Overview
This lecture reviews key concepts from the ends of Chapters 6 and 7, focusing on mass percent composition and its practical applications in chemistry calculations.
Mass Percent Composition
- Mass percent (mass %) is the mass of a component divided by total mass, multiplied by 100.
- Formula: mass % = (mass of part / mass of whole) × 100.
- The sum of mass percent values for all components in a compound equals 100%.
- Example: For a compound formed from 0.358g Cr and 0.523g total mass, mass % Cr = (0.358 / 0.523) × 100 = 68.5%.
Using Mass Percent as a Conversion Factor
- Mass percent can be used to convert between mass of a single element and total compound mass.
- Example: NaCl is 39% Na by mass, so 39g Na per 100g NaCl.
- Conversion example: To stay under FDA's 2.4g Na/day limit, 2.4g Na × (100g NaCl / 39g Na) = 6.2g NaCl per day.
Calculating Mass Percent from Chemical Formulas
- Find mass percent by dividing total mass of a specific element (accounting for number of atoms) by total molar mass, then multiply by 100.
- Example: For C2Cl4F2 (Freon-114), mass % Cl = (4 × 35.453) / (2 × 12.011 + 4 × 35.453 + 2 × 18.998) × 100 = 69.58%.
Mass Percent in Health and Environment
- Fluoride in water helps prevent tooth decay but excess fluoride causes dental or skeletal fluorosis.
- Recommended fluoride intake is 1–4mg/day for adults.
- Example calculation: NaF has one Na and one F; mass % F = (18.998) / (22.990 + 18.998) × 100 = 45.24%.
Applying Mass Percent to Water Fluoridation
- To fluoridate 1500L water to 1mg F⁻/L:
- 1500L × (1mg F⁻/1L) = 1500mg F⁻.
- Convert mg to g: 1500mg × (1g/1000mg) = 1.5g F⁻.
- Find grams of NaF: 1.5g F⁻ × (100g NaF/45.24g F⁻) ≈ 3.32g NaF.
- Using provided mass percent (54.76%), 2.74g NaF needed for 1500L water.
Key Terms & Definitions
- Mass Percent (mass %) — The percentage by mass of a component in a compound: (mass of part / mass of whole) × 100.
- Conversion Factor — A ratio derived from the equality between two different units that can be used to convert from one unit to another.
- Fluoride — The ion F⁻, added to water to reduce tooth decay.
Action Items / Next Steps
- Practice calculating mass percent with given formulas.
- Apply mass percent as a conversion factor in sample problems.
- Review recommended readings from Chapters 6 and 7.