Solution Concentration

Key Concepts

• Solute: The substance being dissolved (e.g., sodium chloride).
• Solvent: The substance doing the dissolving (e.g., water).
• Solution: Combination of solute + solvent.

Formulas

Mass Percent

• Formula: ( \text{Mass Percent} = \frac{\text{Mass of Solute}}{\text{Mass of Solution}} \times 100% )

Volume Percent

• Formula: ( \text{Volume Percent} = \frac{\text{Volume of Solute}}{\text{Volume of Solution}} \times 100% )

Mole Fraction

• Mole fraction of A: ( \frac{\text{Moles of A}}{\text{Total Moles}} )
• Mole fraction of B in a two-substance system: ( \frac{\text{Moles of B}}{\text{Moles of A} + \text{Moles of B}} )

Molarity ((M))

• Formula: ( \text{Molarity} = \frac{\text{Moles of Solute}}{\text{Liters of Solution}} )

Molality ((m))

• Formula: ( \text{Molality} = \frac{\text{Moles of Solute}}{\text{Kilograms of Solvent}} )

Density

• Formula: ( \text{Density} = \frac{\text{Mass}}{\text{Volume}} )
• Units: typically in grams/mL or grams/cm³

Common Conversions

• 1 mL = 1 cm³
• 1 L = 1000 mL
• 1 kg = 1000 g

Practice Problems

Problem 1

• Q: 15g NaCl in 225g water. Find mass percent of NaCl.
  • A: Solution mass = 240g. Mass percent = 6.25%.

Problem 2

• Q: 25mL methanol, 150mL water. Find volume percent of methanol.
  • A: Solution volume = 175mL. Volume percent methanol = 14.29%.
• Q: Find mass percent of water.
  • A: Mass percent water = 88.3%.

Problem 3

• Q: 2 mol KCl, 8 mol water. Find mole fraction of KCl.
  • A: Mole fraction = 0.2.

Problem 4

• Q: 25g NaF, 200g water. Find mole fraction of NaF.
  • A: Mole fraction = 0.051.

Problem 5

• Q: 15g NaBr in 400mL solution. Find molarity.
  • A: Molarity = 0.364 M.

Problem 6

• Q: 10g NaOH in 500g water. Find molality.
  • A: Molality = 0.5 m.

Problem 7

• Q: 15% AlCl3, density = 1.17 g/mL. Find molarity.
  • A: Molarity = 1.316 M.

---

Key Takeaways

• Always identify solute and solvent correctly.
• Understand and apply the formulas correctly for mass percent, volume percent, mole fraction, molarity, and molality.
• Use correct unit conversions and understand the importance of using appropriate units for calculations.
• Pay attention to problem specifications to determine solute and solvent if not directly stated.