Conversions and Dimensional Analysis

SI Prefixes and Unit Conversions

Understanding SI prefixes and unit conversions is essential for solving quantitative problems in chemistry. Dimensional analysis allows you to convert between different units using conversion factors.

• SI Prefixes: Prefixes indicate powers of ten and are used to express very large or very small quantities.

• Common Prefixes:

• Dimensional Analysis: Use conversion factors to change from one unit to another. For example, to convert 5 kilometers to meters:

• Application: Used in density calculations, stoichiometry, and laboratory measurements.

Significant Figures and Rounding

Determining and Using Significant Figures

Significant figures (sig figs) reflect the precision of a measured or calculated quantity. Proper use of sig figs ensures that results are not over- or under-reported in terms of accuracy.

• Rules for Counting Significant Figures:

  • All nonzero digits are significant.

  • Zeros between nonzero digits are significant.

  • Leading zeros are not significant.

  • Trailing zeros after a decimal point are significant.

• Rounding: When performing calculations, round the final answer to the correct number of significant figures based on the operation (multiplication/division: fewest sig figs; addition/subtraction: fewest decimal places).

• Example: (rounded to 2 sig figs)

The Periodic Table

Groups, Regions, and Classification

The periodic table organizes elements by increasing atomic number and similar chemical properties. Understanding its structure is fundamental to predicting element behavior.

• 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).

Chemical Laws

Conservation of Mass, Definite Proportions, Multiple Proportions

Several fundamental laws govern chemical reactions and the composition of substances.

• 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 ratios of the masses of the second element that combine with a fixed mass of the first element are ratios of small whole numbers.

Atomic Structure and Counting Particles

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

Atoms are composed of protons, neutrons, and electrons. Understanding how to determine their numbers is crucial for 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; adjusted for charge in ions.

• Conversions: Use Avogadro's number () to convert between moles, mass, and particles.

Chemical Nomenclature

Naming Compounds and Polyatomic Ions

Chemical nomenclature provides systematic names for compounds. Mastery of naming conventions is essential for communication in chemistry.

• Binary Covalent Compounds: Composed of two nonmetals; use prefixes (mono-, di-, tri-, etc.).

• Ionic Compounds: Composed of metals and nonmetals; name the cation first, then the anion.

• Polyatomic Ions: Ions composed of multiple atoms. Memorize common polyatomic ions and their charges.

• Example: NaNO3 is named sodium nitrate.

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.

• Steps to Balance:

  1. Write the unbalanced equation.

  2. Count atoms of each element on both sides.

  3. Add coefficients to balance atoms, starting with the most complex molecule.

  4. Check your work.

• Example:

Balanced:

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: