D36.1 pH and Ratio of Conjugate Base to Conjugate Acid
The ionization constant expression for a weak acid HA is:
Rearranging gives:
This equation shows that the ratio of concentration of conjugate acid over concentration of conjugate base determines the hydrogen-ion concentration of a solution. The bigger the concentration of conjugate acid relative to conjugate base, the bigger the hydrogen-ion concentration is and the more acidic the solution is.
Conversely, if we know the hydrogen-ion concentration of a solution, the ratio of concentration of conjugate acid over concentration of conjugate base can be calculated, as this rearranged version of the equation shows.
These equations can be rearranged to show how pH is related to composition of a conjugate acid-base pair. Taking the negative logarithm of both sides of the first equation shows that the higher the pH is the greater the ratio of concentration of conjugate base over concentration of conjugate acid:
Exercise: Acid/base Ratio and pH
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