Concentration Units – Molarity & Mass Percent

Molarity (M)

• Definition: Measures concentration as the number of moles of solute per liter of solution.
• Conversion reminder
  • Given volume: 578 mL.
  • Convert milliliters to liters because molarity is defined per liter:
    V_{L} = \frac{578\, \text{mL}}{1000\, \text{mL}\,/\,\text{L}} = 0.578\, \text{L}
• Example calculation in the transcript
  • Amount of solute: 0.224 moles.
  • Volume of solution: 0.578 L (from conversion above).
  • Formula:
    M = \frac{n}{V} = \frac{0.224\, \text{mol}}{0.578\, \text{L}} \approx 0.388\, \text{mol·L^{-1}}
• Units
  • Acceptable to write as mol/L, moles per liter, or simply M (molar).
  • Statement of meaning: "0.388 M" means 0.388 moles of solute per one liter of solution.

Mass Percent (Mass %)

• Context: Commonly used in medical, environmental, and other applied sciences to describe concentration.
• Close relationship to percent composition but focuses specifically on mixtures/solutions rather than pure compounds.
• Symbol: Occasionally seen with a % w/w or simply % when context is clear.
• General formula:
  \text{Mass %} = \Bigg( \frac{\text{mass of component}}{\text{total mass of mixture}} \Bigg) \times 100
• Allowed units
  • Any mass unit is acceptable as long as the numerator and denominator share the same unit.
  • Examples: kilograms, grams, pounds, etc.
• Practical implication: Because the ratio uses the same units top and bottom, the units cancel, leaving a true percentage regardless of which mass unit you select.