Prelaboratory assignment Exp. 7 Determining K Dale, _ Lab Sect 5. Data in the fo

Question

Prelaboratory assignment Exp. 7 Determining K Dale, _ Lab Sect 5. Data in the following table were obtained for the titrntion of a 0.297-g sample of a solid monoprotic weak acid with a 0 150 (abscissa) KOH solution. Plot pH (ordinate) versus Prou 0.00 2.00 4.00 7.00 10.00 12.00 14.00 1600 17.00 18.00 20.00 25.00 1.96 2.22 2.46 2.37 3.06 3.29 3.60 11.08 11.67 12.05 12.40 a. What volume of the KOH solutioes is required so reach the stoichiometric point? b. What is the pH a the stoichiometrie point? e. What is the pKa of the weak acid? d. Calculate the number of moles of weak acid analyzed.

Explanation / Answer

a)

volume of KOH --> the pH must change drastically

this point is between 16 and 17

so choose V = 16.5 mL approx for equivalence

b)

pH in stoichioemtric point, i..e V= 16.5 mL

must be betwen 4.26 and 11.08 --> pH = (11.08+4.26)/2 = 7.70 approx

c)

pKa for the acid --> in 50% half point

pH = pKa + log(A-/HA)

A- = HA is true at 50% so

pH = pK a+ log(1)

pH = pKa

V half = 16.5/2 = 8.25 mL, the point is approx 2.9

then

pH =pKa = 2.9

d)

moles of weak acid -->

mmol of base = MV = 0.15*16.5 = 2.475 mmol = 2.475*10^-3 mol

mol of acid = mol of base in equivalence

so

2.475 mol of base = 2.475*10^-3 mol of acid

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