Thymolphthalein is an indicator that changes color over a range from ph=9.3 to ph=10.5. what is ka of thymolphthalein?

According to Hasselbalch equation pH = pKa at the equal point. When pH equals the pKa value of the indicator, the base and acid forms of indicator are present in the ratio of 1:1 

(9.3 + 10.5) / 2 =9.9
pKa = 9.9
Using the equation pH = pKa at equal point we get
Ka = 10^- pKa
Ka = 10^- 9.9
Ka = 1.26 x 10^-10
Ka = 1 x 10^-10