#AP Chemistry Study Guide: Matter, Composition, and Formulas

Welcome to your ultimate AP Chemistry review! This guide is designed to help you feel confident and prepared for your exam. Let's dive in!

#States of Matter and Their Properties

# 🧊 Solids

• Fixed shape and volume
• Particles are densely packed with strong bonds
• Particles vibrate in place

Solids maintain a defined shape and volume due to their tightly packed particles.

# 💧 Liquids

• Fixed volume, but no fixed shape (takes the shape of its container)
• Particles are less tightly packed than solids, allowing them to flow

Liquids adapt to the shape of their container while maintaining a constant volume.

# ♨️ Gases

• No fixed shape or volume (expands to fill its container)
• Particles are very spread out and in constant, random motion

Gases have neither a definite shape nor volume, with particles moving freely.

#Classifying Matter by Composition

#Pure Substances

• Composed of a single type of atom or molecule
• Composition is consistent throughout

#Elements

• Made up of only one type of atom
• Examples: Gold (Au), Oxygen (O), etc.
• Found on the periodic table 🪙

#Compounds

• Made up of two or more different elements chemically bonded
• Examples: Water (H2O), Salt (NaCl)
• Always have a fixed ratio of elements

#Formula Units

• Represents the lowest whole-number ratio of atoms in a compound
• Example: NaCl has a 1:1 ratio of sodium to chloride ions
• Useful for conversion and understanding proportions

#Example Problem

How many formula units of KCl are in 1.76 mol of KCl?

#Law of Definite Proportions

• Also known as the law of constant composition
• The ratio of elements in a compound is always the same
• Example: Water (H2O) always has a 2:1 ratio of hydrogen to oxygen

#Understanding the Law of Definite Proportions 🍪

#Empirical Formula

#The Empirical Formula of Glucose 🍬

• Molecular formula of glucose: C6H12O6
• Empirical formula of glucose: CH2O
• The empirical formula does not always reflect the actual number or arrangement of atoms in a molecule

The empirical formula is the simplest ratio, while the molecular formula shows the actual number of atoms.

#Empirical Formula - Practice Question #1

(1) A carbohydrate contains C, H, and O, with a % composition of 33.3% C and 7.4% H. Find the empirical formula.

#Step 1: Drop the %

• 33.3% C becomes 33.3 g C
• 7.4% H becomes 7.4 g H
• Calculate % of O: 100% - 33.3% - 7.4% = 59.3% O, which becomes 59.3 g O

#Step 2: Unit conversion

• Convert grams to moles using molar mass:
  • C: 33.3 g / 12.01 g/mol = 2.773 mol
  • H: 7.4 g / 1.008 g/mol = 7.34 mol
  • O: 59.3 g / 16.00 g/mol = 3.706 mol

#Step 3: Finding the lowest whole-number ratio

• Divide each mole value by the smallest mole value (2.773 in this case):
  • C: 2.773 / 2.773 = 1
  • H: 7.34 / 2.773 = 2.66
  • O: 3.706 / 2.773 = 1.33

#Step 4: Multiplying to a whole number

• Multiply by a common factor to get whole numbers (multiply by 3 in this case):
  • C: 1 x 3 = 3
  • H: 2.66 x 3 ≈ 8
  • O: 1.33 x 3 ≈ 4
• Empirical formula: C3H8O4

#Final Exam Focus

• States of Matter: Understand the differences in particle arrangement and motion. Focus on phase changes and their energy requirements.
• Pure Substances vs. Mixtures: Be able to classify matter correctly. Know the difference between elements and compounds.
• Law of Definite Proportions: Understand that compounds have fixed ratios of elements.
• Empirical Formula: Master the steps to calculate empirical formulas from percent compositions. This is a frequent question type.

#Last-Minute Tips

• Time Management: Quickly identify the type of question and apply the correct strategy. Don't spend too long on one question.
• Common Pitfalls: Watch out for rounding errors, especially in empirical formula calculations. Always double-check your work.
• Question Strategies: For multiple-choice, eliminate obviously incorrect answers first. For FRQs, show all your work to get partial credit.

Practice Question

#Practice Questions

Multiple Choice

1. Which of the following best describes a substance that has a fixed volume but not a fixed shape? (A) Solid (B) Liquid (C) Gas (D) Plasma

2. A compound is found to contain 40% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. What is the empirical formula of this compound? (A) CHO (B) CH2O (C) C2H2O (D) C2H4O

3. According to the law of definite proportions, which of the following is true regarding the composition of a pure compound? (A) The ratio of elements can vary depending on the source. (B) The ratio of elements is always the same. (C) The compound can be made up of different elements. (D) The compound can be in different states of matter.

Free Response Question

Consider a compound containing only carbon, hydrogen, and oxygen. A 2.500 g sample of this compound is completely combusted in excess oxygen, producing 3.666 g of CO2 and 1.500 g of H2O.

(a) Calculate the mass of carbon in the 2.500 g sample. (b) Calculate the mass of hydrogen in the 2.500 g sample. (c) Determine the mass of oxygen in the 2.500 g sample. (d) Determine the empirical formula of the compound.

Answer Key and Scoring Guidelines

Multiple Choice

1. (B)
2. (B)
3. (B)

Free Response Question

(a) Mass of C: * Moles of CO2 = 3.666 g / 44.01 g/mol = 0.0833 mol * Moles of C = moles of CO2 = 0.0833 mol * Mass of C = 0.0833 mol * 12.01 g/mol = 1.000 g (1 point for moles of CO2, 1 point for mass of C)

(b) Mass of H: * Moles of H2O = 1.500 g / 18.02 g/mol = 0.0832 mol * Moles of H = 2 * moles of H2O = 0.1664 mol * Mass of H = 0.1664 mol * 1.008 g/mol = 0.1677 g (1 point for moles of H2O, 1 point for mass of H)

(c) Mass of O: * Mass of O = 2.500 g - 1.000 g - 0.1677 g = 1.332 g (1 point for mass of O)

(d) Empirical Formula: * Moles of C = 1.000 g / 12.01 g/mol = 0.0833 mol * Moles of H = 0.1677 g / 1.008 g/mol = 0.1664 mol * Moles of O = 1.332 g / 16.00 g/mol = 0.0833 mol * Divide by smallest (0.0833): C = 1, H = 2, O = 1 * Empirical formula = CH2O (1 point for moles of each element, 1 point for the correct formula)

Good luck! You've got this! 🚀