Bromcresol purple is a weak acid and can act as an acid-base indicator. The yellow acidic form is equal in amount to the purple base form when the pH is equal to 6.3. Determine the pK_a for bromcresol purple.  

• A. pK_a = 6.3
• B. pK_a = 7.7
• C. pK_a = 3.1
• D. pK_a = 12.6

Answer: (A)