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?