The pressure is a widely used quantity; it is found in physics, chemistry, engineering, and biology. Because of the wide usage of pressure, it has adopted various units to express it. And it is necessary for us to know the relationship between various units of pressure.

The mole is a unit of measurement. It is one of the seven fundamental SI base units, others are the kilogram, the second, the metre, the ampere, the kelvin, the candela. The mole is the SI unit of amount of substance. The approximate value of one mole of any substance is 6.022 × 10²³. The symbol mol is used to denote it.

The formula mass is often confused with the molar mass or the molecular mass, but it differs from both. As the name suggests, it has some relation with the formula of a molecule. The formula mass is a mass calculated using the formula. It is determined by adding the average atomic masses of the atoms present in a molecule.

The atomic weight (also known as relative atomic mass) is a quantity used to express the average weight of an atom. Atoms consist of electrons, protons, and neutrons. Protons and neutrons are mainly responsible for the mass of an atom. For a given element, the proton number (more commonly known as the atomic number) is fixed, but the neutron number can vary. Such elements are called isotopes. Because of this variance in the neutron number, an atom of the same element can have a different atomic mass.

An atom comprises electrons, protons, and neutrons. Electrons revolve around the nucleus of an atom just like the Earth around the Sun. Protons and neutrons reside in the centre of an atom called nucleus. Electrons are negatively charged while protons are positively charged. Neutrons are neutral i.e., no charge. In an unbounded stable atom, the number of protons equals the number of electrons. This makes the net charge of an atom zero. These number of protons are unique for a given element, and the number is called the atomic number or proton number.

The molar mass constant is a quantity similar to the atomic mass constant. The constant, as from the name, is related to the molar mass of carbon-12.

The atomic mass constant is used to define the atomic mass unit (or the unified atomic mass). It equals one atomic mass unit. The atomic mass unit is the standard unit used to quantify mass on the atomic scale. Also, the atomic mass unit, the unified mass unit, and the dalton are all synonymous units and are interchangeably used.

The atomic mass unit is also known as the unified mass unit or the dalton. It is a standard non-SI unit of mass. It is used to quantify mass on the atomic scale, for example, the mass of a proton, the mass of an electron, the mass of iron.

The number of nucleons present in an atom is called mass number. Mass number, as from the name, is responsible for the mass of an atom or atomic mass. Since protons and neutrons are much heavier than electrons, the mass of an atom is estimated by the number of protons and the number of neutrons or in other words, mass number. Mass number is always a whole number because the number of protons and neutrons present in an atom is always a whole number.

The Gold Book of the International Union of Pure and Applied Chemistry (IUPAC) defines atomic mass as “rest mass of an atom in its ground state”. In simple words, atomic mass is the mass of an atom of an element.