Question

Volume at equivalance point

Volume at one-half equivalance point

Average Ka (show calculations) ... Standard deviation of Ka

350 Ron Sheet Vetermination of the Dissociation Constant of a Weak Acid Third determination ml NaOH PH 3.33 3.68 mill- sorinis 1513 Oleme royale =- BmL C. Determination of pK, of Unknown Acid First determination Second determination ml NaOH PH ml NaOH pH IML 3.25 IML 2.84 am 3.53 2mL 3.11 SmL 3.74 30 4ML 3.88 4mL Bm 403 5ML 4.01 GML 4.15 6 mL 4.17 63mL 4.23 TML 4.35 Lelmi 4.27 4.49 7.ML 4.33 4. 7.402 One 4.BO 4.43 um - 4.99 3.5m 12ml 5.82 om 13mL 5.63 lomu 13.am 5.71 ML 403 19.40 5.82 516 13. but 3.93 12m IS. Sme 6.18 13.2mL 10.99 13.4001 13.10L HOML 14.20 15 mL 12.31 Tom 18.71 4.35 4.53 4.61 13.2 13.4 13.6 7.91 13.8 18.01 18. as Volume at equivalence point Volume at one-half equivalence point pk K pk. pa Average K, (show calculations) Standard deviation of K,

Answer 1

The equivalence point and one-half equivalence point can be determined by plotting a graph between pH (y-axis) and volume of NaOH (x-axis).

At one-half equivalence point or midpoint pH = pKa and from pKa we can determine Ka. Below I have attached the graphs corresponding to the three determinations and marked the midpoint and equivalence point.
14 First determination Equivalence point Volume 14.01 mL 8 Mid Equivalence point 6 Volume 9.49 mL 4 2 0 2 8 10 12 14 18 Volume of NaOH (mL) 16 12 10 Hd

14 12 Second determination 8 Equivalence point Volume 13.90 mL Mid Equivalence point 6 Volume 8.84 mL 4 2 0 2 2 4 6 8 10 12 14 16 18 Volume of NaOH (mL) 10 Hd

Third determination 10 8 Equivalence point Volume 13.70 mL Mid Equivalence point Volume 8.60 mL 2 0 -2 T 2 4 10 12 14 16 Volume of NaOH (mL) 12 Hd
At half-equivalence or midpoint pH = pKa

For the first determination, pH = pKa = 4.64

Ka = 10^(-pKa) = 10^(-4.64) = 2.29

For the second determination, pH = pKa = 4.63

Ka = 10^(-pKa) = 10^(-4.63) = 2.34

For the third determination, pH = pKa = 4.64

Ka = 10^(-pKa) = 10^(-4.64) = 2.29

Average Ka = (2.29+2.34+2.29)/3 = 2.31