Chapter 2: Chemistry and Measurements

Units of Measurement

  • Metric System

    • Scientists primarily use the metric system, with modifications for specific applications.

    • International System of Units (SI): A globally accepted system that standardizes measurements for length, volume, mass, temperature, and time.

Volume

  • Volume measures the space occupied by a substance.

    • SI Unit: cubic meters (m³)

    • Common units for chemists: liters (L) and milliliters (mL).

    • Tools for Measurement: Graduated cylinders are typically used for measuring small volumes.

Length

  • Length is measured in:

    • SI Unit: meters (m)

    • Common metric unit: centimeters (cm).

Mass

  • Mass reflects the quantity of material in an object.

    • Measured using an electronic balance.

    • SI Unit: kilograms (kg)

    • Commonly measured by chemists in grams (g).

Temperature

  • Indicates how hot or cold an object is.

    • Measured in degrees Celsius (°C) and Kelvin (K).

    • Key Temperatures:

      • Water freezes at 0 °C (32 °F)

      • Water boils at 100 °C (212 °F)

    • The Kelvin scale begins at absolute zero (0 K).

Time

  • Measured in seconds (s), consistent in both metric and SI systems.

    • Tools used: Stopwatches.

Measured Numbers

  • A measuring tool produces measured numbers, quantifying dimensions like length and mass.

Reporting Measurements

  • Example: Length: 4.5 cm

    • Estimation is crucial; report values reflecting the distance between measurement marks.

Significant Figures

  • Significant Figures (SFs): Include all known digits in a measurement plus one estimated digit.

    • General Rules:

      • Nonzero digits are always significant.

      • Zeros between nonzero digits are significant.

      • Leading zeros are not significant.

      • Trailing zeros in a decimal are significant (e.g., 40.00 has 4 significant figures).

      • In numbers without a decimal point, trailing zeros are not significant (e.g., 500 has 1 SF).

Exact Numbers

  • Exact numbers are defined quantities that have no uncertainty:

    • Example: Counting items (e.g., 8 cookies) or definitions (e.g., 1 ft = 12 in).

Rounding Off

  • In calculations, answers should typically be rounded off.

    • Rounding Rules:

      • Dropping digits less than 5 keeps the last retained digit the same.

      • Dropping digits 5 or more increases the last retained digit by 1.

Operations with Significant Figures

  • Multiplication/Division

    • Result should have the same number of significant figures as the measurement with the fewest significant figures.

  • Addition/Subtraction

    • Result should be reported to the same number of decimal places as the measurement with the fewest decimal places.

Metric Prefixes

  • Metric prefixes modify the size of units by factors of 10:

    • Example: 1 kilometer (1 km) = 1000 m; 1 millimeter (1 mm) = 0.001 m.

Measuring Length and Volume

  • Volume of a Cube:

    • Volume can be calculated as side x side x side.

    • 1 cm³ is equivalent to 1 mL.

Equalities and Conversion Factors

  • Equalities compare two units to express the same quantity:

    • Example: 1 lb = 16 oz, 2.20 lb = 1 kg.

  • Conversion Factors:

    • Defined relationships between units, utilized in calculations.