Stoichiometry is the branch of Chemistry that deals with the quantitative relationships between reactants and products in a chemical reaction. It helps us determine how much of each substance is needed or produced in a reaction. It is based on the Law of Conservation of Mass, which states that matter cannot be created or destroyed during a chemical reaction. Therefore, the number of atoms of each element remains the same before and after the reaction.
In general, all chemical reactions depend on one key factor: the amount of substance present. Stoichiometry helps measure and calculate important quantities, such as:
- Mass of reactants and products
- Number of moles involved
- Molecular and molar masses
- Volume of gases participating in reactions
Stoichiometric Coefficient
The stoichiometric coefficient is the number written in front of a chemical formula in a balanced chemical equation. It represents the number of molecules or moles of a substance participating in the reaction.
Consider the following equation:
aA + bB ⇌ cC + dD
The Stoichiometric coefficients of A, B, C, and D, respectively, are a, b, c, and d in this equation.
- Stoichiometric coefficients establish the mole ratio between reactants and products.
- These coefficients are usually written as whole numbers, although fractional coefficients can be used during intermediate steps.
Example:
- 3 moles of iron react with 4 moles of water
- 1 mole of iron oxide and 4 moles of hydrogen gas are formed
Balanced Reactions and Mole Ratios
- In Stoichiometry, calculations are based on a Balanced Chemical Equation.
- A balanced equation is a chemical equation in which the number of atoms of each element is the same on both sides of the equation.
- This follows the Law of Conservation of Mass, which states that matter cannot be created or destroyed during a chemical reaction.
- Balancing an equation ensures that the reactants and products are in the correct proportion.
Example:
2H2+O2→2H2O
The number of atoms on both sides is equal:
- Hydrogen atoms = 4 on both sides
- Oxygen atoms = 2 on both sides
- The Mole Ratio is the ratio of moles of one substance to another substance in a balanced chemical equation.
- The mole ratio is obtained from the coefficients of the balanced equation.
Example:
2H2+O2→2H2O
The mole ratios are:
- H2 : O2 = 2 : 1
- H2 : H2O = 2 : 2
- O2 : H2O = 1 : 2
Order of Balancing Chemical Equations (Stoichiometry)
While balancing a chemical equation, elements are balanced in a systematic order to ensure conservation of mass. The general rule is to start with elements that appear in the fewest compounds and leave the most common elements for the end.
- Write the unbalanced equation and list all atoms on both sides.
- Adjust stoichiometric coefficients (numbers in front of formulas), not subscripts.
- Polyatomic ions that remain unchanged (e.g., SO₄²⁻, NO₃⁻) should be treated as a single unit.
- Hydrogen and oxygen are balanced last because they appear in many compounds, making them harder to isolate.
- Finally, verify atom counts and reduce coefficients to the smallest whole-number ratio.
Limiting Reagent
- The Limiting Reagent (or limiting reactant) is the reactant that is completely used up first in a chemical reaction.
- Because it is used up first, it limits the amount of product that can be formed.
- In most chemical reactions, reactants are not always present in the exact required ratio.
- The reactant that finishes first stops the reaction, even if other reactants are still left.
- When two or more substances react together, the limiting reagent determines how much product will be produced.
- Once the limiting reagent is consumed, the reaction cannot continue further.
Example:
2H2 + O2 → 2H2O
According to the balanced equation:
- 2 moles of hydrogen react with 1 mole of oxygen.
Suppose we have:
- 2 moles of H2
- 2 moles of O2
But the reaction only needs 1 mole of O2 to react with 2 moles of H2.
So:
- Hydrogen will be completely used
- Oxygen will remain in excess
Therefore, hydrogen (H2) is the limiting reagent.
Stoichiometry in Chemical Analysis
Stoichiometric calculations are widely used in chemical analysis to determine the concentration or amount of substances in a sample. There are two main analytical methods:
1. Gravimetric Analysis
This method determines the quantity of a substance by measuring its mass. It is highly accurate because mass can be measured precisely.
Types include:
- Precipitation gravimetry – formation and weighing of a precipitate
- Volatilization gravimetry – separation by heating or decomposition
- Electrogravimetry – deposition of metal ions on an electrode
2. Volumetric Analysis
This method involves measurement based on volume. Where a solution of known concentration reacts with a solution of unknown concentration until the reaction is complete. The principle is:
Important terms:
- Titration – process of determining concentration
- Titrant – solution of known strength
- Titrate – solution of unknown strength
- Indicator – substance that shows completion of reaction by colour change
Solved Examples
Question 1: Calculate how much sodium hydroxide will be needed to make 500 mL of a 0.10 M solution.
Solution:
The molar mass of NaOH = 40g
Volume of NaOH= 500ml = 0.5 L
Molarity = 0.10M
Molarity = moles / volume in litres
So, weight of NaOH = molar mass of NaOH x volume x molarity
= 0.10 x 40 x 0.5
= 2 g
Question 2: To make 3 M 400 ml HCl, how much 11 M HCl should be diluted with water?
Solution:
Given that, M1 = 11M, and M2 = 3M
Also, V1 = ? , and V2= 400ml
Now, M1 x V1 = M2 x V2
V1 = (3×400) / 11
= 109 ml
Question 3: By reacting nitrogen with hydrogen, how many moles of nitrogen are required to make 8.2 moles of ammonia?
Solution:
The balanced chemical equation is N2 + 3H2 → 2NH3
2 mole of NH3 are produced from = 1 mole of N2
8.2 mole of NH3 are produced from = (1/2) x 8.2
= 4.1 mol of N2