Use the following titration curve to answer the next three questions. This curve

Question

Use the following titration curve to answer the next three questions. This curve demonstrates titration of a weak acid, HB, by a 0.24 M solution of sodium hydroxide. the 50 100 150 200 volume NaOH added (mL) 14. Which of the following indicators would be most appropriate for this titration? a. Methyl yellow b. Bromophenol blue c. Methyl red d. Phenolphthalein e. Alizarin yellow R 15. The half-equivalence point is labeled and the equivalence point is labe a. A,C b. A; D c. B; D d. C; D e. D; E 16. Which of the following is/are the major species at point Cof the titration? Weak acid (HB) b. Weak base (B) c. Weak acid (HB) and weak base (B) d. ??' e. H30* a.

Explanation / Answer

This is a titration of Weak Acid and Strong Base.

A1) d)

We choose an indicator which changes colour as close as possible to the equivalence point. Here the equivalence point is D. The pH range about the equivalence point, from the graph , is 8-9. Therefore the indicator we choose is, Phenolphthalein which has a pH range of 8.2-10.

A2) d)  

Equivalence point, by definition is the point where the initially added Acid (solution) is completely neutralized by the base (titrant). (ie, the acid and base are mixed in exactly right proportions depending on the equation ) At equivalence point the colour change of indicator is observed. Here the equivalence point is D.
Half equivalence is C. (Half equivalence is the point where exactly half the acid has been neutralized). Therefore if 100ml NaOh is required to completely neutralize the acid , so 50ml will be required to neutralize half of it by stoichiometry. Hence D.

A3) c)

At half equivalence point exactly half the weak acid is neutralized and [HB]= [B-1] forming a buffer solution with the special condition that pH=pKa.

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