Ka, the acidity constant
pH is a number that is a property of a solution (pH = -log[H3O+]) where [H3O+] is in .
The equilibrium constant Ka is a number that is a property of an acid-base conjugate pair.
Examples of acid-base conjugate pairs are
The relationship between Ka and the concentrations of H3O+, acid and conjugate base in a solution at equilibrium is given at the right.
The form of the reaction quotient expression is defined by reaction of one mole of the acid with water.
acid + H2O
H3O+ + conjugate base
Reaction quotient expressions have products in the numerator and reactants in the denominator. Each concentration is raised to a power equal to its coefficient in the balanced equation for the reaction.
Because the form of the reaction quotient expression is the same for all acid-base conjugate pairs, comparison of Ka for two acid-base conjugate pairs indicates the relative tendency of the two acids to donate a proton.
For stronger acids, Ka is higher as the tendency of the acid to donate H+ is higher.
pH is measured with a pH meter or pH paper
pH depends on which acids and bases are present and their concentration.
pH depends on which acids and bases are present and their concentration.
The equilibrium constant Ka is a number that is a property of an acid-base conjugate pair.
Examples of acid-base conjugate pairs are
CH3CO2H (acid) and CH3CO2– (conjugate base)
NH4+ (acid) and NH3 (conjugate base)
NH4+ (acid) and NH3 (conjugate base)
The relationship between Ka and the concentrations of H3O+, acid and conjugate base in a solution at equilibrium is given at the right.
Solutions containing weak acids and/or their conjugate bases achieve equilibrium on mixing.
The form of the reaction quotient expression is defined by reaction of one mole of the acid with water.
acid + H2O
H3O+ + conjugate base| Ka = | [conjugate base ][H3O+] |
| [acid] | |
| number | reaction quotient |
Reaction quotient expressions have products in the numerator and reactants in the denominator. Each concentration is raised to a power equal to its coefficient in the balanced equation for the reaction.
Because the form of the reaction quotient expression is the same for all acid-base conjugate pairs, comparison of Ka for two acid-base conjugate pairs indicates the relative tendency of the two acids to donate a proton.
For stronger acids, Ka is higher as the tendency of the acid to donate H+ is higher.
The forward reaction is more significant (higher [conjugate base] than [acid] at equilibrium).