## In Part III, the phenolphthalein indicator is used to monitor the equilibrium shifts of the ammonia/ammonium ion system. The phenolphthalein

Question

In Part III, the phenolphthalein indicator is used to monitor the equilibrium shifts of the ammonia/ammonium ion system. The phenolphthalein equilibrium established with water is Hph(aq)(colorless) + H2O (l) H3O+ (aq) + ph-(aq)(pink or red). You compared the color of the solutions in three test tubes that initially contained 3 mL of 0.1 M ammonium hydroxide and a few drops of phenolphthalein indicator. In the first test tube, you added 1 M NH4Cl dropwise. What color change was observed and what did this color change indicate about the shift in the phenolphthalein equilibrium? a. The solution turned a more intense pink or red color indicating that the phenolphthalein equilibrium shifted to the left, producing more of the pink or red colored Hph.

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1 week 2022-11-24T16:01:44+00:00 1 Answer 0 views 0

1. The pink color in the solution fades. Some of the colored indicator ion converts to the colorless indicator molecule.

### Explanation

What’s the initial color of the solution?

$$\text{NH}_4\text{Cl}$$ is a salt soluble in water. $$\text{NH}_4\text{Cl}$$ dissociates into ions completely when dissolved.

$$\text{NH}_4\text{Cl} \; (aq)\to {\text{NH}_4}^{+} \; (aq) +{\text{Cl}}^{-} \; (aq)$$.

The first test tube used to contain $$\text{NH}_4\text{OH}$$. $$\text{NH}_4\text{OH}$$ is a weak base that dissociates partially in water.

$$\text{NH}_4\text{OH} \; (aq) \rightleftharpoons {\text{NH}_4}^{+} \;(aq)+ {\text{OH}}^{-} \; (aq)$$.

There’s also an equilibrium between $$\text{OH}^{-}$$ and $${\text{H}_3\text{O}}^{+}$$ ions.

$${\text{OH}}^{-}\;(aq) + {\text{H}_3\text{O}}^{+} \;(aq) \to 2\; \text{H}_2\text{O} \;(l)$$.

$$\text{OH}^{-}$$ ions from $$\text{NH}_4\text{OH}$$ will shift the equilibrium between $$\text{OH}^{-}$$ and $${\text{H}_3\text{O}}^{+}$$ to the right and reduce the amount of $${\text{H}_3\text{O}}^{+}$$ in the solution.

The indicator equilibrium will shift to the right to produce more $${\text{H}_3\text{O}}^{+}$$ ions along with the colored indicator ions. The solution will show a pink color.

What’s the color of the solution after adding NH₄Cl?

Adding $$\text{NH}_4\text{Cl}$$ will add to the concentration of $${\text{NH}_4}^{+}$$ ions in the solution. Some of the $${\text{NH}_4}^{+}$$ ions will combine with $$\text{OH}^{-}$$ ions to produce $$\text{NH}_4\text{OH}$$.

The equilibrium between  $$\text{OH}^{-}$$ and $${\text{H}_3\text{O}}^{+}$$ ions will shift to the left to produce more of both ions.

$${\text{OH}}^{-}\;(aq) + {\text{H}_3\text{O}}^{+} \;(aq) \to 2\; \text{H}_2\text{O} \;(l)$$

The indicator equilibrium will shift to the left as the concentration of $${\text{H}_3\text{O}}^{+}$$ increases. There will be less colored ions and more colorless molecules in the test tube. The pink color will fade.