stoichiometry

        Using a balanced chemical equation to calculate amounts of reactants and products is called stoichiometry. It is a super technical-sounding word that simply means using ratios from the balanced equation. In this article, we will discuss how to use mole ratios to calculate the amount of reactants needed for a reaction.

  The Mole      
       Given the equation above, we can tell the number of moles of reactants and products. A mole simply represents Avogadro's number (6.022 x 10²³) of molecules. A mole is similar to a term like a dozen. If you have a dozen carrots, you have twelve of them. Similarly, if you have a mole of carrots, you have 6.022 x 10²³ carrots. In the equation above there are no numbers in front of the terms, so each coefficient is assumed to be one (1). Thus, you have the same number of moles of AgNO₃, NaCl, AgCl, NaNO₃.
        Converting between moles and grams of a substance is often important. This conversion can be easily done when the atomic and/or molecular mass of the substance(s) are known. Given the atomic or molecular mass of a substance, that mass in grams makes a mole of the substance. For example, calcium has an atomic mass of 40 atomic mass units. So, 40 grams of calcium makes one mole, 80 grams makes two moles, etc.
      Describing the quantitative relationships among substances as they participate in chemical reactions is known asreaction stoichiometry. In the example above, reaction stoichiometry measures the relationship between the methane and oxygen as they react to form carbon dioxide and water.
Atomic Mass
1. relative atomic mass (Ar)
Measuring the mass is comparing the mass of an object to another, in which the mass of the reference object is called the standard mass.
2. Relative Molecular Mass and Mass Relative Formula (Mr)
The mass of a molecule of a compound is called the relative molecular mass (Mr). The magnitude of the relative molecular mass of a compound is the sum of the relative atomic mass (Ar) of its constituent elements.
Mr AxBy = (x  Ar A + y Ar B)
3. Average atomic mass
Atoms of the same element do not always have the same mass. This is known as isotopes. The atoms in nature can have different masses, then the atomic mass is calculated on the average mass of all the atoms in nature.
Molar proportion
Stoichiometry is often used to balance chemical equations (reaction stoichiometry). For example, the two diatomic gases, hydrogen and oxygen, can combine to form a liquid, water, in an exothermic reaction, as described by the following equation:

2 H
2 + O
2 → 2 H
2O

Reaction stoichiometry describes the 2:1:2 ratio of hydrogen, oxygen, and water molecules in the above equation.
The molar ratio allows for conversion between moles of one substance and moles of another. For example, in the reaction

2 CH
3OH + 3 O
2 → 2 CO
2 + 4 H
2O

the amount of water that will be produced by the combustion of 0.27 moles of CH
3OH is obtained using the molar ratio between CH
3OH and H
2O of 2 to 4.

${\displaystyle \left({\frac {0.27{\mbox{ mol }}\mathrm {CH_{3}OH} }{1}}\right)\left({\frac {4{\mbox{ mol }}\mathrm {H_{2}O} }{2{\mbox{ mol }}\mathrm {CH_{3}OH} }}\right)=0.54\ {\text{mol }}\mathrm {H_{2}O} }$

The term stoichiometry is also often used for the molar proportions of elements in stoichiometric compounds (composition stoichiometry). For example, the stoichiometry of hydrogen and oxygen in H₂O is 2:1. In stoichiometric compounds, the molar proportions are whole numbers.