General Chemistry Study Notes: Measurement, Conversions, Significant Figures, and Classification of Matter

Measurement and Metric Conversions

Metric Prefixes and Unit Conversions

Understanding metric prefixes and unit conversions is essential in general chemistry for accurate measurement and calculation. The metric system uses prefixes to indicate powers of ten, allowing for easy conversion between units.

• Metric Prefixes: Examples include milli- (10-3), micro- (10-6), kilo- (103), mega- (106), nano- (10-9).

• Unit Conversion: To convert between units, multiply by the appropriate conversion factor. For example, to convert milligrams to micrograms, use $1\ \text{mg} = 1000\ \mu\text{g}$.

• Example: Convert 250 milligrams to micrograms: $250\ \text{mg} \times 1000 = 2.5 \times 10^5\ \mu\text{g}$.

Additional info: Always set up conversion factors so that units cancel appropriately, leaving the desired unit.

English to Metric Conversions

Conversions between English and metric units are common in chemistry, especially for mass, length, and volume.

• Common Conversion Factors: 1 inch = 2.54 cm, 1 lb = 453.6 g, 1 mile = 1.609 km, 1 gallon = 3.785 L.

• Example: Convert 21 inches to centimeters: $21\ \text{in} \times 2.54\ \text{cm/in} = 53.34\ \text{cm}$.

Volume and Density Calculations

Density is a physical property defined as mass per unit volume. It is used to convert between mass and volume.

• Formula: $\text{Density} = \frac{\text{Mass}}{\text{Volume}}$

• Example: If the density of Fe is 7.87 g/cm3, the mass of 100 cm3 of Fe is $7.87\ \text{g/cm}^3 \times 100\ \text{cm}^3 = 787\ \text{g}$.

Temperature Conversions

Celsius, Fahrenheit, and Kelvin

Temperature is measured in Celsius (°C), Fahrenheit (°F), and Kelvin (K). Converting between these units is important for laboratory work and chemical calculations.

• Celsius to Kelvin: $K = ^\circ C + 273.15$

• Celsius to Fahrenheit: $^\circ F = (^\circ C \times \frac{9}{5}) + 32$

• Fahrenheit to Celsius: $^\circ C = (^\circ F - 32) \times \frac{5}{9}$

• Example: Convert 25°C to Kelvin: $25 + 273.15 = 298.15\ K$

Additional info: Kelvin is the SI unit for temperature and is used in scientific calculations involving thermodynamics.

Significant Figures

Rules for Significant Figures

Significant figures reflect the precision of a measurement. Correctly identifying and using significant figures is crucial for reporting scientific data.

• Nonzero digits are always significant.

• Zeros between nonzero digits are significant.

• Leading zeros are not significant.

• Trailing zeros in a decimal number are significant.

• Example: 0.00350 has 3 significant figures.

Calculations with Significant Figures

• Addition/Subtraction: The result should have the same number of decimal places as the measurement with the fewest decimal places.

• Multiplication/Division: The result should have the same number of significant figures as the measurement with the fewest significant figures.

• Example: $6.0 \times 10^2 \times 1.5 \times 10^3 = 9.0 \times 10^5$ (2 significant figures)

Classification of Matter

Pure Substances and Mixtures

Matter can be classified as pure substances or mixtures. Pure substances have a fixed composition, while mixtures have variable composition.

• Pure Substance: A material with a constant composition and distinct chemical properties. Examples: graphite (carbon), glucose (C6H12O6).

• Mixture: A combination of two or more substances where each retains its own properties. Examples: koolaid, chocolate chip cookies.

• Homogeneous Mixture: Uniform composition throughout (e.g., salt water).

• Heterogeneous Mixture: Non-uniform composition (e.g., salad).

Elements and Compounds

Elements are pure substances consisting of one type of atom. Compounds are pure substances composed of two or more elements chemically combined in fixed ratios.

• Element: Nitrogen gas (N2), iron rod (Fe).

• Compound: Table salt (NaCl), carbon monoxide (CO), distilled water (H2O).

Classification Table

The following table classifies substances as elements, compounds, pure substances, or mixtures:

Substance	Element	Compound	Pure Substance	Mixture	Homogeneous	Heterogeneous
Nitrogen gas	✔		✔
Table salt (NaCl)		✔	✔
Orange juice without pulp				✔	✔
Orange juice with pulp				✔		✔
Iron rod	✔		✔
Distilled water		✔	✔
Beef stew				✔		✔
Carbon monoxide (CO)		✔	✔
Wine				✔	✔
Hot coffee				✔	✔

Additional info: Homogeneous mixtures are also called solutions; heterogeneous mixtures have visibly different parts.

Summary of Key Concepts

• Metric and English unit conversions are foundational for chemical calculations.

• Temperature conversions between Celsius, Fahrenheit, and Kelvin are necessary for laboratory work.

• Significant figures ensure precision and accuracy in measurements and calculations.

• Matter is classified as elements, compounds, and mixtures, with further distinction between homogeneous and heterogeneous mixtures.