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.