Problem: Methyl orange, HMO, is a common acid-base indicator. Insolution it ionizes according to the equation:                                                HMO ↔  H++ MO-                                                  red          yellowIf methyl orange is added to distilled water, the solution turns yellow. If a drop or two of 6 M HCL is added to the yellow solution, it turns red. If to that solution one adds a few drops of 6 M NaOH the color reverts to yellow.a. Why does adding 6M HCL to the yellow solution of methyl orange tend to cause the color to change to red? (Note that in solution HCL exists as H+ and Cl-ions.)b. Why does adding 6 M NaOH to the red solution tend to make it turn back to yellow? (Note that in solution NaOHexists as Na+ and OH- ions.

FREE Expert Solution

We’re being asked to determine why the color of an indicator changes to red or yellow by adding 6M HCL or 6 M NaOH.


The indicator changes from red to yellow when going from its acidic (nonionized) form to its basic (ionized) form.


HA (acidic form)  A (basic form)



Recall that when:

• pH < pKa: the acidic form is dominant/in greater quantity

• pH = pKa: the acidic and basic forms are in equal quantities

• pH > pKa: the basic form is dominant/in greater quantity


HA (acidic form)  A (basic form)

pH < pKa     pKa   pH > pKa



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Problem Details

Methyl orange, HMO, is a common acid-base indicator. Insolution it ionizes according to the equation:

                                                HMO ↔  H++ MO-

                                                  red          yellow

If methyl orange is added to distilled water, the solution turns yellow. If a drop or two of 6 M HCL is added to the yellow solution, it turns red. If to that solution one adds a few drops of 6 M NaOH the color reverts to yellow.

a. Why does adding 6M HCL to the yellow solution of methyl orange tend to cause the color to change to red? (Note that in solution HCL exists as H+ and Cl-ions.)

b. Why does adding 6 M NaOH to the red solution tend to make it turn back to yellow? (Note that in solution NaOHexists as Na+ and OH- ions.

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