The word stoichiometry has its origin in the Greek words stoicheion (elementary constituent) and metrin (to measure). **The conventional meaning of the word limits its application to calculations involving the quantities of materials involved in chemical reactions.** But in chemical engineering the word stoichiometry is used in a broader sense; it is not just the application of the law of combining proportions of chemical reactions. It includes calculations involving the behavior of gases and gas mixtures, the phase behavior of liquids and solutions, the material and energy balance of unit operations and unit processes etc. The conventional application of stoichiometry is concerned with the mass relations involved in chemical reactions between the reactants on the hand and the products on the other. The strength of the mole concept is in dealing with chemical reactions. Since a mole of a compound reacts with a simple multiple of moles of other reactants, it is easier to use molal quantities such as mol, kmol in mass balance calculations of processes in which chemical reactions are involved.

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Example problems of stoichiometry are given below:

### Solved Examples

**Question 1: **Sulphur trioxide gas is obtained by the combustion of iron pyrites(FeS_{2}). Write down the chemical reaction and balance this equation according to the stoichiometry of each compound?

** Solution: **

According to the question, the equation for chemical reaction is

**FeS**_{2 }+ O_{2 }$\to$ Fe_{2}O_{3 }+ SO_{3}

(While combustion compounds react with the oxygen)

Balancing the above equation according to stoichiometry is,

**4FeS**_{2} + 15O_{2 }$\to$ 2Fe_{2}O_{3} + 8SO_{3}

**Question 2: **The reaction of ammonia with solid copper oxide give the nitrogen gas, solid copper and water vapor. Write this reaction and balancing the equation according to its stoichiometry?

** Solution: **

The described question is in the form of,

NH_{3} + CuO $\to$ Cu + H_{2}O + N_{2}

Balancing the above equation according to its stoichiometry,

2NH_{3 }+ 3CuO $\to$ 3Cu + 3H_{2}O + N_{2}

According to the question, the equation for chemical reaction is

(While combustion compounds react with the oxygen)

Balancing the above equation according to stoichiometry is,

The described question is in the form of,

NH

Balancing the above equation according to its stoichiometry,

2NH