Chapter 2: Chemistry and Measurements

Chapter 2: Chemistry and Measurements

Readiness Key Math Skills

  • Identifying Place Values (1.4A): This involves recognizing the position of digits in a number, which determines their value (e.g., in the number 345, 3 is in the hundreds place, 4 in the tens, and 5 in the ones place).

  • Using Positive and Negative Numbers in Calculations (1.4B): Understanding how to work with positive and negative values in mathematical equations.

  • Calculating Percentages (1.4C): This skill includes converting a fraction to a percentage by multiplying by 100. Example: to find 20% of 50, calculate (0.20imes50=10)(0.20 imes 50 = 10).

  • Writing Numbers in Scientific Notation (1.4F): This involves expressing numbers as a product of a number between 1 and 10 and a power of ten. Example: 4500 can be written as (4.5imes103)(4.5 imes 10^3).

2.1 Units of Measurement

  • Metric System: The standard method of measurement in chemistry.

  • Learning Goal: Recognize names and abbreviations for metric or SI units associated with measurements of:

    • Length: Meter (m)

    • Volume: Liter (L)

    • Mass: Gram (g)

    • Temperature: Celsius (°C), Kelvin (K)

    • Time: Seconds (s)

2.2 Units of Measurement: Metric and SI

  • International System of Units (SI): A universal measurement system based on the metric system, includes standard units for various quantities.

  • Length:

    • Meter (m): 1 m = 100 cm, 1 m = 1.09 yd, 1 m = 39.4 in, 2.54 cm = 1 in.

  • Volume:

    • Liter (L): SI unit is m³ but generally measured in liters (1 L = 1000 mL, 1 L = 1.06 qt, 946 mL = 1 qt).

    • Graduated cylinders are used for small volume measurements.

  • Mass:

    • Gram (g) and Kilogram (kg): 1 kg = 1000 g, 1 kg = 2.20 lb, 454 g = 1 lb. Mass of a nickel is approximately 5.01 g.

  • Temperature:

    • Celsius (°C) in metric; Kelvin (K) in SI. Water freezes at 0 °C (32 °F); Kelvin starts at 0 K, which is absolute zero.

  • Time:

    • Units include years (yr), days, hours (h), minutes (min), seconds (s). SI unit is the second (s).

2.3 Measured Numbers and Significant Figures

  • Measured Numbers: Numbers obtained through measurement, have a certain degree of precision based on tool used:

    • The last digit is always an estimate.

  • Significant Figures: All digits in a measured number that provide useful information about the precision of the measurement, including:

    • All non-zero digits

    • Zeros between significant digits

    • Zeros at the end of a decimal number (e.g., 1.00 has three significant figures).

  • Counting Significant Figures: Key practices in recognizing how to determine significant figures in calculations.

Scientific Notation

  • Significant Zeros: When zeros hold significance (e.g., in large numbers), express using scientific notation.

    • Example: 300 m is denoted as (3.0imes102m)(3.0 imes 10^2 m) if the zero is significant.

  • Conversions & Examples:

    • 400,000 g = 4imes105g4 imes 10^5 g

    • 0.0004 s = 4imes10−4s4 imes 10^{-4} s

Exact Numbers

  • General characteristics:

    • Exact numbers are counted (e.g., 5 oranges) or defined quantities (e.g., 1 inch = 2.54 cm).

    • Do not have significant figures because they are completely accurate.

2.5 Writing Conversion Factors

  • Conversion Factors: Derived from equalities, used to convert between different units:

    • Example: 1m=100cm1 m = 100 cm.

  • U.S. & Metric Unit equalities are included.

2.6 Density

  • Density Calculation: Density = mass / volume.

  • Substances denser than water sink; those less dense float.

  • Example Calculation: Measuring the density of a solid by the volume displaced in water.

  • Specific Gravity: Comparison of a substance's density to water's density, expressed as a unitless ratio.

Temperature Scales

  • Celsius and Kelvin Scales:

    • Celsius has a direct correlation with Kelvin (0 °C = 273.15 K).

    • No negative temperatures on the Kelvin scale (absolute zero = 0 K).

  • Conversion:

    • Formula for converting between Celsius and Fahrenheit: TF=1.8TC+32TF = 1.8TC + 32 (Fahrenheit from Celsius).

  • Temperature Examples:

    • A person’s body temp at 34.8 °C converts to Fahrenheit.

Problem Solving Using Unit Conversion

  • Steps to solve conversion problems in chemistry:

    1. Identify given and needed quantities.

    2. Write a plan to connect the two units.

    3. State equalities and conversion factors.

    4. Set up the calculations to cancel out non-needed units.

2.7 Density Problem Solving

  • Example problem: Calculate mass using known density and volume.

  • Unit Relationships: Understand how to express density and its mass or volume relations clearly.

Concept Map

  • Chemistry measurements involve units for measuring mass, volume, length, density, and temperature, structured around:

    • Measurement significance with significant figures and mathematical operations.