SI Units, Prefixes, and Dimensional Analysis

SI Prefixes and Unit Conversions

Understanding SI prefixes and unit conversions is essential for solving quantitative problems in chemistry. Prefixes indicate powers of ten and are used to express very large or very small quantities.

• SI Prefixes: Common prefixes include kilo (k, ), centi (c, ), milli (m, ), micro (μ, ), nano (n, ), and mega (M, ).

• Unit Conversions: Use conversion factors to switch between units (e.g., hours to seconds, grams to kilograms). Set up conversion factors so that units cancel appropriately.

• Dimensional Analysis: A systematic approach to problem-solving that uses conversion factors to move from one unit to another.

Example: Convert 2.5 kilometers to meters.

• 1 kilometer = 1000 meters, so

Significant Figures and Rounding

Rules for Significant Figures

Significant figures (sig figs) reflect the precision of a measured or calculated quantity. Proper use of sig figs is important for reporting scientific data accurately.

• Nonzero digits are always significant.

• Zeros between nonzero digits are significant.

• Leading zeros are not significant.

• Trailing zeros are significant only if there is a decimal point.

• When performing calculations, round your answer to the correct number of significant figures based on the operation:

  • Multiplication/Division: The result should have as many sig figs as the measurement with the fewest sig figs.

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

Example: (rounded to two sig figs)

The Periodic Table and Chemical Classification

Groups, Regions, and Types of Elements

The periodic table organizes elements by increasing atomic number and similar chemical properties.

• Groups: Vertical columns; elements in the same group have similar properties.

• Regions: Metals (left and center), nonmetals (upper right), metalloids (bordering the staircase line).

• Common Groups: Alkali metals (Group 1), alkaline earth metals (Group 2), halogens (Group 17), noble gases (Group 18).

Example: Sodium (Na) is an alkali metal; chlorine (Cl) is a halogen.

Basic Chemical Laws

Conservation of Mass, Definite Proportions, and Multiple Proportions

Several fundamental laws govern chemical reactions and compounds:

• Law of Conservation of Mass: Mass is neither created nor destroyed in a chemical reaction.

• Law of Definite Proportions: A chemical compound always contains the same elements in the same proportion by mass.

• Law of Multiple Proportions: When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in ratios of small whole numbers.

Example: Water (H2O) always contains hydrogen and oxygen in a 2:1 ratio by atom count.

Atomic Structure and Counting Particles

Protons, Neutrons, Electrons, Atomic Number, and Mass Number

Atoms are composed of protons, neutrons, and electrons. Understanding their numbers is key to identifying elements and isotopes.

• Atomic Number (Z): Number of protons in the nucleus; defines the element.

• Mass Number (A): Total number of protons and neutrons.

• Number of Neutrons:

• Number of Electrons: Equal to protons in a neutral atom; differs in ions.

Example: Carbon-14 has 6 protons, 8 neutrons, and 6 electrons (if neutral).

Counting Atoms, Moles, and Particles

Conversions Between Mass, Moles, and Particles

The mole is the SI unit for amount of substance. It allows conversion between atoms/molecules and grams.

• Avogadro's Number: particles per mole.

• Mole-Mass Conversion:

• Mole-Particle Conversion:

Example: 18 g of water () is 1 mole, or molecules.

Chemical Nomenclature

Binary Covalent and Ionic Compounds

Chemical nomenclature is the system for naming compounds. There are different rules for covalent and ionic compounds.

• Binary Covalent Compounds: Composed of two nonmetals. Use prefixes (mono-, di-, tri-, etc.) to indicate the number of atoms.

• Ionic Compounds: Composed of metals and nonmetals or polyatomic ions. Name the cation first, then the anion. Use Roman numerals for metals with multiple charges.

• Polyatomic Ions: Memorize common polyatomic ions and their charges.

Example: CO2 is carbon dioxide; NaCl is sodium chloride.

Common Polyatomic Ions

Balancing Chemical Equations

Law of Conservation of Mass in Reactions

Balancing chemical equations ensures that the same number of each type of atom appears on both sides of the equation, reflecting the law of conservation of mass.

• Adjust coefficients (whole numbers in front of formulas) to balance atoms for each element.

• Never change subscripts in chemical formulas.

Example:

Calculating Molar Mass

Molar Mass of Molecules

The molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). It is calculated by summing the atomic masses of all atoms in a molecule.

• Formula:

Example: For H2O: