Question 14.6: Consider bromthymol blue (Ka = 1 × 10^-7). At pH 6.5, (a) ca...

Consider bromthymol blue (K_{a} = 1 \times 10^{-7}). At pH 6.5,
(a) calculate the ratio [In^{-}]/[HIn].
(b) what is the color of the indicator at this point?

ANALYSIS

Information given:      K_{a} for bromthymol blue (1 \times 10^{-7})                                                                                                                                                       pH (6.5)
Information implied:                  color of bromthymol blue at different pH values
Asked for: (a)                                        [In^{-}]/[HIn]                                                                                                                                             (b) color of bromthymol blue at pH 6.5

STRATEGY

(a) Convert pH to [H^{+}] and substitute into Equation 14.1 to find   [In^{-}]/[HIn].

[H^+]=K_{a} \times \frac{[HB]}{[B^-]}          (14.1)
(b) Find the color of bromthymol blue at pH 6.5 by using the information in the above discussion of the colors for bromthymol blue at different pH values.