Acid and Bases

Arrhenius definition

Acids will increase the concentration of H+ in water.

Bases will increase the concentration of OH- in water.

A strong acid will completely dissociates in water, e.g.

HCl -> H+ + Cl-


HCl + H2O -> H3O+ + Cl-

A strong base will complete dissociate in water, e.g.

LiOH -> OH- + Li+

Brønsted-Lowry definition

An acid is a proton donor.

A base is a proton acceptor.

Hydrogen ion, H+, is a proton. It has no electron.

HCl + H2O -> H3O+ + Cl-

HCl has donated a proton to the water molecule, it is the acid.

The conjugate base is the original acid with the H+ removed. In this case the conjugate base in Cl-

Water accepted the proton, it is the base.

The conjugate acid is the original base with the H+ accepted. In this case the conjugate acid is H3O+

This also works for non water examples.

Lewis definition

An acid is an electron acceptor

A base is an electron donor.

Acid base introduction at Khan Academy.

Weak acids

Do not dissociate completely.

HF ⇌ H+ + F-


HCl -> H+ + Cl-

HF bond is much stronger than the HCl bond.


For HF dissolving in water at 25 degress celcius,

Dissociation constant for HF acid, KaHF is the given by

$$ K_{aHF} = \frac{[H^+][F^-]}{[HF]} $$

This is used for measuring concentrations of the different units.

autoionization of water

$$ K_w = [H^+][OH^-] = 10^{-7} * 10^{-7} = 10^{-14} $$

$$ pKw = 14 $$

This is all at 25 degrees celsius.


If you know the molarity of H+ ions (or hydronium ions, then you can calculate pH by:

pH = -log([H+])

log is in base 10, and [H+] is the concentration of H+ ions, measured is moles per litre.

pH < 7 means it is an acidic solution

pH > 7 means it is a basic solution.

pH of 7 is neutral solution. It is the pH of water at 25 degrees celsius.

pOH can also be calculated.

Negate pH is possible.

Definition of pH at Khan Academy.