mass in chemistry



We explain the concept of mass from chemistry and what its units of measurement are. Also, the difference between weight and mass.

Mass basically refers to the amount of matter in a body.

What is mass in chemistry?

In chemistry, Mass (m) is understood as the amount of matter that a body possesses, or in the case of a chemical reaction, the specific amount of matter that comprises each of the reagents or products involved.

All bodies have a mass, whether they are in the state solid, liquid or gaseous, and this mass is composed of atoms, joined by chemical links to form more complex structures. The more atoms there are in a body, the greater its mass.

In most fields of chemistry, mass is considered a uniform and constant dimension, although in the physical Quantum has been theoretically shown that it is not an absolute value, but that it increases according to the speed of the body, as it approaches the speed of light (according to studies by Albert Einstein and the relativity).

According to the physics approach, mass can be defined in the following ways:

  • Inertial mass. It is a property that bodies have, which is that they cannot change their speed by themselves. Inertial mass gives a measure of the reaction of a body to any applied force.
  • Gravitational mass. It is a property of bodies that makes them have a gravitational interaction between them.

The numerical values ​​of gravitational mass and inertial mass are the same, although their physical definitions are different. That is why we speak only of mass, without specifying what mass it refers to.

So, since mass is the measure of the inertia of a body (resistance that matter puts to change its state of relative rest or relative motion), when the speed of a body increases, its inertia increases and it needs more and more energy to change your state of movement, that is, it is increasingly difficult to change your state of movement (its inertia is greater). Since mass is associated with inertia, it is said that mass increases with increasing speed. But none of this means that when a body increases its speed, the amount of matter that makes it up increases.

On the other hand, according to the chemical approach, the mass never varies during a given reaction, as established by the Law of Conservation of Mass (also called the Lomonosov-Lavoisier Law). Thus, the exact amount of matter that was had at the beginning of the chemical reaction and that obtained at the end must be exactly the same, although it will be organized differently: some will have changed state, others will have regrouped into different compounds. , etc.

In chemical sciences, different masses can be calculated depending on the compound, element or utility that the calculation requires, for example:

  • Atomic mass. It is the mass of an atom. It is defined as the mass of an isotope of chemical element at the same time, and not as the average mass of all the isotopes that make up the chemical element.
  • Molar mass It is the mass of a substance per unit quantity of substance. It is usually expressed in kg / mol or g / mol
  • Molecular mass. It is equal to the molar mass, but is defined for a single molecule, and not for 1 mole of molecules, that is, 1 mole of substance, as is the case of molar mass. It is expressed in atomic mass units or kg / mol.

Units of measurement for mass

The International System of Weights and Measures (SI) proposes the kilogram (kg) as the fundamental unit of measurement of mass, together with its equivalent units, such as the gram, milligram, etc. It is usually measured using scales, both traditional pan and modern electronic.

There is also the unit of the mole (mole), used to refer to the amount of substance based on the amount of its particles constitutive. One mole of any substance equals the quantity of itself that contains so many elemental units, such as atoms there are in twelve grams of carbon-12. Therefore,

1 mole = 6.0221429 × 1023 elemental units.

This unit is fundamental for chemistry, since it allows to study the reactions from the proportion of the participating subjects, many of which cannot be weighed ordinarily.

Difference between mass and weight

Although in common life they are considered more or less synonymous, mass (m) and weight (p) are different quantities. While mass refers to the amount of matter, weight has to do with the intensity with which said matter is attracted by a gravitational field. Therefore, mass is a more or less absolute dimension, while weight is a force which will vary according to the amount of mass and the distance the body is from the origin of gravity.

Similarly, weight as a force is measured in newtons (N), as opposed to mass, and is measured with a dynamometer (instrument for measuring forces), rather than a balance.

For example, an astronaut in his spacesuit registers a mass of 120 kg and a weight of 1200 N on the Earth, but after being transported to the Moon, where there is much less gravity, its weight decreases to 200 N, while its mass remains intact.

!-- GDPR -->