Molarity (M): Moles per Liter
Molarity (M) is the most common way to express the concentration of a solution in chemistry. It tells you the number of moles of solute (the substance being dissolved) per liter of the total solution. For example, a 1 M (one molar) solution of salt means there's one mole of salt dissolved in enough water to make one liter of solution.
Key points about Molarity:
- It's based on the volume of the solution, which can change slightly with temperature. So, molarity is temperature dependent.
- It's widely used in labs for reaction calculations and preparing standard solutions because it directly relates to the number of particles involved in a chemical reaction.
- Essential for dilution series where you make weaker solutions from stronger ones.
Normality (N): Equivalents per Liter
Normality (N) is another concentration unit that focuses on the "reactive power" of a solution. It tells you the number of gram equivalents of solute per liter of solution. An "equivalent" is the amount of a substance that can react with or provide one mole of hydrogen ions (H⁺) in an acid-base reaction, or one mole of electrons in a redox reaction.
Important aspects of Normality:
- It's reaction dependent: The value of 'equivalents' changes based on the specific chemical reaction (acid-base, redox, precipitation).
- Often used in acid-base reactions and redox reactions because it simplifies calculations for these types of reactions.
- It directly relates to the reacting capacity of a substance.
Equivalence: The Reactive Unit
The concept of equivalence is central to normality. It represents the amount of a substance that can participate in a specific chemical reaction. The number of equivalents per mole ('n' factor) depends on the type of reaction:
- For acids: The number of replaceable H⁺ ions (e.g., HCl has 1 equivalent/mol, H₂SO₄ has 2 equivalents/mol).
- For bases: The number of replaceable OH⁻ ions (e.g., NaOH has 1 equivalent/mol, Ca(OH)₂ has 2 equivalents/mol).
- For redox reactions: The number of electrons gained or lost per mole of the substance.
- For precipitation reactions: The charge of the ion involved.
Understanding equivalence helps ensure that reactants are mixed in the correct proportions for a complete reaction.
Titration: Measuring Unknown Concentrations
Titration is a common laboratory method used to find the unknown concentration of a solution. It involves slowly adding a solution of known concentration (the titrant) to a solution of unknown concentration (the analyte) until the reaction is complete. This point is called the equivalence point.
Applications of Titration:
- Quality Control: Ensuring products meet specific concentration standards in industries (e.g., food, pharmaceuticals).
- Environmental Monitoring: Measuring pollutants in water or air samples.
- Clinical Chemistry: Analyzing components in blood or urine.
- Research and Development: Determining reaction yields and purity of synthesized compounds.
- Acid-Base Neutralization: Finding the strength of acids or bases.