The titration curve of a 25.00-ml. sample of a monophonic acid (HA) titrated wit

Question

The titration curve of a 25.00-ml. sample of a monophonic acid (HA) titrated with NaOH is shown below. For questions through circle the best response for each question. Is the acid strong or weak? Strong Weak W hat is the approximate pH at the equivalence point? 5.0 7.0 9.0 What is the approximate pKa of this acid? 5.0 9.0 12.0 Which of the following indicators would be the best choice to determine the endpoint? The pH ranges of the respective indicators are provided below. Bromocresol Green (3.8-5.4) o-Cresolphthalein (8.5-10.0) What volume of NaOH was required to completely react with the acid? 12 30 mL 24.60 mL 31.00 mL

Explanation / Answer

a) This is a weak acid, since there is a small change of pH in "buffered" zones (V = 5 ml to 20 ml approx)

b)

Equivalence point is approx found in V = 22 ml

the middle poiunt value is about 8.5-9.0... The only valid option is 9 which is basic

c)

Ka value may be found when

pH = pKa + log(A-/HA)

This is when we arte in the half equivalence point, therefore V2 = 22ml / 2 = 11 ml approx

pH = pKa = 4.5 - 5 approx

choose 5

d)

you need an indicator in the pH apprx 9, therefore choose o -Cresolphtalein

e)

This is calculated in the equivalence point... which is about V = 22 ml, nearest anser is 24.6 ml, you can see this in the drastic chagne of pH

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