BackChapter 2: Chemistry and Measurements – Study Notes
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Chapter 2: Chemistry and Measurements
2.1 Metric and SI Units: Names and Abbreviations
This section introduces the fundamental units and abbreviations used in chemistry for measuring volume, length, mass, temperature, and time. The metric and International System of Units (SI) are standard systems for scientific measurements.
Metric Units: Commonly used units include liter (L) for volume, meter (m) for length, gram (g) and kilogram (kg) for mass, degree Celsius (°C) for temperature, and second (s) for time.
SI Units: The SI system is based on multiples of ten and uses standardized prefixes.
Examples: 1 mL = 0.001 L; 1 kg = 1000 g
Example: The abbreviation for meter is "m" and for kilogram is "kg".
2.2 Significant Figures in Measurement
Significant figures reflect the precision of a measured or calculated quantity. The number of significant figures in a measurement is determined by the measuring instrument and the method used.
Measured Number: Obtained by using a measuring device; the number of significant figures depends on the instrument's precision.
Exact Number: Obtained by counting or by definition; has an infinite number of significant figures.
Significant Figures: All nonzero digits and zeros between nonzero digits are significant. Leading zeros are not significant; trailing zeros are significant only if there is a decimal point.
Example: 2.50 has three significant figures; 0.0045 has two significant figures.
2.3 Determining Significant Figures in Calculations
When performing calculations, the result should reflect the precision of the measurements used. The number of significant figures in the answer is determined by the least precise measurement.
Multiplication/Division: The answer should have the same number of significant figures as the measurement with the fewest significant figures.
Addition/Subtraction: The answer should have the same number of decimal places as the measurement with the fewest decimal places.
Example: 2.5 × 3.42 = 8.6 (two significant figures)
2.4 Prefixes in Metric Units
Prefixes are used to indicate multiples or fractions of metric units. Each prefix represents a specific power of ten.
Kilo- (k): or 1,000 times the base unit
Centi- (c): or 0.01 times the base unit
Milli- (m): or 0.001 times the base unit
Prefix | Symbol | Factor |
|---|---|---|
Kilo | k | 1,000 |
Centi | c | 0.01 |
Milli | m | 0.001 |
Example: 1 kilometer (km) = 1,000 meters (m)
2.5 Conversion Factors
Conversion factors are ratios used to express a quantity in different units. They are essential for solving problems involving unit conversions.
Definition: A conversion factor is a ratio that expresses how many of one unit are equal to another unit.
Example:
Percent as Conversion Factor: Percent can be used to relate parts to a whole, such as 10% meaning 10 parts per 100.
Example: To convert 5 inches to centimeters:
2.6 Using Conversion Factors for Unit Changes
Conversion factors allow quantities to be expressed in different units by multiplying by the appropriate ratio.
Multiple Conversion Factors: Sometimes, more than one conversion factor is needed to reach the desired unit.
Dimensional Analysis: The process of multiplying by conversion factors to cancel units and obtain the correct answer.
Example: To convert 2.0 hours to seconds:
2.7 Density and Its Use in Calculations
Density is a physical property that relates the mass of a substance to its volume. It is commonly used to calculate mass, volume, or to identify substances.
Definition: Density () is the ratio of mass () to volume ():
Units: Common units for density are g/mL or g/cm3.
Specific Gravity: The ratio of the density of a substance to the density of water (1.00 g/mL).
Example: If a substance has a mass of 25 g and a volume of 5 mL, its density is:
