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Ascorbic acid (H2C6H6O6), also known as Vitamin C, is a polyprotic acid found in fruit, tomatoes, potatoes and leafy vegetables. The pKa's of the ascorbic acid are pKa1 = 4.10 and pKa2 = 11.80 at 25 °C. When ascorbic acid is titrated with NaOH and it takes 60.0 mL to remove all protons possible. In the titration curve, how many mL of NaOH are required to reach the first pKa? Another way of asking this is when presented with a titration curve of ascorbic acid where on the x-axis should one look (which volume of NaOH), if one wants to determine the first pKa?

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Answer:

as when we add 60 ml it removes all the protons it means it is 2nd equivalence point of Ascorbic acid

and we know that pH = pKa1 when moles of NaOH is half of the 1st equivalence point

1st equivalence point = 2nd equivalence point / 2 = 60/2 =30ml

1st half equivalence point = 30/2 = 15ml

so when we add 15ml of NaOH

pH = pKa1 =15mL