The mole fraction of a substance in the mixture is the ratio of the mole of the substance in the mixture to the total mole of the mixture. Since it is the ratio of moles to moles, it is a dimensionless quantity. The mole fraction is sometimes called the amount fraction.

The mass fraction of a substance in a mixture is the ratio of the mass of the substance to the total mass of the mixture. It is also known as mass per cent or percentage by mass when expressing in percentage. Since the mass fraction is a ratio of mass to mass, it is a dimensionless quantity.

The empirical formula is the formula formed by atoms of a compound (or a molecule) such that atoms of an element are in the simplest relative ratio.

A compound is a pure substance made up of different elements. So, it consists of atoms of different elements. Examples of compounds are water (H₂O), carbon dioxide (CO₂), hydrogen chloride (HCl), sulphuric acid (H₂SO₄), potassium hydroxide (KOH), methane (CH₄), acetic acid (CH₃COOH). All these compounds contain at least two atoms of elements.

In chemistry, the molar mass is an important quantity. It measures the mass of a mole of a given substance.

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.