Overview
This lecture explains the law of definite proportions, demonstrates how to calculate the mass ratios of elements in compounds, and provides practice problems using these concepts.
Law of Definite Proportions
- The law of definite proportions states that a compound always contains the same elements in the same mass ratio.
- In any sample of a compound, the ratio by mass of the constituent elements is constant.
Carbon Dioxide Mass Ratios
- Carbon dioxide (CO₂) consists of one carbon atom (atomic mass 12) and two oxygen atoms (atomic mass 16 each).
- For every 12 g of carbon in CO₂, there are 32 g of oxygen (16 × 2).
- Regardless of sample size, the mass ratio of oxygen to carbon in CO₂ remains about 2.67 (32/12).
Using Mass Ratios and Conversions
- To find the mass of one element when given another, multiply the given mass by the ratio (e.g., 30 g of C in CO₂ means 80 g of O, since 30 × 32/12 = 80).
- Proportions or conversion factors based on atomic mass and formula subscripts can solve for unknown masses.
Practice Problems
Sodium Fluoride (NaF)
- NaF contains sodium (Na, atomic mass 23) and fluorine (F, atomic mass 19).
- For every 23 g Na in NaF, there are 19 g F.
- If a sample has 35 g Na, the mass of F is 28.9 g (35 × 19/23).
Methane (CH₄)
- Methane has one carbon (12) and four hydrogen atoms (1 each; total 4).
- For every 12 g C, there are 4 g H in CH₄.
- If there are 42 g H, the mass of C is 126 g (42 × 12/4).
Aluminum Oxide (Al₂O₃)
- Al₂O₃ contains 2 Al atoms (27 each, total 54) and 3 O atoms (16 each, total 48).
- For every 54 g Al, there are 48 g O.
- If there are 19 g O, the mass of Al is 21.5 g (19 × 54/48).
Key Terms & Definitions
- Law of Definite Proportions — A chemical compound always contains the same elements in the same proportion by mass.
- Atomic mass — The mass of an atom, typically from the periodic table, used to calculate ratios in compounds.
- Conversion factor — A ratio derived from the formula and atomic masses to relate the masses of elements in a compound.
- Proportion — An equation stating two ratios are equal, used to solve for unknown masses.
Action Items / Next Steps
- Practice additional problems using mass ratios and conversion methods.
- Review atomic masses and compound formulas in the periodic table.
- Prepare for quizzes on using proportions to solve for unknown element masses in compounds.