:
- FG (weight force) = m (mass) x g (gravity acceleration, about 9.81 m/s²).
The mass is the same everywhere, but the weight is not – on the Moon, for example, it is only 16% of the weight on Earth. Incidentally, a normal body scale measures the weight force and calculates the mass from it. In science, we should always speak of “mass” and not “weight”.
Meaning in pharmacy
Weighing substances is one of the everyday activities in a pharmacy. Mass is of great importance in pharmacy for the manufacture of drugs, for weighing open goods, for dosage, and as an indication of size in sales. The dose is often given in milligrams, grams and, less frequently, micrograms. Examples of single doses of active ingredients:
- Mesalazine 1.5 g (1500 mg).
- Acetylsalicylic acid 500 mg
- Ibuprofen 400 mg
- Perindopril 5 mg
- Salbutamol 200 µg
- LSD (hallucinogen) 25 µg
The mass of excipients in a drug usually exceeds the mass of active ingredients.
Determination of a mass
The mass can be determined with a balance. This, as mentioned earlier, measures the weight force and converts it to mass. If the volume and density are known, the mass can be calculated from these two quantities.
Density
Through density, there is a close relationship of mass to its volume (m3):
Accordingly:
The unit of density is kilograms per cubic meter (kg/m3), the unit of volume is the cubic meter (m3). Alternatively, density is also frequently used to mean g/cm3 is often used for density. See also under the article Density.
