Chemistry: Amount of Substance

Chemistry: Amount of Substance

Basics of Amount of Substance

• Amount of substance is a standard measure in chemistry and is expressed in the unit moles.
• A mole provides a bridge between the atomic world (microscopic) and the laboratory (macroscopic) scales.
• The term mole refers to the number of atoms in exactly 12g of pure carbon-12. This is also known as Avogadro’s number.

Molar Mass

• Molar mass of a substance is a physical property defined as the mass of a given substance (chemical element or chemical compound) divided by the amount of substance.
• The molar mass of individual atoms can be determined by the periodic table. For example, the molar mass of carbon is approximately 12g/mol.
• For compounds, the molar mass is calculated by adding up the molar mass of all the atoms in a molecule.

Mole Calculations

• The amount of substance can be calculated using the formula: n = m/M where n = number of moles, m = mass and M = molar mass.
• Measurements of mass and volume can be converted to moles by using their respective molar masses or volumes.

Avogadro’s Law

• Avogadro’s Law states that equal volumes of gases, at the same temperature and pressure, contain an equal number of molecules.
• This law aids calculations between volume and amount of substance (in moles) in reactions involving gases.

Balanced Chemical Equations

• Balanced chemical equations provide the ratio of moles of one substance to another in a chemical reaction.
• This molar ratio can be used to convert moles of one substance in a reaction to moles of another.

Stoichiometry

• Stoichiometry is a branch of chemistry that deals with the quantitative relationships that exist among the reactants and products in chemical reactions.
• It can be used to calculate the amount of product that should form in a reaction, given quantities of starting materials.

Limiting Reactants

• In a chemical reaction, the limiting reactant is the substance that is completely consumed when the chemical reaction is complete.
• The amount of product formed is limited by this reactant because the reaction cannot continue without it. Identifying the limiting reactant is necessary for stoichiometric calculations.

Concentration

• In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. It can be expressed in terms of mass per unit volume.
• Concentration can be used to determine the amount of a substance when given the volume (or vice versa) using the formula: c = n/V where c = concentration, n = number of moles, and V = volume.

Solution Preparation

• Solutions of known concentrations can be prepared by dissolving a known mass of solute in a solvent, then diluting the solution to a known final volume.
• Serial dilution is a stepwise dilution of a substance in solution, and is commonly used to decrease a known concentration to a necessary, lesser concentration.