Measurement and Significant Figures

Unit Conversions and Significant Figures

Understanding how to convert between units and report answers with the correct number of significant figures is fundamental in chemistry.

• Unit Conversion: To convert between units, use appropriate conversion factors. For example, to convert centimeters to inches:

• Significant Figures: The number of significant figures in your answer should reflect the least number of significant figures in the values used in the calculation.

• Example: 5.99 cm / 2.54 cm = 2.36 in (3 significant figures)

Reporting Measurements

• When reporting measurements, round to the correct number of significant figures based on the precision of the measuring instrument.

• Example: 0.1457 cm and 1.0457 cm should be reported as 0.146 cm and 1.05 cm (3 significant figures).

Measured vs. Exact Numbers

• Measured Number: Obtained from a physical measurement (e.g., length, mass).

• Exact Number: Defined value, not measured (e.g., 1 dozen = 12).

Density and Specific Gravity

• Density: The mass of a substance per unit volume.

• Example: 3.00 g / 3.00 mL = 1.00 g/mL

• Specific Gravity: The ratio of the density of a substance to the density of water at the same temperature.

Chemical and Physical Changes

Definitions and Examples

• Chemical Change: A process in which substances are transformed into new substances with different properties.

• Physical Change: A process that affects the form of a chemical substance, but not its chemical composition.

Atomic Structure and Electron Configuration

Subatomic Particles

• Protons: Inside the nucleus, positive charge

• Neutrons: Inside the nucleus, no charge

• Electrons: Outside the nucleus, negative charge

Electron Configuration

• Describes the arrangement of electrons in an atom.

• Example: 1s22s22p2 is the configuration for carbon (C), which has 6 electrons.

• Abbreviated Configuration: Uses the previous noble gas to shorten notation.

• Extended Configuration: Lists all electrons in each orbital.

Valence Electrons and Dot Structures

• Valence Electrons: Electrons in the outermost shell, important for chemical bonding.

• Lewis Dot Structure: Uses dots to represent valence electrons around the element symbol.

• Example: Carbon (C) has 4 valence electrons; its dot structure has 4 dots.

Isotopes

• Isotope: Atoms of the same element with different numbers of neutrons.

• Example: Carbon-13 has 6 protons and 7 neutrons (mass number 13).

Classification of Matter

Elements, Compounds, and Mixtures

• Element: Pure substance made of only one kind of atom (e.g., Al - aluminum).

• Compound: Substance made of two or more elements chemically combined (e.g., water).

• Mixture: Physical combination of two or more substances (e.g., blood, urine).

Physical and Chemical Properties

• Physical Properties: Can be observed without changing the substance (e.g., melting point, boiling point).

• Chemical Properties: Describe how a substance reacts with other substances (e.g., sodium ignites in water).

Temperature and Unit Conversions

Temperature Conversion

• To convert Celsius to Kelvin:

• Example: 25°C = 298.15 K

Length Conversion

• To convert inches to centimeters:

• Example: 12 in × 2.54 cm/in = 30 cm

Qualitative vs. Quantitative Statements

• Qualitative: Describes qualities or characteristics (e.g., "The plants have green leaves").

• Quantitative: Involves numerical measurements (e.g., "The plant is 15 cm tall").

Periodic Table and Atomic Properties

Metals, Nonmetals, and Metalloids

• Metal: Good conductor, malleable, shiny (e.g., Na, Al).

• Nonmetal: Poor conductor, brittle (e.g., Cl, F).

• Metalloid: Properties intermediate between metals and nonmetals.

Atomic Radius

• Atomic radius decreases across a period from left to right.

• Example: Fluorine (F) has a smaller atomic radius than sodium (Na).

Similar Chemical Properties

• Elements in the same group (column) of the periodic table have similar chemical properties (e.g., Li and K).

Problem-Solving Techniques

Dimensional Analysis

• Use conversion factors to ensure units cancel appropriately, leaving the desired unit.

• Example: To find how many mL to infuse: (800 mg/5.0 mL) / 200 mg = 20 mL every 8 hours.

Volume by Displacement

• Used to determine the volume of an irregular solid by measuring the volume of water displaced.

Conversion Factors

• Conversion factors are ratios used to express the same quantity in different units.

Summary Table: Key Properties and Classifications