AP Chemistry Flashcards: Elemental Composition of Pure Substances
Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026
Review key ideas with interactive flashcards. This set includes 10 cards to help you master important concepts.
Two samples of pure sodium chloride (NaCl) are analyzed. How will their elemental composition by mass compare according to the Law of Definite Proportions?
Their mass ratios of sodium to chlorine will be identical because the mass ratio of elements in a pure compound is always the same.
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Two samples of pure sodium chloride (NaCl) are analyzed. How will their elemental composition by mass compare according to the Law of Definite Proportions?
Their mass ratios of sodium to chlorine will be identical because the mass ratio of elements in a pure compound is always the same.
Define 'empirical formula'.
The empirical formula is the chemical formula that lists the lowest whole-number ratio of atoms of the elements in a compound.
If a compound's molecular formula is C₄H₈, what is its empirical formula?
The empirical formula is CH₂, as this represents the lowest whole-number ratio of carbon to hydrogen atoms (1:2).
How does a compound's elemental composition by mass relate to its empirical formula?
The elemental composition by mass provides the quantitative data needed to calculate the empirical formula of a pure substance.
What is meant by a 'formula unit'?
A formula unit describes the fixed proportions of atoms or ions that compose a pure substance, particularly ionic compounds.
What are pure substances composed of?
Pure substances are composed of individual molecules, or atoms and ions held in fixed proportions as described by a formula unit.
What is the Law of Definite Proportions?
This law states that the mass ratio of constituent elements in any pure sample of a compound is always the same.
Why is understanding the elemental composition by mass crucial for determining a substance's identity?
Because the fixed mass composition is a defining characteristic of a pure substance, it allows for the calculation of its empirical formula.
What is the key difference represented by an empirical formula compared to a molecular formula?
The empirical formula shows only the lowest whole-number ratio of atoms, not the actual number of atoms in an individual molecule.
What fundamental principle guarantees that water (H₂O) will always have the same mass percentage of hydrogen and oxygen?
The Law of Definite Proportions guarantees that the mass ratio of constituent elements in any pure sample of a compound is always the same.