A certain indicator HA has a Ka value of 25105 The protonate

A certain indicator, HA, has a Ka value of 2.5×105. The protonated form of the indicator is yellow and the ionized form is red.

What is the pKa of the indicator?

What is the color of this indicator in a solution with pH = 7?

orange

yellow

red

= -log(2.5 x 10^-5)

= 4.60

Pka of indicator = 4.60

pH = pKa + log([A-]/[HA])

6 = 4.60 + log([A-]/[HA])

log([A-]/[HA]) = 1.40

[A-]/[HA] = 10^1.40 = 25.11

[A-] = 25.11 x [HA]

From the above equation , it is evident that the concentration of the ionized form A- is 25.11 times to the concentration of the protonated form HA.

It shows red color