Stoichiometry Fundamentals: Chemical Calculations & Concentrations

Stoichiometry: Quantitative Chemical Relationships

In chemistry, stoichiometry (from the Greek "στοιχεῖον" = stoicheion (element) and "μέτρον" = metron (measure)) is the calculation of the quantitative relationships between reactants and products during a chemical reaction. These relationships can be deduced from atomic theory, although historically they were formulated without direct reference to the composition of matter, based on various laws and principles.

Molarity (Molar Concentration)

Molarity (M), or molar concentration, is the number of moles of solute per liter of solution. For example, if 0.5 mole of solute is dissolved in 1000 mL of solution, the concentration of that solute is 0.5 M (0.5 molar).

Molality

Molality (m) is the number of moles of solute per kilogram of solvent. To prepare solutions of a given molality in a solvent, a volumetric flask is not used, unlike with molarity. Instead, it can be prepared in a beaker by weighing with an analytical balance, after weighing the empty beaker to subtract its corresponding value.

Normality (N)

Normality (N) is the number of equivalents (eq-g) of solute (sto) per liter of solution (VSC).

The number of equivalents is calculated by dividing the total mass by the mass of an equivalent: n = m / m_eq, or as the product of the total mass and the number of equivalents per mole divided by the molar mass:

Mole Fraction

The mole fraction is a chemical unit that expresses the proportion of moles of solute relative to the total moles of the solution, calculated by adding the moles of solute(s) and solvent. To calculate the mole fraction of a homogeneous mixture, the following expression is used:

Covalent Bonds: Simple and Double Examples

• Simple covalent bond: Cl₂ (Cl-Cl)
• Double covalent bond: O₂ (O=O)

The Mole Concept

The mole is the amount of substance that contains Avogadro's number of any kind of unit (atoms, molecules, ions, etc.). This number is approximately 6.022 x 10²³.

Atomic Weight

Atomic weight (or molar mass) is the mass in grams of one mole of atoms of an element, typically expressed in units like g/mol. Other related units include mg/mmol or kg/kmol.