Molarity Podcast Lecture Notes

Introduction to Molarity

• Concept: Molarity is a unit used to quantify the concentration of a solution.
• Visual Aid: Solutions with varying molarities can be visually identified (more concentrated solutions appear darker).
• Definition: Molarity = moles of solute ÷ liters of solution.
  • Units: Moles per liter (M).

Importance of Molarity

• The most common concentration unit in chemistry.
• Essential for solving chemistry problems involving solutions.

Example Calculations

Example 1: Sodium Chloride Solution

• Problem: Calculate the molarity of a 2-liter solution containing 0.4 moles of NaCl.
• Equation:
  • Molarity = Moles of solute / Liters of solution
  • M = 0.400 moles / 2.00 liters = 0.200 M
• Notation: Use capital M for molar.
• Significant Figures: Maintain the least number of significant figures from values used (3 sig figs).

Example 2: Potassium Nitrate Solution

• Problem: Find the molarity of 56.7 grams of KNO3 dissolved in 3.50 liters of solution.
• Conversion:
  • Convert grams to moles (KNO3 = 101.11 g/mol).
  • Moles = 56.7 g / 101.11 g/mol = 0.561 moles
• Calculation: M = 0.561 moles / 3.50 liters = 0.160 M
• Significant Figures: 3 significant figures.

Example 3: Finding Grams from Molarity

• Problem: How many grams of KNO3 are needed for 1.50 liters of 0.5 M solution?
• Steps:
  • Solve for moles: 0.5 M = x moles / 1.50 liters → x = 0.750 moles
  • Convert moles to grams: 0.750 moles × 101.11 g/mol = 75.8 grams

Example 4: Sodium Hydroxide Solution

• Problem: Determine molarity of a solution with 450 mL volume containing 12.45 grams of NaOH.
• Conversions:
  • Convert grams to moles: NaOH (~40.00 g/mol)
  • Moles = 12.45 g / 40.00 g/mol = 0.3113 moles
  • Convert mL to liters: 450 mL = 0.4500 liters
• Calculation: M = 0.3113 moles / 0.4500 liters = 0.6918 M
• Significant Figures: 4 significant figures maintained.

Conclusion

• The lecture covered various examples of how to calculate molarity.
• Encouraged to practice with the packet provided and ask questions in class.