Determining Charge State of Predominant Species

[Maddy_Rotkosky_3I]

Hi there! I was wondering how the pH and pKa of an acid can help you determine if that acid is charged, neutral, or unknown?

[Joah Lee]

When pH < pKa then HA is dominant in solution, making it neutral

When pH> pKa then A- is dominant in the solution, making it charged

HA + H2O <--> H3O+ + A-

[kaitlyn nguyen 2F]

It is also helpful to look at the Henderson Hasselbalch equation: pH = pKa + log [A-]/[HA] which is log of a conjugate base/weak acid. When the components of the equation are rearranged, we get [A-]/[HA]=10^pH-pKa. From there, the values of pH and pKa can be put into the equation. If pH is greater, 10 will be raised to a greater positive value which means there is more [A-] or conjugate base, so it is dominated by charged species. If pKa is greater, the value will be smaller, meaning [HA] dominates which is neutral.

[Tamara Andrijanic 1L]

"When pH < pKa then HA is dominant in solution, making it neutral

When pH> pKa then A- is dominant in the solution, making it charged"

It is important to note that this is true when the solution is acidic, but when the solution is basic it is the other way around, when pH<pKa then the charged particle is dominant in the solution, and if pH>pKa then the uncharged one is, because in the opposite case, we're looking at pOH and pkB. pOH+pH=14, so if the pOH is greater, this means that the pH is lesser, so this relationship is shown in the two hw questions relating to this- 9/10 on week 2 online homework.