What is the point called on a graph of pH vs log {[A-]/[HA]} when [A-] = [HA]? Graphs are listed as “y vs. x”.
That's the half-way point in the neutralization of acid/base or base/acid.
Sometimes called the mid-point. That's the point at which pH = pKa for the titration of an acid or pKb for the titration of a base.
The point on a graph of pH vs. log {[A-]/[HA]} where [A-] (concentration of the anion) is equal to [HA] (concentration of the acid), represents the equivalence point in an acid-base titration. In this case, the ratio {[A-]/[HA]} is equal to 1, meaning that the concentrations of both the acid (HA) and its conjugate base (A-) are equal.
To understand how to find this point on the graph, we first need to understand the relationship between pH and the ratio {[A-]/[HA]}.
The pH of a solution is a measure of its acidity or basicity. It is calculated using the negative logarithm (base 10) of the concentration of hydrogen ions (H+). Thus, pH = -log[H+].
In an acid-base reaction, when an acid (HA) reacts with water, it donates a proton (H+) to form its conjugate base (A-) and hydronium ions (H3O+). The equilibrium between HA and A- can be represented by the equation: HA ⇌ A- + H+.
The ratio {[A-]/[HA]} is a measure of the extent of ionization of the acid. When this ratio is low (close to 0), it indicates that most of the acid remains in its undissociated form (HA), making the solution acidic. On the other hand, when this ratio is high (close to infinity), it suggests that the acid has completely ionized into its conjugate base (A-), making the solution basic.
Now, in terms of the graph of pH vs. log {[A-]/[HA]}, the x-axis represents the logarithm (base 10) of the ratio {[A-]/[HA]}, while the y-axis represents the pH.
At the equivalence point of an acid-base titration, the moles of acid (HA) are equal to the moles of its conjugate base (A-) added to the solution. This means that the ratio {[A-]/[HA]} becomes 1.
To find the point on the graph corresponding to this equivalence point, you would look for the value of log {[A-]/[HA]} that gives a pH of 7. This is because a pH of 7 is considered neutral, indicating an equal concentration of H+ and OH- ions in the solution.
Therefore, on the graph of pH vs. log {[A-]/[HA]}, the point where [A-] = [HA] is equivalent to the point where the pH is equal to 7, indicating a neutral solution.