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2) Phenolphthalein (C20H1404) is a dye very commonly used as indicator in acid-base titrations. The simplified
generic expression for the dissociation of this molecule in water is shown in the equation below, where Hin
represents the phenolphthalein molecule and In is the phenolphthalein ion. Their appearance and relevant
properties are also shown below.
HIn (S) + H20 (1) = H30* (aq) + In" (aq)
a. Can this equilibrium be controlled by changing the pH of the solution? Briefly explain.
b. What color do you expect to see in the solution when you add acid to it? Briefly explain.

Sagot :

Abibatibrahim11idn

Answer:

methyl orange is an indicator

The pKa of phenolphthalein is 9.3.

Who discovered phenolphthalein?

By combining phenol and phthalic anhydride in the presence of sulfuric acid or zinc chloride, German chemist Adolf von Baeyer created phenolphthalein in 1871.

What is phenolphthalein?

It is a weak acid that is colorless. Its base due to loss of proton ,[tex]C_{20} H_{13} O_{4}^{-}[/tex] is an intense shade of pinkish-red.

What is the molecular formula of phenolphthalein?

The molecular formula is C₂₀H₁₄O₄.

What is the structure of phenolphthalein?

  • Three hexagonal structures, one pentagonal structure, two alcoholic groups, and one ketone group make up the structure of phenolphthalein.
  • Aside from that, the carbon, hydrogen, and oxygen chains make up the structure of phenolphthalein.

What is the equation at equilibrium?

The equation is:

[tex]HIn +H2O----- > In^{-} +H3O^{+}[/tex]

Can the equilibrium change with change is pH?

  • If [tex]H_{3} O^{+}[/tex] is introduced (acidic solution), the equilibrium will be moved to the left.
  • Thus, in a very acidic solution, almost all of the pink [tex]In^{-}[/tex] will be converted into colorless [tex]HIn[/tex].
  • On the other hand, if the solution is made extremely basic, [tex]OH^{-}[/tex] ions will react with  [tex]HIn[/tex] molecules and change them into  [tex]In^{-}[/tex] , which will cause the equilibrium to shift to the right.
  • The phenolphthalein solution will turn pink as a result.

What color do you expect to see in the solution when you add acid to it?

  • When acid is added to the solution containing the indicator the color of the solution will begin to decrease in intensity.
  • It is because If [tex]H_{3} O^{+}[/tex] is introduced (acidic solution), the equilibrium will be moved to the left.
  • Thus, in a very acidic solution, almost all of the pink [tex]In^{-}[/tex] will be converted into colorless [tex]HIn[/tex].

To learn more about phenolphthalein, acidic solution and weak acid here,

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