I mixed 4.50mL of 10.0mM NaOH with .50mL phenol red. I need to calculate the pH of this solution. I have more information in my lab but I am not sure what data I need to look for. As part of the experiment I calculated pKa of Phenol Red. Do I use that pKa in the henderson-hasselbalch equation. If that is correct than I am not sure how to calculate the [HA]/[A-] ratio. Can anyone point me in the right direction?

Question

I mixed 4.50mL of 10.0mM NaOH with .50mL phenol red. I need to calculate the pH of this solution. I have more information in my lab but I am not sure what data I need to look for. As part of the experiment I calculated pKa of Phenol Red. Do I use that pKa in the henderson-hasselbalch equation. If that is correct than I am not sure how to calculate the [HA]/[A-] ratio. Can anyone point me in the right direction?

Answer 1

To calculate the pH of your solution, you can indeed use the Henderson-Hasselbalch equation. Before we go into detail, let's gather the necessary information:

1. pKa of Phenol Red: This will be used in the Henderson-Hasselbalch equation. Please provide this value.

Now, let's discuss the Henderson-Hasselbalch equation:

pH = pKa + log ([A-]/[HA])

In this equation:
- pH: The pH value you want to calculate.
- pKa: The dissociation constant of Phenol Red.
- [A-]: The concentration of the deprotonated form (A-) of Phenol Red.
- [HA]: The concentration of the protonated form (HA) of Phenol Red.

Here's how you can proceed:

1. Calculate the moles of NaOH: To determine the concentration of NaOH, you need to know the volume (4.50 mL) and molarity (10.0 mM). Using the formula:
Moles of NaOH = Volume (L) x Molarity

2. Determine the amount of Phenol Red: As you have mentioned, you mixed 0.50 mL of Phenol Red. To calculate the concentration, you need to know the initial concentration. Check your lab notes or provided information to find this.

3. Calculate the [A-]/[HA] ratio: Once you have the concentration of the deprotonated form ([A-]) and the protonated form ([HA]) of Phenol Red, you can determine their ratio.

4. Substitute the values into the Henderson-Hasselbalch equation: Use the pKa value you calculated for Phenol Red, along with the [A-]/[HA] ratio, and solve for the pH.

Note: The pKa value is important because it helps us understand the degree of ionization of a substance at a given pH.

Once you provide the pKa value of Phenol Red, I can assist you further with the calculations and obtaining the pH of your solution.