You considered the properties of two acid-base indicators, phenolphthalein and methyl orange. Many indicators are weak acids in water and establish the equilibrium: HIn(aq)(Color 1)+ H2O(l) → H3O (aq) + In-(aq)(Color 2). Indicators change color depending on whether they are in a protonated (HIn) or unprotonated (In-) form. What is the equilibrium expression for the phenolphthalein indicator in water and what colors are the protonated and unprotonated forms of the indicator?

Equilibrium constant in terms of concentration is defined as the ratio of concentration of products to the concentration of reactants each raised to the power their stoichiometric ratios. It is expressed as [tex]K_{c}[/tex]

Equilibrium reaction of phenolphthalein indicator in water:

[tex]PhtH(aq)(Colorless)+ H_2O(l)\rightleftharpoons H_3O^+ (aq) + Pht^-(aq)(pink)[/tex]

An expression for equilibrium constant is given as:

[tex]K_c=\frac{[H_3O^+][Pht^-]}{[PhtH]}[/tex]

When protonated it shows no color that is it is colorless but when it is unprotonated form it shows pink color.