Overview
This lecture explains how to calculate the mass percent of an element in a compound using mole conversions and provides a worked example with carbon dioxide.
Calculating Mass Percent
- Mass percent is the ratio of the mass of the element to the mass of the compound, multiplied by 100%.
- Formula: Mass percent = (mass of element / mass of compound) × 100%
- To find mass of an element in a compound, use mole conversions starting from grams of compound.
- Chemists often convert grams to moles because many calculations are based on moles.
Worked Example: CO₂
- Given: 88.2 g of carbon dioxide (CO₂).
- Find the mass percent of carbon in CO₂.
- Step 1: Convert grams CO₂ to moles CO₂ using molar mass (44.1 g CO₂/mol CO₂).
- Step 2: Use the subscript ratio to convert moles CO₂ to moles carbon (1 mol C per 1 mol CO₂).
- Step 3: Convert moles carbon to grams carbon (12.01 g/mol C).
- Result: 88.2 g CO₂ contains 24.02 g carbon.
- Mass percent calculation: (24.02 g C / 88.2 g CO₂) × 100% = 27.29% carbon by mass.
Properties of Mass Percent
- Mass percent is constant for a compound, regardless of total sample size.
- All CO₂ samples have 27.29% carbon by mass.
Key Terms & Definitions
- Mass percent — The percentage by mass of an element in a compound.
- Mole — A unit representing 6.022×10²³ particles, used for counting atoms or molecules.
- Molar mass — The mass (in grams) of one mole of a substance.
- Subscript ratio — The relationship between the number of moles of elements and the compound in a formula.
Action Items / Next Steps
- Practice solving mass percent problems in preparation for next class.
- Review how to convert between grams, moles, and atoms for different compounds.