In order to choose an appropriate indicator, you must know what the pH should be

Question

In order to choose an appropriate indicator, you must know what the pH should be at the equivalence point of your titration. Which of the following indicators would be most appropriate to determine the equivalence point for a titration of a weak base [25 mL 0.10 M morpholine (Kb= 3.1 x10^-6)] with 0.10 M HCl?

Disregard the circled answer, the correct answer is C, but I am not sure how to get that.

Explanation / Answer

The reaction is a titration of weak base and strong acid (HCl). Thus the resultant reaction will have the pH < 7. Among the given options, there are two indicators which indicate the pH < 7.

To choose an appropriate one -

Let B is a weak acid.

Thus -

B (aq) + HCl (aq) <--------> BH+ (Aq) + Cl- (aq)

Hence, at the equivalence point there will be BH+ to be existed.

Therefore, BH+ (aq) + H2O (l) <--------> B (aq) + H3O+ (aq)

The number of mols of morpholine = (0.10 M)(0.025 L) = 0.0025 mol

Thus, volume of HCl required = 0.0025 mol / 0.10 M = 0.025 L = 25 mL

Thus, the resultant concentration of a salt = 0.0025 mol / 0.05 L = 0.05 M

By using ICE table -

[H3O+] = (Ka*C)0.5  [where, C is the concentration of salt, Ka = Kw / Kb = 10-14 / 3.1*10-6 = 3.225*10-9 )

By substituting all values -

[H3O+] = [(3.225*10-9) (0.05)]0.5 = 1.27*10-5 M

pH = -log[H3O+] = - log (1.27*10-5) = 4.9

The value obtained here is approximately equals to 5.0. HEnce the indicator at pH = 5 has to be chosen.

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