Overview
This lecture covers Chapter 2: Chemistry and Measurements, focusing on units of measurement, significant figures, conversions, and the concept of density.
Units of Measurement
- The metric and SI systems are used for scientific measurements (length, mass, volume, temperature, and time).
- Length is measured in meters (m); 1 meter = 100 centimeters (cm); 1 inch = 2.54 cm.
- Mass is measured in grams (g) or kilograms (kg); 1 kg = 1000 g; 1 pound = 454 g.
- Temperature: Metric uses degrees Celsius (°C), SI uses Kelvin (K); 0°C = 273 K; 0°C = 32°F; 100°C = 212°F.
- Time is measured in seconds (s), minutes (min), and hours (h).
Significant Figures (Sig Figs)
- Significant figures include all certain digits plus the estimated last digit.
- Zeros between nonzero digits or after the decimal and a nonzero digit are significant.
- Scientific notation shows only the significant digits.
- Exact numbers (e.g., 1 kg = 1000 g) do not count toward significant figures.
Calculations with Significant Figures
- For multiplication/division, the result should have the same number of sig figs as the least precise measurement.
- For addition/subtraction, round to the least number of decimal places in any number of the operation.
Prefixes and Equalities
- Common prefixes: kilo (k, 10³), centi (c, 10⁻²), milli (m, 10⁻³), micro (µ, 10⁻⁶).
- 1 cm = 10 mm; 1 cm = 0.01 m; 1 cm³ = 1 mL.
- 1000 g = 1 kg; 1000 mg = 1 g.
Writing and Solving Conversions
- Set up conversion factors so units cancel until only the desired unit remains.
- Use exact conversion values where appropriate for sig fig calculation.
- Multi-step conversions may be needed for some problems.
Problem Solving with Conversions
- Always start with the quantity given and use conversion factors to reach the desired unit.
- Example: Converting 164 lbs to kg uses 1 kg/2.20 lbs or steps through grams.
- Multi-step conversions combine several conversion factors.
Density and Volume Displacement
- Density = mass ÷ volume (d = m/v).
- Volume displacement calculates volume for irregular objects (final volume - initial volume).
- Both mass and volume must be known to find density.
Key Terms & Definitions
- SI System — International System of Units for measurement.
- Significant Figures (Sig Figs) — All measured digits including the uncertain last digit.
- Exact Number — A value with no uncertainty (definitions or counts).
- Conversion Factor — A ratio used to convert from one unit to another.
- Density — Mass per unit volume, d = m/v.
- Volume Displacement — Method to measure volume by fluid rise.
Action Items / Next Steps
- Practice homework problems on sig figs, conversions, and density.
- Review section problems 2.13, 2.27, 2.39, sample problems 2.7, 2.8, and study check 2.9.
- Visit office hours if struggling with any concepts.