Stoichiometry

Definition: The branch of chemistry which deals with the quantitative relationship between reactants and products in a balanced chemical equation is called Stoichiometry.

Principle: It is based on the Law of Conservation of Mass.

Importance:

• It helps to calculate the amounts of reactants required to produce a desired amount of product.
• It helps to determine the unknown masses using known masses from a balanced equation.

Statement: Matter can neither be created nor destroyed during a chemical reaction, but it can change from one form to another.

Explanation: The total mass of reactants must be equal to the total mass of products in a chemical reaction.

Definition: The representation of a chemical compound in terms of symbols and numbers (subscripts) is called chemical formula.

Types:

• Molecular Formula: Shows the actual number of atoms of each element in a molecule. E.g., Benzene ($C_6H_6$), $H_2O_2$.
• Empirical Formula: Shows the simplest whole number ratio between atoms in a compound. E.g., Benzene ($CH$), $H_2O_2$ ($HO$).
• Formula Unit: The simplest ratio of oppositely charged ions in an ionic compound. E.g., $NaCl$, $KBr$. (Ionic compounds are represented by empirical formulas).

Relationship: $Molecular Formula = n \times Empirical Formula$

Where $n = \frac{Molecular Mass}{Empirical Formula Mass}$

Example: For Benzene, Molecular Mass = 78, Empirical Mass (CH) = 13. So, $n = 78/13 = 6$. Molecular Formula = $6 \times (CH) = C_6H_6$.

Mole: The atomic mass, molecular mass, or formula mass of a substance expressed in grams is called a mole.

Avogadro’s Number ($N_A$): The number of particles (atoms, ions, or molecules) present in one mole of a substance. Its value is $6.022 \times 10^{23}$.

Examples:

• 1 mole of C atoms = 12g = $6.022 \times 10^{23}$ atoms
• 1 mole of $H_2O$ molecules = 18g = $6.022 \times 10^{23}$ molecules
• 1 mole of $NaCl$ formula units = 58.5g = $6.022 \times 10^{23}$ ions

Definition: The mass of one mole of a substance is called molar mass. Unit: g/mol.

Calculations:

• $H_2SO_4$: $(1\times 2) + 32 + (16\times 4) = 98$ g/mol
• $C_6H_{12}O_6$ (Glucose): $(12\times 6) + (1\times 12) + (16\times 6) = 180$ g/mol

Chemical Equation: Representation of a chemical reaction using symbols and formulas of reactants and products. E.g., $2H_2 + O_2 \to 2H_2O$.

Parts:

• Reactants: Substances that react (Left side).
• Products: Substances formed (Right side).

Balancing: Adjusting coefficients to ensure the number of atoms of each element is equal on both sides (Law of Conservation of Mass).

Mole Calculations:

• $Mass = Moles \times Molar Mass$
• $Moles = Mass / Molar Mass$
• $Particles = Moles \times N_A$