Part A) What is the pH at the equivalence point for the titration of 0.17 M solu

Question

Part A) What is the pH at the equivalence point for the titration of 0.17 M solutions of the following acids and bases.

1) HNO2 and NaOH

2) HI and NaOH

3)CH3NH2 (methylamine) and HCl

Part B) Which of the following indicators would be suitable for each titration?

Which of the indicators in Figure14.4 in the textbook would be suitable for this titration?

Methyl orange. Alizarin yellow. Bromcresol green. Methyl red. Chlorphenol red. Thymol blue. Phenol red. Thymolphthalein. Bromthymol blue. Phenolphthalein. Methyl violet.

Explanation / Answer

Ans. #1. HNO2 is a weak acid. Titration of weak acid with strong base forms the salt at equivalent point. The resultant cation of weak acid acts as conjugate base and accepts a proton from water as follow-

            NO2- + H2O <------> HNO2 + OH-

Formation of OH- ions makes the aqueous solution basic.

So, at equivalence point, the pH of solution in BASIC.

# Choice of Indicator: Initial pH of weak acid is acidic. The pH at equivalence point is basic. So, choose a pH which changes color when pH is increased, or an indicator that is colorless in acid and neutral pH but shows color in basic pH. Moreover, the color change shall be closest to the post-equivalence pH.

#Example: Thymol blue, Phenolphthalein, Thymolphthalein, Alizarin Yellow,

#2. HI is a strong acid, and NaOH is a strong base. The pH at equivalence point of strong acid-strong base titration is 7.0 because the resultant salt undergoes complete dissociation and none of the resultant ions react with H2O.

So, at equivalence point, the pH of solution in NEUTRAL.

# Choice of Indicator: Methyl orange, Thymol blue, Phenolphthalein,

#3. Methylamine is a weak base. Titration of a weak base with strong acid yields the equivalence point with acidic pH. The cation of salt of weak acid acts as conjugate acid and donates a proton to the solution, thus making the solution at equivalence point acidic.

            CH3NH3+ + H2O <-------> CH3NH2 + H3O+

Formation of H3O+ ions makes the aqueous solution basic.

So, at equivalence point, the pH of solution in ACIDIC

# Indicator: Methyl orange, Methyl red, Chlorophenol red,

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