To determine the pKa of the acid tested using a graph of NaOH drops added versus pH, follow these steps:
- Prepare the Experiment: Start by setting up a titration with the acid solution. Gradually add NaOH while measuring the pH after the addition of each drop.
- Plot the Data: Once you have your pH readings recorded for each drop of NaOH added, create a graph. The x-axis will represent the number of drops of NaOH added, and the y-axis will show the corresponding pH values.
- Identify the Equivalence Point: As you plot the points, you’ll notice a region where the pH changes dramatically with the addition of NaOH. This steep part of the graph indicates the equivalence point of the titration.
- Locate the Half-equivalence Point: The pKa can be found at the half-equivalence point, which is when half of the acid has been neutralized. On the graph, this point can be found by taking the volume (or number of drops) where the pH is about halfway between the initial pH of the acid and the final pH at the equivalence point.
- Read the Corresponding pH: At the half-equivalence point, read the pH value from the y-axis. This value is the pKa of the acid you tested.
In summary, to find the pKa, you need to plot the pH against the number of NaOH drops, locate the equivalence and half-equivalence points, and read the corresponding pH value at the half-equivalence point. This method provides a direct visual representation of the acid’s dissociation properties, giving you an accurate determination of the pKa.