Overview
This lecture explains why water is a powerful solvent, introduces concepts like polarity, electrolytes, and solution concentration, and demonstrates how to calculate and dilute solutions using molarity.
Properties of Water
- Water (dihydrogen monoxide) makes up about 60% of the human body and is crucial for chemical reactions.
- Water excels as a solvent due to its abundance, liquid state range, and polarity.
- Polarity arises because oxygen in water is more electronegative than hydrogen, creating partial charges on the molecule.
- Polar molecules like sugar dissolve easily in water as water molecules surround and separate them.
Solutions and Solvents
- A solution consists of a solvent (e.g., water) and a solute (e.g., hydrogen peroxide) that is dissolved.
- Aqueous solutions are water-based solutions, important for many types of chemistry.
- Water dissolves polar and ionic substances but does not dissolve nonpolar substances like oil.
Electrolytes and Conductivity
- Water’s dielectric property reduces attraction between charged ions, helping dissolve salts into ions.
- Electrolytes are ions in water that conduct electricity, essential for body functions and solution classification.
- Pure water does not conduct electricity, but dissolved ions allow conductivity.
- Strong electrolytes (e.g., salts, strong acids/bases) dissociate completely; weak electrolytes (e.g., acetic acid) only partially.
- Some substances (e.g., sugar, alcohol) dissolve but do not conduct electricity (nonelectrolytes).
Solution Concentration: Molarity and Molality
- Molarity (M) = moles of solute per liter of solution; molality = moles of solute per kilogram of solution.
- Example: 3% (weight/volume) hydrogen peroxide solution contains 30g H2O2 per liter; molarity = 0.88 M.
- Molar mass of H2O2 is 34.014 g/mol.
Dilution of Solutions
- To dilute a solution, add more solvent while the moles of solute remain constant.
- Dilution equation: M₁V₁ = M₂V₂ (M = molarity, V = volume).
- Example: To prepare 38L of 0.1M H2O2 from 0.88M stock, add 0.24L (240mL) of stock to water.
Key Terms & Definitions
- Solvent — the liquid in which another substance is dissolved.
- Solute — the substance dissolved in a solvent.
- Aqueous Solution — a solution with water as the solvent.
- Polarity — uneven distribution of electrical charge in a molecule.
- Electrolyte — a substance dissolved in water that conducts electricity.
- Molarity (M) — moles of solute per liter of solution.
- Molality — moles of solute per kilogram of solution.
- Strong Electrolyte — fully dissociates in water and conducts electricity well.
- Weak Electrolyte — partially dissociates in water; weakly conductive.
- Nonelectrolyte — dissolves in water but does not conduct electricity.
Action Items / Next Steps
- Review definitions and formulas related to solutions, electrolytes, and molarity.
- Practice calculating molarity and using the dilution equation.
- Prepare for upcoming lessons on oxidation and chemical reactions.