Summary of "5.2 Mass Percents and Empirical and Molecular Weights | High School Chemistry"
Summary of “5.2 Mass Percents and Empirical and Molecular Weights | High School Chemistry”
This lesson by Chad from Chad’s Prep focuses on understanding mass percents, empirical formulas, and molecular formulas in chemistry, particularly as they relate to stoichiometry.
Main Ideas and Concepts
1. Mass Percent Composition
- Mass percent (mass %) is calculated as:
[ \text{Mass percent of element} = \frac{\text{mass of element in sample}}{\text{total mass of sample}} \times 100 ]
- Mass percent is an intensive property, meaning it does not depend on the sample size.
- Using a 1 mole sample (based on molar mass) simplifies calculations because the total mass corresponds to the molar mass.
- Example: Calcium carbonate (CaCO₃) has a molar mass of about 100 g/mol, making it easy to calculate:
- %C = (12 g C / 100 g CaCO₃) × 100 = 12%
- %O = (3 × 16 g O / 100 g CaCO₃) × 100 = 48%
2. Empirical vs. Molecular Formulas
- Molecular formula: The exact number of atoms of each element in a molecule (e.g., benzene C₆H₆).
- Empirical formula: The simplest whole-number ratio of atoms in a compound (e.g., benzene’s empirical formula is CH).
- Most compounds have the same empirical and molecular formulas, but some differ (e.g., P₄O₁₀ empirical formula is P₂O₅).
- Ethanol (C₂H₆O) has the same empirical and molecular formula.
3. Determining Empirical Formula from Mass Percent
- When given mass percents (e.g., 80% C, 20% H), assume a convenient sample size (usually 100 g) to convert percents directly to grams.
- Convert grams to moles using molar masses:
- Moles C = grams C / 12 g/mol
- Moles H = grams H / 1 g/mol
- Calculate mole ratio of elements.
- To get the empirical formula, divide all mole values by the smallest mole number to get the simplest whole number ratio.
- If the ratio is not a whole number:
- Multiply all ratios by a common factor (e.g., 2 for halves, 3 for thirds) to get whole numbers.
- Write the empirical formula using these whole numbers.
4. From Empirical to Molecular Formula
- The molecular formula is a whole-number multiple of the empirical formula.
- To find the molecular formula, you need the molecular weight (molar mass) of the compound.
- Calculate the empirical formula mass.
- Divide the molecular weight by the empirical formula mass to find the multiple.
- Multiply the subscripts in the empirical formula by this multiple to get the molecular formula.
- Example: Empirical formula CH₃ has a mass of 15 g/mol. Given molecular weight = 30 g/mol, the molecular formula is C₂H₆.
Step-by-Step Methodology to Find Empirical and Molecular Formulas from Mass Percent Data
- Assume a 100 g sample (or any convenient mass).
- Convert mass percent to grams of each element.
- Convert grams to moles using molar masses.
- Calculate mole ratio by dividing all mole values by the smallest mole number.
- Adjust ratios to whole numbers by multiplying by a common factor if necessary.
- Write the empirical formula from these whole numbers.
- If molecular weight is known, divide molecular weight by empirical formula mass to find the multiple.
- Multiply empirical formula subscripts by this multiple to find the molecular formula.
Additional Notes
- Mass percents are intensive properties and independent of sample size.
- Empirical formulas represent the simplest ratio, molecular formulas represent actual numbers.
- Molecular formula determination requires additional information (molecular weight).
- The lesson uses calcium carbonate, benzene, ethanol, and hypothetical carbon-hydrogen compounds as examples.
- The instructor emphasizes careful conversion between mass and mole quantities and the importance of whole number ratios.
Speakers/Sources Featured
- Chad (Instructor and creator of Chad’s Prep channel)
This summary captures the key chemistry concepts and stepwise problem-solving methods presented in the video lesson.
Category
Educational
