1.3 Elemental Composition of Pure Substances

• What is a pure substance?

  Fixed chemical composition
• Match the example with its type of substance:

  Water ↔️ Pure substance

  Air ↔️ Mixture
• Water always consists of two hydrogen atoms and one oxygen atom.


  True
• What does the law of multiple proportions state?

  Simple whole-number ratio
• What does the empirical formula represent?

  Simplest whole-number ratio
• What is the empirical formula of glucose (C₆H₁₂O₆)?

  CH₂O
• Match the formula type with its example for glucose:

  Empirical formula ↔️ CH₂O

  Molecular formula ↔️ C₆H₁₂O₆
• The subscript in a compound's formula is multiplied by the element's atomic mass.


  True
• If a compound contains 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen, the ratio of carbon to hydrogen to oxygen is 1:2:1
• What operation converts grams to moles?

  Division
• Match the concept with its description:

  Mole ↔️ Measures amount of substance

  Conversion ↔️ Involves molar mass
• What does the law of definite proportions state?

  Fixed ratio by mass
• What is the ratio of oxygen in CO to CO₂ when carbon is fixed at 12g?

  1:2
• What is the molar mass of water (H₂O) calculated using atomic masses from the periodic table?

  18.016 g/mol
• What is the molar mass of water (H₂O)?

  18.016 g/mol
• What is the first step in determining empirical and molecular formulas from experimental data?

  Convert percent composition to grams
• To convert grams to moles, you divide the mass in grams by the molar mass of the substance.


  True
• Steps to solve percentage composition problems

  1️⃣ Convert the masses of the elements to moles

  2️⃣ Find the mole ratios of the elements

  3️⃣ Calculate the percent composition
• To analyze experimental data, the first step is to convert percent composition to grams.
• A mixture is a combination of two or more pure substances that are not chemically bonded
• How do the properties of mixtures differ from those of pure substances?

  They can vary
• Match the element with its ratio in water:

  Hydrogen ↔️ 1

  Oxygen ↔️ 8
• The law of multiple proportions applies when two elements form multiple compounds
• The molecular formula indicates the actual number of each type of atom
• The empirical formula shows the simplest whole-number ratio of atoms
• What is the molar mass expressed in?

  g/mol
• What is the molar mass of water (H₂O)?

  18.02 g/mol
• What is the molecular formula of a compound with an empirical formula of CH₂O and a molar mass of 180 g/mol?

  C₆H₁₂O₆
• Converting 120 grams of NaCl to moles requires dividing by the molar mass of NaCl
• What is a pure substance composed of?

  One type of molecule
• The mass ratio of hydrogen to oxygen in water is always 1:8.


  True
• Match the formula type with its description:

  Empirical formula ↔️ Simplest ratio of atoms

  Molecular formula ↔️ Exact number of atoms
• Steps to calculate the molar mass of a compound

  1️⃣ Identify the elements in the compound and their atomic masses

  2️⃣ Multiply each element's atomic mass by its subscript

  3️⃣ Add up the masses of all the elements
• The subscript of an element in a compound's formula is multiplied by its atomic mass to find its contribution to the molar mass.

  True
• One mole contains Avogadro's number, which is approximately 6.022 × 10^23.
• How is the percent composition of an element calculated in a compound?

  Mole ratio / Total mole ratios × 100
• What does the empirical formula of a compound represent?

  Simplest whole-number ratio of atoms
• Pure substances have a variable composition.

  False
• The law of definite proportions states that a chemical compound always contains elements in a fixed ratio
• Carbon monoxide (CO) and carbon dioxide (CO₂) have the same oxygen-to-carbon ratio.

  False