The chemical formula is the record made with use of the standard symbols which characterizes structure of a molecule of any substance. For example, a formula all of the known sulfuric acid – H2SO4. It is easily possible to see that each molecule of sulfuric acid contains two atoms of hydrogen, four atoms of oxygen and one atom of sulfur. It is necessary to understand that it is only an empirical formula, it characterizes structure of a molecule, but not its "degree of structure", that is arrangement of atoms relatively each other.

Equivalent of any chemical element is called its such quantity which enters interaction with one moths of atoms of hydrogen. Interaction can consist in connection with hydrogen, or its replacement (in replacement reactions). The molar mass of an equivalent of an element is called, respectively, the mass of one of an equivalent.

The order of chemical reaction on any substance is an indicator of degree which concentration of this substance in the kinetic equation of reaction has. The order is zero, the first and second. How to define it for concrete reaction?

Practically everyone in school or student's days faced the solution of tasks in chemistry, and some continue to be engaged in them to this day, working in the field or just helping the child on study. Types of chemical tasks are tasks with equivalents, calculating which you can meet some difficulties.