Stoichiometry

Jeremias Richter, a German chemist, coined the term Stoichiometry that refers to the quantitative analysis of the  involved reactants and products of a chemical reaction. This word is derived from Greek words, “Stoikhein”, meaning element and “metron” meaning measure. 

What Is Stoichiometry?

We can define Stoichiometry in simple words as the calculation of reactants as well as products involved in a particular chemical reaction. 

Stoichiometry is a concept that helps in using balanced chemical equations to calculate amounts of reactants and products. In general, all the reactions are dependent on how much substance is present.

Stoichiometry helps in determining how much substance is needed for a reaction or how much is present in it. It helps measure the mass of reactants and products, their molecular weight, chemical equations and formulas. 

Balanced Reactions and Mole Ratios

Atoms or molecules are so tiny in size that their numbers will be larger, even for a small quantity of a substance. In order to represent such atoms or molecules in large quantities, a mole concept was evolved. 

One mole of any substance will have 6.022 x 1023 numbers of that particular substance. This number is called Avogadro’s number.

Molar mass is the mass of a substance in grams per one mole. It will be numerically equivalent to the atomic/molecular mass.

3Fe(s) + 4H₂O(l) ⇾ Fe₃O₄ (s)+ 4H₂ (g)

The quantitative evaluation of this balanced chemical equation is

1. 3 molecules of Fe when reacted with 4 moles of H₂O will produce one mole of Fe₃O₄ and 4 moles of H₂.
2. 168g ( 56×3) of Fe reacted with 72g( 18×4) of H₂0 to produce 231g of Fe₃O₄ and 8g of H₂ gas.

When both the reactants and products are in their gaseous form, their molar volume will be taken into account. One mole of any gas occupies a volume of 22.4 Liters.

CH₄(g) + 2O₂(g)⇾ CO₂(g)+ 2H₂0 (g)

In the given reaction, 22.4 Litres of CH₄ is reacting with 44.8 (2 x 22.4) liters of O₂ leading to the production of 22.4 Liters of CO₂ and 44.8 litres of H₂O. 

Limiting Reagent

In a chemical reaction, one of the reactants might be present in excess amounts. The  reaction stops immediately when one of the reactants is totally utilized. This leads to excess reactants being left over at the end of the reaction. When a substance is consumed completely in a reaction it is called the limiting reagent.

Let us check an example to understand this concept.

N₂ + 3H₂ ➝ 2NH₃

When 1 mole of N₂ is reacting with 1 mole of H₂, to balance the chemical equation, we need three moles of H₂ reacting with one mole of N₂. Therefore, the limiting reagent here is H₂.

Stoichiometry in Chemical Analysis

Stoichiometric calculations that follow the methodology of quantitative analysis are usually used to determine the substance concentration in a sample. There are two types of analysis which are discussed below.

1. Gravimetric Analysis

The gravimetric analysis is the quantitative determination based on the mass of an analyte. It gives the most accurate results when compared to all other analytical analysis because the weight of the substance is measured with better accuracy in comparison to other fundamental quantities. Gravimetric analysis are of the following types.

1. Precipitation gravimetry – It is the isolation of ions in a solution by a precipitation reaction, followed by filtering, cleaning the precipitate to remove contaminants, and finally weighing it and determining its mass by difference.
2. Volatilization gravimetry – It involves separation of components in a mixture with the help of heat or chemical decomposition of the sample.
3. Electrogravimetry – It is the electrochemical reduction of metallic ions and their deposition on the cathode. Weighing the cathode before and after the electrolysis process reveals their weight. Their  weight difference indicates the mass of the analyte that was present in the sample before the process. 

2. Volumetric Analysis

The process of volumetric analysis deals with the quantitative measurement of any substance in volume.

Principle: In volumetric analysis, a substance whose volume (V1) and concentration (N1) are known is reacting with a substance whose volume is unknown (V2) and whose concentration(N2) needs to be calculated. The volume V1 is noted at the end of the reaction. The concentration N2 is calculated using the following equation.

N1x V1 = N2 x V2

The endpoint of this reaction is marked by the colour change or precipitation.

Common terms associated with volumetric analysis;

1. Titration – This process is used to determine the quantity of a particular substance by adding another substance to it in parts till the equivalence point has been achieved. 
2. Titrant – The solution whose strength is known, is called a titrant.
3. Titrate – The solution whose concentration you need to estimate. 
4. Indicator – The reagents that change their colour at the end of the reaction which indicates the completion of the reaction.