Question

Bromocresol green, Ka=2.0×10−5, is yellow in its protonated form (HX) and blue in its ionized form (X−). At what pH will bromocresol green be a perfect green color?

Answer 1

pH = 4,70

Step-by-step explanation:

For bromocresol, as HX is yellow and X⁻ is blue, the indicator will be a perfect green color when HX = X⁻.

Using Henderson-Hasselbalch formula:

pH = pka + log [X⁻]/[HX] (1)

Where pka is -log ka = -log2,0x10⁻⁵ = 4,70

As HX = X⁻, [HX] / [X⁻] = 1

Replacing in (1):

pH = 4,70 + log 1

pH = 4,70 + 0

pH = 4,70

I hope it helps!