Name Lab Daytime Titration of a Weak Acid Pre-Lab Assignment The titration curve
ID: 1024858 • Letter: N
Question
Name Lab Daytime Titration of a Weak Acid Pre-Lab Assignment The titration curve shown was obtained for the titration of a 0.297 g sample of a solid acld with 0.150 M KOH 0 S S 20 Volume of KOH added (ml) 1. What volume of KOH was added to reach the equivalence point? 2. How many moles of acid are present? Assume a 1:1 stoichiometry between acid and base. 3. What is the molar mass of this acid? 4. What is the pKa? 5. In Part A of the experiment, what volume of unknown acid will you use? 6. In Part B of the experiment, what mass of unknown acid will you use?Explanation / Answer
1.
The equivalence point is indicated by the center of the steep part of the titration curve. At this point, moles of base added equals the moles of acid originally present in the sample.
So, locate the center point on the steep part of the curve and draw a vertical line to the x-axis to get the volume.
volume of KOH at equivalence point = 17 mL.
2.
Calculating moles of acid present:
we have 0.150 M KOH and volume of KOH =17 mL
thus, moles of KOH = 0.150 mol/L * 17 mL *1L/1000mL = 2.55*10-3 moles
As the stoichiometry between acid and base is assumed to be 1:1,
moles of KOH = moles of acid
So, moles of acid = 2.55*10-3 moles
3.
Now we can determine the molar mass of the acid using moles and mass of the acid(0.297g)
Molar mass of acid = mass of acid(g) /moles of acid = 0.297g/2.55*10-3 moles = 116.47 g/mol
4.
Ka for an acid is given by,
Ka = [H+aq][A-aq]/[HAaq]
The volume of the titrant halfway from the origin to equivalence point corresponds to the point where half of the acid has been titrated, or converted to its conjugate base. So, at this point, [A-] = [HA] and thus according to Ka expression, pH = pKa.
So, pKa can be determined from the titration curve just by noting the pH at the volume halfway to the equivalence point. For given acid, pKa = pH at 8.5mL that is 1/2 of 17mL.
Mark the equivalence point on the curve. Then mark the midpoint volume on x-axis. Draw a vertical line from the point on x-axis to the curve and then draw a horizontal line from the point on curve to the y-axis. Note down the pH.
So, pKa = 2.9
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.