2.5 grams of a ferrous sample ore are dissolved with acid and diluted to a total
ID: 1011193 • Letter: 2
Question
2.5 grams of a ferrous sample ore are dissolved with acid and diluted to a total volume of 100 mL. Then, 0.75 grams of K_2Cr_2O_7 are dissolved and diluted to 250 mL. 9.00 mL of the dichromate solution are required to reach the endpoint when titrating 20.0 mL of the previously prepared ferrous solution. The reduction-oxidation reaction is Cr_2O_7^2- + 6 Fe^2+ + 14H^+ rightarrow 2 Cr^3+ + 6Fe^3+ + 7H_2O Determine the percent iron in the unknown ore. A potentiometric titration of the previous dichromate and ferrous solutions was performed. Explain how to determine the endpoint of this titration using (1) the first derivative curve and (2) the second derivative curve. thank you^-^Explanation / Answer
Part A:
Mass of K2Cr2O7 = 0.75 g
Volume of K2Cr2O7 solution = 250.0 ml
In 250 ml, K2Cr2O7 present = 0.75 g
In 9.00 ml, K2Cr2O7 present = (0.75 g*9.00)/250 = 0.027 g
Molar mass of K2Cr2O7 = 294.19 g/mol
Moles of K2Cr2O7 used = mass/molar mass = 0.027/294.19 = 9.18*10-5 mol
moles of Cr2O72- = moles of K2Cr2O7 = 9.18*10-5 mol
In the reaction, we can see that 1 mol of Cr2O72- requires 6 mol of Fe2+ for titration.
Moles of Fe2+ = 6 moles of Cr2O72- = 9.18*10-5 mol*6 = 5.5*10-4 mol
For Ferrous sample, 100 ml solution contained 2.5 g Ferrous sample
20 ml of solution will contain ferrous sample = 2.5*20/100 = 0.5 g
Number of moles of Fe2+ = 5.5*10-4 mol = mass/molar mass
molar mass = mass/number of moles = 0.5/5.5*10-4 mol = 909 g/mol
molar mass of compound = 909 g/mol
atomic mass of Fe2+ = 56 g/mol
% iron in ore = (56/909)*100 = 6.16%
Part B:
The titration curve of Fe2+ vs dichromate would be sigmoidal shape. To get the end point, first derivative curve will be plotted. The plot will give a sharp peak. It will show a spike at the end point. The spike is the end point of the titration.
In the 2nd derivative plot, there will be two peaks, one in +ve axis and one in negative axis. The point which intersects with the volume axis is the end point.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.