Overview
This lecture covers how to determine the percentage composition of a compound using its chemical formula or experimental mass data, an important skill for chemical analysis.
Percentage Composition Concept
- Percentage composition expresses the proportion by mass of each element in a compound.
- It is calculated as (mass of element in compound / total molar mass of compound) Ă— 100%.
- Knowing percentage composition helps in finding empirical formulas in advanced studies.
Calculating from Chemical Formula
- Identify the elements present in the compound and their atomic masses from the periodic table.
- Count the number of atoms of each element in the chemical formula.
- Calculate the compound's total molar mass by summing the masses of all atoms present.
- Use the formula: (mass of element in one mole / molar mass of compound) Ă— 100%.
- Example: For HCl (hydrochloric acid), hydrogen is 2.74% and chlorine is 97.26% by mass.
Calculating from Mass Data
- Record the mass of the whole compound and the mass of each element present.
- If one element’s mass is unknown, subtract known element(s) from the total mass to find it.
- Calculate each element’s percentage using (mass of element / total compound mass) × 100%.
- Example: In a 30g sample with 24.12g zinc, Zn is 80.40% and O is 19.60% by mass.
Key Terms & Definitions
- Percentage Composition — the percent by mass of each element in a compound.
- Molar Mass — the mass of one mole of a compound, calculated from atomic masses.
- Empirical Formula — the simplest whole-number ratio of elements in a compound.
Action Items / Next Steps
- Practice calculating percentage composition from both chemical formulas and mass data.
- Review atomic masses on the periodic table for future calculations.