Bromophenol blue is a weak acid (represented as BH) that acts as an acid-base indicator - VCE - SSCE Chemistry - Question 2 - 2007 - Paper 1

At pH 7, the concentration of hydrogen ions is:

$[H^+] = 10^{-7} \text{ M}$

Using the equilibrium expression again:

$K_a = 6.3 \times 10^{-5} = \frac{[B^-][H^+]}{[BH]}$

Let [BH] = y and [B^-] = x. Rearranging gives:

$x = 6.3 \times 10^{-5} \cdot \frac{y}{10^{-7}}$

At pH 7, we can substitute pK_a:

$pK_a = -\log(6.3 \times 10^{-5}) \approx 4.20$

Thus:

$\text{Ratio} = \frac{[B^-]}{[BH]} = \frac{x}{y} = \frac{6.3 \times 10^{-5}}{10^{-7}} = 630$

At pH 7, we determined the ratio of [B^-] to [BH] to be 630:1, indicating that the concentration of the conjugate base (B^-) significantly exceeds that of the acid (BH). Since bromophenol blue is blue in its base form (B^-), the solution at pH 7 will appear blue.