Topic 12.1.2 Hydrogen Ions and pH

• pH of an aqueous solution is a measure of acidity and alkalinity. It is related to the hydrogen ion concentration of the solution, which is measured in mol dm ^-3.
• To calculate the pH of a solution using the concentration of H+ ions, pH = -log v 10 [H^+] (pH = - log x [H+ concentration].
• Hydrogen ion concentration is measured in mol dm^-3.
• To calculate the hydrogen ion concentration from a given pH value, use [H^+ (aq)] = 10^-pH (hydrogen ion concentration = 10 to the power of -pH).
• pH is a logarithmic scale from 0-14 that gives the concentration of H+ ions in a solution. 0 is an acidic solution with a high concentration of H+ ions, whilst 14 is a basic solution with a low concentration of H+ ions.
• When a weak acid partially dissociates in an aqueous solution, there is a significant amount of undissociated acid in the solution. Therefore, it is necessary to refer to the acid dissociation constant, Ka.
• If we use HA to represent a weak acid, then its equation for dissociating in an aqueous solution is HA ⇌ H+ (aq) + A- (aq).
• To calculate the constant Ka, Ka = [H+][A-]/[HA].
• The concentrations in the expression for Ka are the equilibrium concentrations. However, if the value of Ka is very small, then the concentration of the undissociated acid at equilibrium is very similar to the initial concentration of the acid. Therefore, we can assume that concentration at equilibrium = initial concentration.
• The larger the Ka value, the stronger the acid.
• The smaller the pKa value, the stronger the acid.
• pKa is a logarithmic acid dissociation constant, representing how acidic something is. A low value indicates a strong acid.