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Chemistry Reference
Periodic Table of Elements
Generating table...
Common Formulas
Density: D = m / V
Molarity: M = moles / L
Mole Conversion (Mass): Moles = Grams / Molar Mass
Mole Conversion (Particles): Moles = Particles / NA
Ideal Gas Law: PV = nRT
Combined Gas Law: (P₁V₁) / T₁ = (P₂V₂) / T₂
pH: pH = -log[H⁺]
pOH: pOH = -log[OH⁻]
pH + pOH: pH + pOH = 14 (at 25°C)
Percent Composition: % Element = (Mass of Element / Molar Mass of Compound) * 100%
Significant Figures (Sig Figs)
Indicate the precision of a measurement.
Counting Rules:
Non-zero digits are always significant.
Zeros between non-zero digits are significant.
Leading zeros are NOT significant.
Trailing zeros are significant ONLY IF there's a decimal point.
Exact numbers have infinite significant figures.
Calculation Rules:
Add/Subtract: Result has same number of decimal places as value with fewest decimal places.
Multiply/Divide: Result has same number of sig figs as value with fewest sig figs.
Constants & Common Ions
Constants:
Avogadro's Number (NA): ≈ 6.022 x 10²³ mol⁻¹
Ideal Gas Constant (R): ≈ 0.08206 L·atm/(mol·K) or 8.314 J/(mol·K)
Common Polyatomic Ions:
Nitrate: NO₃⁻
Sulfate: SO₄²⁻
Phosphate: PO₄³⁻
Carbonate: CO₃²⁻
Hydroxide: OH⁻
Ammonium: NH₄⁺
Acetate: C₂H₃O₂⁻ / CH₃COO⁻
Permanganate: MnO₄⁻
Cyanide: CN⁻
Bicarbonate: HCO₃⁻
Key Definitions
Mole (mol)
The SI unit for the amount of substance, containing Avogadro's number (approx. 6.022 x 10²³) of particles (atoms, molecules, ions, etc.).
Molar Mass (M)
The mass (in grams) of one mole of a substance (g/mol). Calculated by summing the atomic masses of all atoms in a chemical formula.
Molarity (M)
A unit of concentration, defined as moles of solute per liter of solution (mol/L).
Stoichiometry
The study of the quantitative relationships (ratios) between reactants and products in chemical reactions, based on balanced equations.
Limiting Reactant
The reactant in a chemical reaction that is completely consumed first, thereby limiting the amount of product that can be formed.
Atom
The smallest unit of an element that retains the chemical properties of that element, consisting of a nucleus (protons and neutrons) and electrons.
Molecule
An electrically neutral group of two or more atoms held together by covalent chemical bonds.
Compound
A pure substance formed when two or more different chemical elements are chemically bonded together in a fixed ratio.
Element
A pure substance consisting only of atoms that all have the same number of protons in their nuclei (same atomic number).
Ion
An atom or molecule that has gained or lost one or more electrons, giving it a net positive (cation) or negative (anion) electrical charge.
Isotope
Atoms of the same element (same number of protons) that have different numbers of neutrons, resulting in different mass numbers.
Acid
A substance that produces hydrogen ions (H⁺) or hydronium ions (H₃O⁺) when dissolved in water (Arrhenius), or a substance that donates a proton (H⁺) (Brønsted-Lowry).
Base
A substance that produces hydroxide ions (OH⁻) when dissolved in water (Arrhenius), or a substance that accepts a proton (H⁺) (Brønsted-Lowry).
pH
A measure of the acidity or alkalinity (basicity) of an aqueous solution, based on the concentration of hydrogen ions ([H⁺]). pH = -log[H⁺]. Scale typically 0-14, with 7 being neutral.
Valence Electrons
Electrons in the outermost energy shell of an atom, which are involved in forming chemical bonds.
Electronegativity
A measure of the tendency of an atom to attract a bonding pair of electrons.
Ionic Bond
A chemical bond formed through the electrostatic attraction between oppositely charged ions, typically formed by the transfer of electrons between a metal and a nonmetal.
Covalent Bond
A chemical bond formed by the sharing of one or more pairs of electrons between atoms, typically between nonmetals. Can be polar (unequal sharing) or nonpolar (equal sharing).
Solution
A homogeneous mixture composed of two or more substances.
Solute
The substance that is dissolved in a solvent to form a solution; usually present in a smaller amount.
Solvent
The substance in which a solute dissolves to form a solution; usually present in a larger amount.
Catalyst
A substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change.
Activation Energy
The minimum amount of energy required for reactants to undergo a chemical reaction.
Exothermic Reaction
A chemical reaction that releases energy, usually in the form of heat or light.
Endothermic Reaction
A chemical reaction that absorbs energy from its surroundings, usually in the form of heat.