Question

Question 8 For the polyprotic acid histidine (HisH3), the hydrogens in bold can be lost. When HisH3t is titrated w to reach the second pka? Another way of asking this is when presented with a titration curve of Hisha A. 45 ml B. 37.5 mL C. 30 mL OD. 15 mL E. 22.5 mL

Answer 1

Answer:

In addition to 45 mL of NaOH, all protons have been removed. This shows that the third equivalence point is attained at 45 mL of NaOH.

Hence, the three equivalence point will be obtained at 15 mL, 30mL, and 45mL volume of NaOH.

Now, the half volume of equivalence volume results into the pKa value. Hence, the three pKa values will be obtained at 7.5 mL, 22.5mL, and 37.5 mL volume of NaOH as explained below.

First equivalence point = 15 mL

First Mid Point = 15/2 =7.5 mL

Second Equivalence point = 30 mL

Second mid point = 15+ ( 30-15/2)= 15+7.5 = 22.5 mL

Third equivalence point = 45 mL

Third Mid point/pKa = 30+ (45-30/2) = 30+7.5 = 37.5 mL

Hence, the correct option is B.

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