Chem 4/1/2025

Class Reminders

  • No new information presented at the start of class.

  • Study sheets available on the Canvas site:

    • One study sheet per lecture with reflective questions.

    • Not required but useful for motivated students.

    • Includes a selection of useful videos from the "Crash Course" series for review.

Tutoring Resources

  • Tutoring available for students needing extra help:

    • Plus tutor contacted recently, looks like a good resource.

    • Chemistry department tutors available in Science 232, known to be effective.

Significant Figures Overview

Definitions

  • Exact Numbers:

    • No uncertainty, related to definitions (e.g., 1 kilometer = 1000 meters).

    • Counting (e.g., 5 dimes) also provides exact numbers with no uncertainty.

  • Inexact Numbers:

    • Based on measurements and have inherent uncertainty. Examples include weights and dimensions depending on measuring tools.

Measurement Examples

  • Weight of a dime could be measured differently based on the scale's precision.

  • E.g., 2.2403 grams ± 0.0001 is an inexact number with uncertainty.

Significant Figures Convention

  • Purpose: A communication method that indicates how well a measurement is known.

Rules for Significant Figures

  1. Non-zero digits are always significant.

  2. Leading zeros are not significant (e.g., 0.0022 has 2 significant figures).

  3. Trailing zeros in a decimal number are significant (e.g., 2.300 has 4 significant figures).

Examples of Calculating Significant Figures

  • 501 kg: 3 significant figures (± 1 kg uncertainty).

  • 5.500: 5 significant figures (determines its precision).

Addition and Subtraction Rule

  • Result should match the most uncertain measurement.

  • Use this when dealing with addition problems (e.g., converting Celsius to Kelvin).

Multiplication and Division Rule

  • Result should match the measurement with the least significant figures involved in the calculation.

  • Example: 12 cm, 13 cm, and 6.205 cm give a volume calculated to the lowest significant figures.

Dimensional Analysis

  • Definition: Ensure you are careful with units when combining measurements.

  • Practice converting using units (e.g., inches to centimeters) and ensuring units cancel appropriately (e.g., 20 inches to centimeters).

Example Problem

  • Converting 20 inches using 1 inch = 2.54 cm gives you 50.8 cm. Both significant figures and units are taken into consideration.

Important Remarks

  • Exact numbers (like defined conversions) do not affect the measurement's significant figures.

  • Significant figures must be carefully considered in every calculation, especially in laboratories and exams.