Concentration and Solution Stoichiometry

Importance

• Many reactions in general chemistry occur in solutions, typically aqueous (water as solvent).
• Solutions are critical in lab work.

Key Concepts

Aqueous Solutions

• Solutions often in water (aqueous), indicated by "(aq)".
• Example: Sodium nitrate in solution is written as NaNO₃(aq).

Concentration

• Definition: Amount of solute in solvent.
• Can be qualitative (dilute/concentrated) or quantitative.
• Quantitative Measurement: Molarity (M)
  • Moles of solute per liter of solution.
  • Important to differentiate between volume of solvent and solution.

Molarity

• Most common concentration unit in general chemistry.
• Example: For sodium chloride solution, molarity indicates moles of NaCl per liter of solution.
• Preparation: Use a volumetric flask to measure precise volume.
  • Involves dissolving solute in stages, ensuring complete dissolution before filling to mark.
  • Use deionized water for purity.

Example Calculation

• Calculate molarity for a solution with 45.4g of sodium nitrate in 2.50L of solution.
  • Convert grams to moles using molar mass (84.995 g/mol).
  • Moles = 0.534.
  • Molarity = 0.534 moles / 2.50 L = 0.214 M.

Dilution

• Solutions often stored in concentrated form.
• Dilution Equation: M₁V₁ = M₂V₂
  • Used to find concentration/volume of diluted solution.
  • Example: Dilute 200mL of 15.0 M NaOH to 2.50 M.
    • Solved by finding final volume needed.

Stoichiometry in Solution

• Dissolution Examples:
  • Sugar (C₁₂H₂₂O₁₁): Dissolves as a whole molecule without ionization (non-electrolyte).
  • Sodium chloride (NaCl): Dissolves into ions (electrolyte).
• Ionic compounds (e.g., NaNO₃) break into ions but keep polyatomic ions intact.

Concentration of Ions

• Example: Sodium hydroxide (NaOH) produces Na⁺ and OH⁻ ions.
  • If 2.50 M NaOH, then concentration of OH⁻ is also 2.50 M.