CHEM 218 ASSIGNMENT #1 – Precipitation titrations
Section: 01
Term: Winter 2014
Instructor: Dr. Dajana Vuckovic
Due date: 11:40 am before the beginning of the class on January 29, 2014 in CC-320
TOTAL POINTS: 55
General instructions:
§ You must show full/detailed calculations to receive full marks for each problem.
§ Please show your FINAL answer to correct number of significant figures. During intermediate steps, keep all digits in your calculator to avoid large rounding errors in the final answer. In other words, do not round off your answers during intermediate calculation steps.
§ Write in pen.
§ Use KSP values from textbook – Harris 8th edition
1. (8points) An analytical lab needed to determine the content of phosphorus in a commercial sample of plant food. The phosphorus in a 4.258 g sample of this plant food was converted to PO43- and precipitated by adding 50.00 mL of 0.0820 M AgNO3. The excess AgNO3 was then titrated using 4.06 mL of 0.0625 M KSCN. What was % P2O5 in the original plant food sample?
P2O5 + 9 H2O → 2PO43- + 6H3O+
2PO43- + 6 Ag+→ 2Ag3PO4(s)
Ag+ + SCN– → AgSCN(s)
2. A solution of volume 25.00 mL containing 0.0311 M Na2C2O4was titrated with 0.0257 M La(ClO4)3 to precipitate lanthanum oxalate:
2 La3+ + 3 C2O42-
La2(C2O4)3(s)
La2(C2O4)3(s)
(a) (2 points) What volume of La(ClO4)3 is required to reach the equivalence point?
(b) (3 points) Find pLa3+ when 10.00 mL of La(ClO4)3has been added.
(c) (3 points) Find pLa3+ at equivalence point.
(d) (3 points) Find pLa3+ when 25.00 mL of La(ClO4)3has been added.
(e) (5points) Plot the titration curve for this titration using Excel or other spreadsheet software
(f) (8points) Plot the titration curve for the same titration if the concentration of the titrant is increased to 0.0500 M. What effect does this have on the shape of the titration curve?
(g) (3points) Another analytical chemist performed the titration of the same oxalate sample using calcium to precipitate calcium oxalate. Is this a better approach to this analysis? Why or why not?
3. (a) (10points) Calculate pCu+ for the titration of 10.00 mL of 0.05000 M of I–with 0.02500 M Cu+ standard solution when 4.500 mL, 20.00 mL and 21.40 mL of titrant have been added. Plot the titration curve for this titration using Excel or other spreadsheet software.
(b) (10points) Calculate pCu+ for the titration of 10.00 mL of 0.05000 M of Cl– with 0.02500 M Cu+ standard solution when 4.500 mL, 20.00 mL and 21.40 mL of titrant have been added. Plot the titration curve for this titration using Excel or other spreadsheet software on the same graph as (a). How does Ksp value influence the shape of the observed titration curves?
