The word ‘stoichiometry’ is derived from two Greek words- ‘stoicheion’ (meaning element) and ‘Metron’ (meaning measure).
Stoichiometry, thus deals with calculation of masses (sometimes volumes also) of the reactants and the products involved in the chemical reaction.
Let us consider the combustion of Methane. A balanced equation for this reaction is as given below.
Here Methane and Dioxygen are called reactants and Carbon Dioxide and Water are called products.
indicates that all the reactants and products are gases.
The coeffcients for and are called stoichiometric coeffcients.
Thus, according to the above chemical reaction:
One mole of reacts with two Moles of to give one mole of and two moles of .
One molecule of reacts with molecules of to give one molecule of and molecules of .
of reacts with of to give of and of .
of reacts with of to have of and of .
From the above relationships;
Limiting Reactant / Reagent
Sometimes, in a chemical reaction, the reactants present are not the amount required according to the balanced equation. The amount of products formed then depends upon the reactant which has reacted completely. This reactant reacts completely in the reaction is called Limiting Reactant/Reagent. The reactant which is not consumed completely in the reaction is called an Excess Reactant.
Reactions in Solutions
The concentration of a solution can be expressed in any of the following ways:
It is the mass of the solute in grams per of the solution.
2. Volume Percent
It is the number of units of volume of the solute per units of the volume of the solution.
Parts per Million (PPM) and Parts per Billion (PPB)
It is convenient to measure the concentration of trace quantities of solutes in parts per million and parts per billion. It is independent of temperature.
The normality of a solution is equal to the number of gram equivalents of solute present per liter of solution.
The molarity of a solution is equal to the number of moles of the solute present per liter of solution. It can be calculated from the following formulas:
(ii) If the Molarity and Volume of the solution are changed from to then (Molarity equation)
(iii) In balanced chemical equation, if moles of reactant one react with moles of reactant two, then
(iii) Volume of water added to get a solution of molarity from of molarity is
(iv) It two solutions of the same solute are mixed then molarity of resulting solution,
Here, are molarity and volume of initial solution and are molarity and volume of final solution.
The Relation between Normality and Molarity
Mathematically, Molality can be calculated by
Relation between Molarity (M) and Molality (m)
The Formality of a solution may be defined as the number of gram formula masses of ionic solute dissolved per liter of the solution.
Mole Fraction (X)
To find the Mole Fraction, divide moles of a constituent (either Solute or Solvent) by the total moles of both the constituents (Solute or Solvent)
Let in a Solution A is Solute and B is Solvent.
Mole fraction of Solute
Mole fraction of Solvent
and are number of moles of Solute and Solvent respectively.