Understanding Acids, Bases, and the pH Scale

Acids

Substances that dissolve in water to produce hydrogen ions (H+).

Bases

Ionic compounds that dissociate into cations and hydroxide ions (OH-) in water.

Arrhenius acids

Acids that produce hydrogen ions (H+) in aqueous solution.

Arrhenius bases

Bases that produce hydroxide ions (OH-) in aqueous solution.

Bronsted-Lowry acids

Substances that can donate a hydrogen ion (H+) to another substance.

Bronsted-Lowry bases

Substances that can accept a hydrogen ion (H+).

Conjugate acid

The molecule or ion that forms when one H+ ion is added to a base.

Conjugate base

The molecule or ion that differs by one H+ ion from an acid.

Strong acids

Acids that dissociate completely in water, e.g., HI, HBr, HCl, H2SO4, HNO3.

Weak acids

Acids that do not dissociate completely in water, e.g., HF, H2CO3, H2S, H2O.

Strong bases

Bases that dissociate completely in water, e.g., NaOH, LiOH, Ca(OH)2, Ba(OH)2, KOH.

Weak bases

Bases that do not dissociate completely in water, e.g., NH3.

pH scale

A scale that goes from 0 to 14, used to represent the concentration of acids and bases.

pH calculation

pH = -log[H+], where [H+] is the molar concentration of hydrogen ions.

Acidic solutions

Solutions with a pH less than 7.0.

Basic solutions

Solutions with a pH greater than 7.0.

Neutral solutions

Solutions with a pH equal to 7, having equal concentrations of acid and base.

pOH calculation

pOH = -log[OH-], where [OH-] is the molar concentration of hydroxide ions.

pH and pOH relationship

pH + pOH = 14.

Example of pH calculation

For [H+] = 1.0 x 10^-2 M, pH = 2.

[H+] concentration calculation

For pH = 8.25, [H+] = 10^-8.25 = 5.6 x 10^-9 M.

HCl dissociation

HCl (g) → H+ (aq) + Cl- (aq) in water.

NaOH dissociation

NaOH (s) → Na+ (aq) + OH- (aq) in water.