Summary of "3.3 Mass Percent Composition and Empirical and Molecular Formulas | General Chemistry"
Summary of "3.3 Mass Percent Composition and Empirical and Molecular Formulas | General Chemistry"
This lesson, taught by Chad from Chad’s Prep, covers the concepts of Mass Percent Composition, empirical formulas, and molecular formulas within the context of Stoichiometry in General Chemistry. The main focus is on understanding the differences and relationships between empirical and molecular formulas, calculating Mass Percent Composition from chemical formulas, and determining empirical and molecular formulas from mass percent data.
Main Ideas and Concepts
- Empirical vs. Molecular Formulas
- Molecular Formula: Shows the exact number of atoms of each element in a molecule (e.g., P₄O₁₀ means 4 phosphorus and 10 oxygen atoms).
- Empirical Formula: The simplest whole-number ratio of atoms in a compound, often a reduced form of the Molecular Formula (e.g., P₂O₅ is the Empirical Formula for P₄O₁₀).
- Sometimes the molecular and empirical formulas are the same (e.g., propane C₃H₈).
- Mass Percent Composition
- Mass percent = (mass of element / total mass of compound) × 100.
- Mass percent is an intensive property; it does not depend on sample size.
- It is often easiest to calculate mass percent using a 1 mole sample, as molar masses from the periodic table simplify calculations.
- Example: For Calcium Carbonate (CaCO₃), the molar mass is 100 g/mol; carbon is 12 g/mol, oxygen is 16 g/mol.
- %C = (12 g / 100 g) × 100 = 12%
- %O = (3 × 16 g / 100 g) × 100 = 48%
- %Ca = 100% - (12% + 48%) = 40%
- Determining Empirical Formulas from Mass Percent
- Convert mass percentages to grams assuming a 100 g sample (e.g., 80% C = 80 g C).
- Convert grams to moles using molar masses (C: 12 g/mol, H: 1 g/mol).
- Calculate mole ratio of elements.
- Convert mole ratios to the smallest whole numbers by dividing by the smallest mole value.
- If ratios are not whole numbers, multiply all ratios by an integer to get whole numbers (e.g., multiply by 2 if you have 0.5, multiply by 3 if you have 1.33).
- Example: For 80% C and 20% H:
- Moles C = 80 g / 12 g/mol = 6.67 mol
- Moles H = 20 g / 1 g/mol = 20 mol
- Ratio C:H = 6.67 : 20 → divide both by 6.67 → 1 : 3 → Empirical Formula = CH₃.
- Determining Molecular Formulas from Empirical Formulas
- Molecular Formula is a whole-number multiple of the Empirical Formula.
- Use given molecular weight (molar mass) to find the multiplier:
- Calculate Empirical Formula mass.
- Divide molecular weight by Empirical Formula mass to get the multiplier.
- Multiply subscripts in Empirical Formula by this multiplier.
- Example:
- Empirical Formula CH₃ has mass = 15 amu.
- Given molecular weight = 30 amu.
- Multiplier = 30 / 15 = 2.
- Molecular Formula = C₂H₆.
Methodology / Step-by-Step Instructions
Calculating Mass Percent from a Known Formula:
- Step 1: Calculate molar mass of the compound.
- Step 2: Calculate mass of each element in 1 mole of compound.
- Step 3: Calculate mass percent = (mass of element / molar mass) × 100.
Finding Empirical Formula from Mass Percent:
- Step 1: Assume a 100 g sample to convert percentages directly to grams.
- Step 2: Convert grams of each element to moles using molar mass.
- Step 3: Find the mole ratio by dividing all mole values by the smallest mole value.
- Step 4: If ratios are not whole numbers, multiply all ratios by the smallest integer that converts them to whole numbers.
- Step 5: Write the Empirical Formula with the resulting whole number subscripts.
Finding Molecular Formula from Empirical Formula:
- Step 1: Calculate the molar mass of the Empirical Formula.
- Step 2: Divide the given molecular weight by the Empirical Formula mass to find the multiplier.
- Step 3: Multiply all subscripts in the Empirical Formula by this multiplier to get the Molecular Formula.
Speakers / Sources Featured
- Chad — Instructor
Category
Educational
