Purity & The Law of Definite Proportion
Chemistry: IB Diploma Higher Level, topic 1.6.3
Purity & The Law of Definite Proportion
Pure substances can be elements composed of atoms or compounds of atoms or ions in fixed proportions.
Pure substances
- As discussed earlier, pure substances are only made of one type of atom or molecule.
- Examples of pure substances made up of only one type of atom include elements like Tin (Sn), Oxygen (O2), Copper (Cu), Argon (Ar), and diamond (diamond is made entirely of carbon atoms).
- Examples of pure substances made up of only one type of molecule include sucrose (table sugar) and sodium chloride (NaCl, table salt).
Joseph Proust
- Joseph Proust was a French chemist who demonstrated that all samples of a pure compound contain the same elements in the same proportion by mass.
- This statement is known as the law of definite proportions or the law of constant composition.
- The suggestion that the numbers of atoms of the elements in a given compound always exist in the same ratio is consistent with these observations.
- An example of this is given on the next slide.
Law of definite proportions
- The law of definite proportions can be demonstrated experimentally.
- When different samples of isooctane (a component of gasoline) are analyzed, they are found to have a carbon-to-hydrogen mass ratio of 5.33:1.
- This is demonstrated in the table (fully visible on the next slide).
Empirical formula
- The law of definite proportions can be applied to get the empirical formula of compounds.
- The empirical formula is the simplest whole-number ratio of the atoms in a molecule.
- E.g. Ethane has two carbon atoms and six hydrogen atoms.
- CH3 is the empirical formula for ethane.
Empirical formula - experiment
- The empirical formula of a compound is often very easy to determine experimentally.
- For instance, you could combust a hydrocarbon and find that you get twice as much water vapor as carbon dioxide.
- This would tell you that its empirical formula is CH4 (remember H2O contains two H atoms).
- A molecular formula is relatively difficult to determine experimentally.
Test yourself on Purity & The Law of Definite Proportion
What is the empirical formula of benzene (C6H6)?