0.25 po f Figure 1 pH curve above gives the following equivalence point volumes

Question

0.25 po f Figure 1 pH curve above gives the following equivalence point volumes of strant Equiv pt. volume 1 = 10.25 mL Equiv. pt. volume 2 20.15 mL Equiv. pt. volume 3-30.20 mL (a) calculate the volume (Iin mL) at half equivalence (yellow dots on the pH curve) for the following (express your answer in correct number of significant fgure Midpoint 1 Midpoint 2 = Midpoint 3- If, half equivalence Midpoint 2 pH is 5.5, (b) what is Ka2? Ka2- the theoretical value of Kaz is 3.12 x 106, () what is the percent erro? %Error (Show all calculations to be able to get a full credit. Each answer is worth 0.05 point)

Explanation / Answer

Equivalence point is the point where the concentration of acid and base is equal. In a polyprotic acid titration multiple equivalence points represents the volume of base requred to neutralise each proton respecitvely and the volume at the half equivalence point is the reaction in titration reaches halfway.

So,

a) Midpoint 1 = 10.25 ml / 2 = 5.125 ml

second half equivalence point is the midpoint between first and second equivalence point

Midpoint 2 = (10.25 + 20.15) / 2 = 15.200 ml

third half equivalence point is the midpoint between second and third equivalence point

Midpoint 3 = ( 20.15 + 30.20) / 2 = 25.175 ml

b) At second half equivalence point PH = 5.5 = PKa2

5.5 = -log Ka2

Ka2 = 10-5.5 = 3.16 * 10-6

c) percent error = ((calculated value - therotical value) / therotical value) * 100

% error = (( 3.16 * 10-6 - 3.12 * 10-6) / 3.12 * 10-6) * 100 = 1.282 %

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