Elements & Mixtures (College Board AP® Chemistry): Flashcards
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Define formula unit.
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Define formula unit.
A formula unit is the arrangement of atoms or ions in fixed proportions held together by ionic bonds. It represents the simplest repeating unit of an ionic compound, such as NaCl.
True or False?
A molecular compound is composed of atoms held together by ionic bonds.
False.
A molecular compound is composed of atoms held together by covalent bonds. Ionic bonds hold formula units together in ionic compounds such as sodium chloride (NaCl).
What does the Law of Definite Proportions state?
The Law of Definite Proportions states that the ratio of the masses between the elements in a compound is always the same, regardless of the source or sample size of the compound.
The Law of Definite Proportions means that a pure compound always has the same .......... and properties, no matter where it comes from.
The Law of Definite Proportions means that a pure compound always has the same composition and properties, no matter where it comes from.
Define empirical formula.
An empirical formula is the simplest formula for a compound, showing the smallest whole-number ratio of atoms present. For example, the empirical formula of butene (C4H8) is CH2.
Why is the empirical formula of an ionic compound also its molecular formula?
Ionic compounds do not exist as discrete molecules — they form extended lattices of formula units in fixed ratios. Because there is no larger repeating molecular unit to simplify, the formula unit ratio is already the simplest whole-number ratio, making it both the empirical formula and molecular formula simultaneously.
To find the empirical formula from percent composition, assume a .......... g sample and convert each percentage directly to grams.
To find the empirical formula from percent composition, assume a 100 g sample and convert each percentage directly to grams.
How do you determine the molecular formula from an empirical formula and a given molar mass?
Calculate the molar mass of the empirical formula.
Divide the molar mass of the molecular compound by the molar mass of the empirical formula to get a whole number.
Multiply each subscript in the empirical formula by that whole number.
For example: empirical formula CH2O (M = 30.03 g mol-1), molecular molar mass = 180 g mol-1 → whole number = 6 → molecular formula C6H12O6.
Define pure substance.
A pure substance is matter that has a fixed composition, containing atoms, molecules or formula units of a single type. Examples include aluminum (Al), water (H2O) and sodium chloride (NaCl).
True or False?
A mixture has a fixed composition.
False.
A mixture has a composition that is not fixed — it contains two or more types of atoms, molecules or formula units, and the proportions of its components can vary.
What is the difference between a homogeneous and a heterogeneous mixture?
A homogeneous mixture has the same composition throughout because its components are mixed uniformly (e.g. air). A heterogeneous mixture varies in composition from one region to another because its components separate into distinct regions (e.g. water and oil).
The individual substances that make up a mixture are called its .......... .
The individual substances that make up a mixture are called its components.
Why does a homogeneous mixture have uniform composition throughout?
In a homogeneous mixture, the components are mixed at the molecular level so evenly that no distinct regions of different composition form. Because every part of the mixture contains the same proportions of components, the overall composition is the same at any point you sample.
True or False?
Air is an example of a homogeneous mixture.
True.
Air is a homogeneous mixture in which components such as oxygen, nitrogen and other gases are uniformly distributed, giving it the same composition throughout.
Define elemental analysis.
Elemental analysis is a technique used to determine the relative composition of elements in a substance and to evaluate its purity.
True or False?
Purity is always expressed as a percentage.
True.
Purity is expressed as a percentage. It can be calculated using either the mass of the desired compound relative to the total sample mass, or by comparing the percent composition of the desired compound to the total sample.
Purity = (mass of .......... / total mass of sample) × 100
Purity = (mass of desired compound / total mass of sample) × 100
Why is it valid to use mole ratios from a compound's formula when calculating the mass of that compound from elemental analysis data?
The Law of Definite Proportions means elements combine in fixed ratios in a compound, so the subscripts in the formula directly reflect the mole ratio of each element to the compound. This allows a direct conversion — for example, 1 mol Ca per 1 mol CaCl2 — when the ratio is 1:1.
A sample contains 0.83 g of CaCl2 in a 4.1 g mixture. What is the percent composition of CaCl2 in the sample?
percent of CaCl2 = (mass of CaCl2 / total mass of sample) × 100
percent of CaCl2 = (0.83 g / 4.1 g) × 100
percent of CaCl2 = 20%
True or False?
Elemental analysis can be used to determine the purity of a substance.
True.
Elemental analysis identifies the relative amounts of each element in a sample. By comparing the measured composition with the expected composition of the pure compound, the purity of the substance can be calculated.
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