Solution A is yellow when alizarin yellow is added and blue when thymol blue is added. Which of the following correctly identifies Solution A?
A. 3.2 x 10-4 mol/L HCI
B. 3.2 x 10-4 mol/L NaOH
C. 6.3 x 10-5 mol/L HCI
D. 6.3 x 105 mol/L NaOH

D. 6.3 x 10-5 mol/L NaOH

Explanation:

Alizarin yellow is an indicator that is yellow when pH < 10.1. In the same way, thymol blue is blue when pH > 9

That means the pH of the solution is between 9 - 10.1

Any acid as HCl could have a pH of these.

The solution of 3.2x10⁻⁴M NaOH has a pH of:

pOH = -log[OH-] = 3.49

pH = 14-pOH = 10.51. The pH of the solution is not 10.5

Now, the solution of 6.3x10⁻⁵M NaOH has a pH:

pOH = -log[OH-] = 4.2

pH = 14-pOH = 9.8

The pH of the solution could be 9.8. Right option is: